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BIOL 181: Life in the Oceans – Lecture Notes

9. Sponges, Cnidarians, and Comb Jellies

(The majority of the text below originally appeared as chapter 6 ofIntroduction to Marine Biology)9.1. Animals

Animals are organisms that are multicellular, eukaryotic, and lack rigid cell walls. They are heterotrophs: cannot produce their own food and need to consume other organisms. Except for adult sponges, animals can actively move, even those that live a sessile lifestyle. Animals that don’t have a backbone are called invertebrates; they represent the vast majority of animal species. Animals with a backbone are called vertebrates.

9.2. Sponges: Phylum Porifera

Sponges are the simplest of multicellular animals. They have no tissues, organs or nervous system and their cells show little differentiation and specialization. They are asymmetric and sessile, and show a variety of growth forms and colors.

9.2.1. Structure of Sponges

Sponges are filter feeders, and constantly circulate water through their bodies. They have many small incurrent holes called ostia, and one or few excurrent holes called the osculum (Figure 9.1). The cavity inside the sponge is called the spongocoel.


BIOL 181: Life in the Oceans – Lecture Notes

Diagram of a Syconoid Sponge, by Kelvinsong, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 9.1. Parts of a typical sponge, with main types of cells.

Sponges lack specialized tissues; instead, they have certain cell types that are specialized to perform a function. Choanocytes (collar cells) line the spongocoel and use a flagellum to move water through. They also trap suspended food particles. Archaeocytes are amoebalike cells that move throughout the body to transport food and aid in repair and regeneration.

Sponges have spicules, skeletal elements made of calcium carbonate, silica, or spongin that provide structural support. Spongin is a protein that forms a flexible fiber found in sponges of the class Desmospongia, the sponges that are harvested commercially.

9.2.2. Size and Body Form of Sponges

The size of a sponge is limited by its ability to circulate water through its body, which in turn is limited by its body form. The simplest of sponges are small and tubular. Increasing folding of the body wall in more complex sponges allows for an increased surface area for choanocytes and allows those sponges to grow to a bigger size (Figure 9.2).

Porifera Body Structure, by Philcha, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 9.2. Main growth forms in sponges. More complex folding (e.g., leuconoid) allows increased surface area for collar cells and a bigger size.


BIOL 181: Life in the Oceans – Lecture Notes

9.2.3. Nutrition and Digestion in Sponges

Sponges are suspension feeders (or filter feeders) and create a current of water through their body by the beating of the flagella in the choanocytes. In this way, they capture small particles in the water including bacteria, plankton, and detritus. Food particles get trapped in the collar cells (choanocytes) and are transported through the sponge by the archaeocytes, which also function in food storage. Undigested materials and waste products exit the sponge through the osculum.

9.2.4. Reproduction in Sponges

Sponges can reproduce asexually by budding, where a group of cells on the outer surface of the sponge develop and grow into a new sponge. When these are developed, the new sponges drop off and settle nearby. Another method of asexual reproduction is fragmentation, where pieces broken off by waves and storms develop into new individuals.

Sponges also reproduce sexually. Most species are hermaphrodites, and produce both male and female gametes, although not usually at the same time. Sperm is released in the water column and captured by another sponge. The sperm is engulfed by a choanocyte and transported to an egg. The fertilized eggs are released in the water column, where the larvae develop until they are ready to settle on the benthos (Figure 9.3).

Stove-Pipe Sponge-Pink Variation, by Nick Hobgood, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.


BIOL 181: Life in the Oceans – Lecture Notes

Figure 9.3. A sponge.

9.2.5. Ecological Roles of Sponges

Sponges are important inhabitants of shallow water ecosystems and compete for space with animals like corals and bryozoans. They produce chemicals (toxins) to kill or inhibit their competitors. Other sponges create their own habitat, such as boring sponges that burrow into coral, thereby playing an important role in calcium recycling. The spicules and toxins of sponges deter most predators, though hawksbill sea turtles and angelfish have evolved adaptation to withstand them. Sponges form symbiotic relationships with a variety of other organisms. Many bacteria and some dinoflagellates live within their tissues, and the spongocoel offers shelter for a variety of organisms.

Chemicals have been isolated from Caribbean sponges that block DNA synthesis in tumors, and there is the prospect of utilizing them in cancer treatment. The antibacterial properties of sponges are also being studied.

9.3. Cnidarians

9.3.1. Structure of Cnidarians

Cnidarians have radial symmetry, with their body organized around a central mouth. This type of symmetry is common in organisms that are sessile or do not live an active lifestyle, and allows them to respond to stimuli equally from all sides. Cnidarians have two main body plans, a benthic polyp and a pelagic medusa (Figure 9.4). Many groups of cnidarians exhibit an alternation of generation, alternating between the two body plans, while others have lost one or the other.

Two Basic Body Forms of Cnidaria: Medusa and Polyp, by Philcha, is in the public domain in the United States.88

BIOL 181: Life in the Oceans – Lecture Notes

Figure 9.4. Typical medusa (left) and polyp (right) stages of cnidarians.

The phylum Cnidaria is characterized by stinging cells called cnidocytes, which contain a stinging organelle called a cnida. The spearing type of cnida, called nematocysts, can be triggered by touch or chemical stimulus. They are found mostly in the tentacles but also in the outer body walls and gastrodermis. They are used both in catching prey and in defense. The sting of some species, including the Portuguese man-o-war and box jellyfish, is so powerful that it can be deadly to humans. However some animals (e.g., anemone fish) are immune to cnidarian stings.

9.3.2. Cnidarian Classes

There are four classes in the phylum Cnidaria, each showing different life histories. The hydrozoans (class Hydrozoa) have an alternation of polyp and medusa stages, both of which are small. The polyps are mostly colonial. The colony includes feeding polyps and reproductive polyps (Figure 9.5). Fire coral (Figure 9.6a) is a type of hydrozoan that secretes a calcium carbonate skeleton and contributes to reef building, and because of that is often confused with true corals (class Anthozoa). The Portuguese man-o-war is another type of hydrozoan in which the polyp colony, rather than being benthic, is attached to a float (Figure 9.6b).


BIOL 181: Life in the Oceans – Lecture Notes

Putative Scheme of the Life Cycle of H. Antarctius, by Lucilia S. Miranda, Allen G. Collins, and Antonio C. Marques, is available under aCreative Commons Attribution-ShareAlike 2.5 Generic license.

Figure 9.5. The reproductive polyps of release medusa through asexual reproduction.


BIOL 181: Life in the Oceans – Lecture Notes

Fire Coral, Looking Up, by Derek Keats, is available under aCreative Commons Attribution 2.0 Generic license.

Portuguese Man-of-Warby NOAA is in the public domain in the United States.91

BIOL 181: Life in the Oceans – Lecture Notes

Figure 9.6a and b. Hydroids (class Hydrozoa); fire coral (a) and Portuguese man-o-war (b).

The true jellyfish (class Scyphozoa) have a dominant medusa stage, with a reduced or absent polyp stage (Figure 9.7a). Scyphozoan medusae are larger than hydrozoan medusae, and they can swim better. They swim by pulsating their bodies, but are still considered part of the plankton because they cannot swim against a current. Jellyfish, as all cnidarians, have a very simple nervous system without a brain or eyes, but some species have photoreceptors that allow them to tell light from dark. Several species avoid sunlight and come to the surface on cloudy days and at twilight.

Box jellies (class Cubozoa) are closely related to scyphozoans. They are voracious predators and feed primarily on fish (Figure 9.7b).

Jellyfishes Floating in the Sea


is available under a

Creative Commons Attribution-ShareAlike 2.0 Generic license. © stefani.drew.

BIOL 181: Life in the Oceans – Lecture Notes

Cubozoa, by Arthurfogo7, is in the public domain in the United States.Figure 9.7a and b. Typical scyphozoan (a) and cubozoan (b).

The anthozoans (class Anthozoa) are benthic cnidarians with a polyp stage only, and all adults are sessile. This class includes corals, gorgonians, and sea anemones (Figure 9.8). In sea anemones, the polyps are larger and more complex than hydrozoan polyps. They are typically solitary and attach to the substrate. Sea anemones catch prey with their tentacles, which they can move and withdraw into the gastrovascular cavity. Hard corals (Hexacoralia) are polyps that secrete a hard skeleton, most often of calcium carbonate. Most hard corals are large colonies made of smaller polyps, where all polyps are interconnected. Some others are solitary. Polyps of hard corals exhibit a six-part radial symmetry. Hard corals form the extensive calcium carbonate reefs found in many tropical regions (see Chapter 17). Soft corals (Octocoralia) have an eight- part radial symmetry and produce a flexible skeleton. They include gorgonians (sea fans and sea whips) as well as sea pansies and sea pens (in areas of softer sediments).


BIOL 181: Life in the Oceans – Lecture Notes

Hard Coral, by Nick Hobgood, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.


BIOL 181: Life in the Oceans – Lecture Notes

Sea Anemone, by Brocken Inaglory, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Fan Coral, by Anders Poulsen, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 9.8 a, b, and c. Representative anthozoans: hard coral (a), sea anemone (b) and gorgonian (c).


BIOL 181: Life in the Oceans – Lecture Notes

9.3.3. Nutrition and Digestion in Cnidarians

Many hydrozoans and anthozoans are suspension feeders. Scyphozoans, cubozoans, and sea anemones are mainly carnivorous and feed on fish and invertebrates, although many also feed on plankton. Cnidarians trap food on their tentacles with their nematocysts and bring it to their central mouth by movement of cilia on the tentacles. Cnidarians do not have a complete digestive tract with an anus; they digest their prey in the central gastrovascular cavity, and then expel their waste back through their mouth (Figure 9.9).

Coral Polypby NOAA is in the public domain in the United States.


BIOL 181: Life in the Oceans – Lecture Notes

Figure 9.9. Cross-section of a coral polyp showing the gastrovascular cavity and the central mouth.

Many cnidarians have symbiotic algae within their tissues. The upside-down jellyfish has algae in its tentacles and swims upside down to expose the algae to higher light levels. Corals obtain most of their nutrition from their symbiotic zooxanthellae (a dinoflagellate) and also feed on plankton.

9.3.4. Reproduction in Cnidarians

Cnidarians can reproduce both sexually and asexually. Asexual reproduction usually occurs in the polyp stage through budding, fission, fragmentation, or the production of medusae.

Sexual reproduction occurs in different ways for the different groups of cnidarians. Hydrozoans have both an asexual polyp stage and a sexual medusa stage, and sexual reproduction happens in the medusa stage, which release gametes in the water column to be fertilized and develop into a new polyp stage (Figure 9.10). Scyphozoans have a similar reproduction to hydrozoans, but the polyp stage is much reduced.


BIOL 181: Life in the Oceans – Lecture Notes

Obelia GeniculataFigure 9.10.Obelia, a typical hydrozoan showing alternation of generation. The benthic polyp

reproduces asexually to produce medusae, which reproduce sexually.

Anthozoans have a well-developed polyp stage and a reduced or absent medusa stage. Asexual reproduction is common for most anthozoans, and fragmentation is an important mode of reproduction in some tropical corals. Moreover, large coral colonies are created by the asexual reproduction of polyps through budding. As there is no medusa stage, sexual reproduction is conducted by the benthic polyps, which release their gametes in the water column. This is often timed with lunar cycles and other environmental cues to form mass spawning that may include several different species (e.g., in tropical corals). Mass spawning of several species has the


BIOL 181: Life in the Oceans – Lecture Notes

advantage of overwhelming predators with a large amount of eggs at the same time, thereby reducing the mortality of eggs and larvae.

9.3.5. Ecological Roles of Cnidarians

Many cnidarians are important predators and catch prey with their tentacles and stinging cells; therefore, they have few predators themselves. However some species are adapted to eat jellyfish, including several sea turtles and some fish, and the crown-of-thorns starfish is an important predator of corals in the Indo-Pacific.

Reef-building corals create the largest living structure on the planet, which is a three-dimensional habitat for many other species and offers a solid substrate for attachment of benthic organisms. Tropical coral reefs are one of the environments with the highest species diversity on earth. Moreover, they are important wave buffers and protect the coast from storm damage.

Cnidarians form symbiotic relationships with many other organisms. The most obvious is probably the zooxanthellae that live in the tissues of hard corals and provide them with food and oxygen in exchange for shelter, carbon dioxide, and nutrients. Sea anemones are often found with anemone fish (clown fish) or cleaner shrimp. These organisms are immune to the sting of the anemone and find shelter in its tentacles. Many sea anemones and hydrozoans are found on the shell of hermit crabs. As they move around with the crab, they are exposed to more sources of food than they would by remaining in the same location.

9.4. Phylum Ctenophora: Comb Jellies

Ctenophores are planktonic, nearly transparent and exhibit radial symmetry (Figure 9.11a and b). At first glance, they look similar to jellyfish, but they lack stinging cells and have eight rows of comb plates (ctenes). Ctenes are composed of large cilia that propel the animal with their beating. Ctenophores do not have rings of tentacles that surround their mouth like jellyfish, although some have two long tentacles that are lined with adhesive cells and used in catching prey. Many ctenophores are iridescent during the day and luminescent at night, and their bioluminescence is thought to be used to attract mates or scare predators.


BIOL 181: Life in the Oceans – Lecture Notes

Comb Jelly, by Bruno C. Vellutini, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Ctenophore Body Vert SectionFigure 9.11a and b. A ctenophore (a) showing rows of ctenes and tentacles lined with sticky

cells; vertical cross-section of the same (b).


BIOL 181: Life in the Oceans – Lecture Notes

Ctenophores are carnivorous and are important predators of smaller plankton. Ctenophores catch prey with the adhesive cells on their tentacles and carry them to their mouth. Ctenophores have the beginnings of a complete digestive tract, but although they have anal pores, most expel undigested food back through their mouth like cnidarians. Some ctenophores prey on jellyfish, and can incorporate the cnida of their prey and use them to capture subsequent prey.

Almost all ctenophores are hermaphrodites. Most release their gametes in the water column, where fertilization occurs. Some brood the eggs in their bodies.

9.5. Review Questions: Sponges, Cnidarians, and Ctenophores

What are four distinctive characteristics of animals?What is the phylum that contains sponges?What is the name of the incurrent and excurrent pores in sponge?Are the cells in sponges organized into tissues and organs?What are choanocytes (collar cells)?What is the role of spicules?What are spicules made of?What is spongin?How do sponges reproduce asexually?How do sponges prevent predation?Give an example of commensalism involving sponges.Which organisms are members of the phylum Cnidaria?What is the benefit of radial symmetry in the cnidarians?What are the two types of cnidarian body plans?What is an example of a spearing type of stinging organelle?What triggers the stinging cells in cnidaria?Give an example of a member of the class Hydrozoa.What is the skeleton of fire coral made of?Is the Portuguese man-of-war an example of a solitary or colonial hydrozoa?Give an example of a member of the class Scyphozoa.Is jellyfish part of the plankton?Give an example of a member of the class Anthozoa.What is the skeleton of hard coral made from?Do cnidarians have an anus?How does the upside-down jellyfish obtain its food?How do hard corals feed?What are the advantages of mass spawning in hard corals?How do corals reproduce asexually?Give an example of a serious coral predator in the Pacific.Give an example of a mutualistic symbiotic relationship involving Cnidaria.Can a ctenophore sting you?


BIOL 181: Life in the Oceans – Lecture Notes

10. Worms, Bryozoans, and Mollusks

(The majority of the text below originally appeared as chapter 7 ofIntroduction to Marine Biology)10.1. Bilateral Symmetry

There are three types of symmetry found in the animal kingdom: radial, bilateral, and biradial (Figure 10.1a, b, and c). The Porifera and Cnidaria are characterized by radial symmetry, in which there are an infinite number of planes that can divide an animal into two mirror-image halves, each one passing through a central axis. (In fact, most sponges and corals display no obvious symmetry at all, but their bodies are built of repeated units which are radially symmetric and so can properly be considered to have radial symmetry.) Almost all the rest of the Animal Kingdom, comprising 98% of animal species, is characterized by bilateral symmetry, in which there is one and only one plane that can divide an animal into two mirror-image halves.

Radial symmetry is characteristic of sessile animals, which need to be able to respond to stimuli coming from any direction. Bilateral symmetry is characteristic of animals which move actively through their environment. As a bilaterally symmetric animal moves, one end tends to encounter novel stimuli first, and sensory and feeding structures develop at that end, forming a head. This process is known as cephalization.

The Ctenophora have their own unique kind of symmetry, known as biradial symmetry, in which there are two planes which can divide an animal into two mirror-image halves, each plane perpendicular to the other.

Haeckel Actiniaeis in the public domain in the United States.


BIOL 181: Life in the Oceans – Lecture Notes

Haeckel Platodesis in the public domain in the United States.

Haeckel Ctenophoraeis in the public domain in the United States.103

BIOL 181: Life in the Oceans – Lecture Notes

Figure 10.1a, b, and c. Types of symmetry found in various groups of animals. Sponges and cnidarians: radial symmetry (a); almost all animals: bilateral symmetry (b); ctenophores: biradial symmetry (c).

10.2. Marine Worms

Marine worms have elongated bodies and lack any external skeleton. They gain support for their body from fluid contained in body compartments, known as a hydrostatic skeleton. Marine worms are a very large group comprising many different phyla. This chapter will cover only a few of the important groups of worms.

10.2.1. Platyhelminthes (Flatworms)

Structure and Description

Flatworms, as their name suggests, have a flattened body (Figure 10.2). They show bilateral symmetry, with a head and a posterior end. They have eye spots that allow them to sense differences in light intensity (for example, the shadow of a predator above). Flatworms have a blind gut, and like cnidarians, their waste products are expelled back through their mouth. Some flatworms are free-living while others are parasites. Free-living flatworms move by gliding on their ventral side through the action of cilia.

Bedford's Flatworm, by Jan Derk, is in the public domain in the United States.10.2. Typical platyhelminthe, showing bilateral symmetry and cephalization.

Groups of Platyhelminthes


BIOL 181: Life in the Oceans – Lecture Notes

Turbellarians are a group of free-living flatworms, which measure from a few millimeters to 50 cm long. Most are benthic, and they are often found in the meiofauna, occupying interstitial spaces between sediment grains. They are common in the Arctic and Antarctic waters, where they feed on sea ice diatoms.

Flukes are parasitic worms with a complex life cycle, frequently involving several hosts. Tapeworms are parasites that live in the digestive tract of their host. They can get quite large, and may attain 30 m long in whales (Figure 10.3a, b, and c).

Marine Flatworm, by Jens Petersen,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.


BIOL 181: Life in the Oceans – Lecture Notes

A Giant Digenean Parasite, by Anilocra, is in the public domain in the United States.

Proglottids of Diphyllobothrium LatumFigure 10.3a, b, and c. Representatives of the Phylum Platyhelminthes: turbellarian (a), fluke (b),

and tapeworm (c).

Nutrition and Digestion

Most free-living flatworms are carnivorous and feed on small invertebrates. They have chemoreceptors to help locate their prey. Flatworms have various strategies to catch and eat their prey; some entangle their prey in mucus to suffocate it, others stab their prey with a sharp penis (stylet) that comes out of their mouths. The food is digested externally or in the gastrovascular cavity. Wastes from digestion are ejected through the mouth.


Flatworms can reproduce both asexually or sexually. Asexual reproduction enables regeneration of missing body parts. Flatworms are typically hermaphrodites, and sexual reproduction involves reciprocal copulation and internal fertilization.

10.2.2. Nematodes (Round Worms)

Nematodes are the most numerous animals on earth, and can be found in densities of up to 4,400,000 individuals/m2in some sediments. The body is cylindrical and elongated, tapered at both ends (Figure 10.4). Most of them are small (less than 5 cm), but some can reach lengths of over 1 m. Nematodes may be scavengers, parasites, predators, or eat algae and bacteria. Most are hermaphrodites.


BIOL 181: Life in the Oceans – Lecture Notes

Anguillicola Crassus, by Bill.bessmer,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 10.4. Nematode worms.

10.2.3. Annelids (Segmented Worms)

The bodies of segmented worms are divided internally and externally into repeating segments (Figure 10.5), which allows them to be more mobile with a more efficient hydrostatic skeleton. Their body wall has longitudinal and circular muscles for movement: crawling, swimming, and burrowing. Their skin often has bristles (setae) that can be used for locomotion, digging, or protection. Annelids have a complete digestive tract with a mouth and an anus. The most common marine annelids are polychaetes, which live in various habitats: sand, mud, under rocks, in crevices, or in tubes they create. Many are mobile, or errant; others are sedentary.


BIOL 181: Life in the Oceans – Lecture Notes

Bearded Fireworm, by Colin Ackerman,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 10.5. Annelid worm showing repeated segments. This bearded fireworm (Hermodicesp.) has calcareous chaetae that break off and release a poison for defense.

Feeding and Digestion

Errant polychaetes tend to be predators or deposit feeders. Active predators feed on small invertebrates, dead animals, and algae. Many have jaws or teeth to catch their prey. Nonselective deposit feeders (e.g., lugworms,Arenicola) ingest sediment and digest the organic matter in it, releasing the nondigestable mineral particles in their feces as mounds of sediments called fecal casts (Figure 10.6). Selective deposit feeders (e.g., spaghetti worm,Eupolymnia crassicornis) separate organic materials from minerals and ingest the organic matter.

Lugworm Cast, by Nveitch,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.


BIOL 181: Life in the Oceans – Lecture Notes

Figure 10.6. Faecal casts produced by the lugworm, a nonselective deposit feeder.

Sedentary polychaetes are either burrowers that live in sediments or tube dwellers that construct a tube. Sedentary polychaetes are usually suspension feeders, and the head is usually modified with special feeding structures to collect detritus and plankton from the water (Figure 10.7a, b, and c). Food particles are then passed to the mouth.

Spirobrancheus Giganteus, by Nick Hobgood,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.


BIOL 181: Life in the Oceans – Lecture Notes

Christmas Tree Worms, by Nick Hobgood,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Tubeworm, by Nick Hobgood,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 10.7a, b, and c. Sedentary polychaetes showing the modified head appendage used in feeding. Christmas tree worms (a and b) and tubeworms (c).


BIOL 181: Life in the Oceans – Lecture Notes


Some polychaetes can reproduce asexually, through the process of budding or division of the body into fragments. They therefore have a high ability to regenerate lost body parts, e.g., tentacles, the head, even nearly their entire body. However, most species only reproduce sexually and they are gonochoristic (single sexes). The gametes accumulate within the body cavity and are released through ducts or the rupture of the body cavity, in which case the adult subsequently then dies.

10.2.4. Ecological Roles of Marine Worms

Nutrient Cycling

Burrowing worms can be important in recycling nutrients. Most decomposition occurs on the benthos, and through their burrowing, worms bring nutrients back to the surface where they can be used by other organisms.

Predator-Prey Relationship

Many small worms consume organic matter that is too small and therefore not available to bigger organisms, and they are an important link in the food chain. For example, marine nematodes, which are the most abundant meiofauna, feed on microorganisms and detritus in the sediment and are an important source of food for fish, birds, and marine invertebrates.

Symbiotic Relationships

Some tube-dwelling or burrowing polychaetes provide a home for other organisms such as small crabs, bivalves, or scale worms, in a commensal symbiotic relationship.

10.3. Bryozoans (Phylum Ectoprocta)

Bryozoans are part of a group called the lophophorates. Lophophorates have bilateral symmetry, but lack a distinct head. They possess a feeding and gas exchange appendage called a lophophore, composed of ciliated tentacles that surround the mouth. They have a completed digestive tract shaped like a U, which means that they excrete their wastes close to their mouths (Figure 10.8a and b). There are three phyla of lophophorates, but here we will only consider the most important, the bryozoans.

Bryozoans are sessile and live in a variety of habitats, including on seaweeds and are very abundant, especially in shallow water. They form colonies of small animals called zooids that live in box-like chambers. They are filter feeders and are important fouling organisms.


BIOL 181: Life in the Oceans – Lecture Notes

Haeckel Bryozoais in the public domain in the United States.

Flustra Foliacea, by Hans Hillewaert,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 10.8a and b. Bryozoan diversity.


BIOL 181: Life in the Oceans – Lecture Notes

10.4. Mollusks

Mollusks are a large and varied group of animals that have a soft body and are most often covered with a calcium carbonate shell. Their body is divided into two major parts (Figure 10.9). The head-foot region includes the head, mouth, sensory organs, and the foot, which is usually used for locomotion. The visceral mass includes all the other organ systems. The mantle of mollusks covers the visceral mass and hangs from the sides of the body. The mantle produces the shell and in some groups is used in locomotion or gas exchange. All mollusks except bivalves have a radula (Figure 10.10), a ribbon of tissue that contains teeth, used for scraping, tearing, piercing, and cutting.

Anatomical Diagram of a Hypothetical Ancestral Molluscis available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 10.9. Generalized mollusk showing the head-foot region, visceral mass, and mantle.


BIOL 181: Life in the Oceans – Lecture Notes

Radula Diagramis available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.Figure 10.10. Radula of mollusks.

10.4.1. Chitons (Class Polyplacophora)

Chitons have a flattened body covered by eight calcareous plates held together by a girdle (Figure 10.11a and b). Chitons are common in the intertidal zone, where they attach to rocks and scrape off algae and other organisms with their radula.


BIOL 181: Life in the Oceans – Lecture Notes

Lined Chiton, by Kirt L. Onthank, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Cryptoconchus Porosus, by Graham Bould, is in the public domain in the United States.Figure 10.11a and b. Chitons (Class Polyplacophora).

10.4.2. Gastropods (Class Gastropoda)

Gastropods move by sliding along the bottom on their foot. Most gastropods have a shell, which may be coiled (e.g., snails) or not (e.g., limpets). Others, the nudibranchs, lack a shell. Many snails can retreat in their shell and close the aperture with the operculum (Figure 10.12a and b), which can be used as defense or to reduce water loss in the intertidal zone. The shape of the shell reflects the environment a snail is adapted to live in; for example, snails that live in exposed intertidal zones often have low, broad shells that reduce drag and allow them to cling to rocks.


BIOL 181: Life in the Oceans – Lecture Notes

Sea Snail on Findhorn Beachis available under aCreative Commons Attribution 2.0 Generic license.© Chris Pearson.

Common Limpets, by Tango22,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 10.12a and b. Gastropods: snail (a) and limpet (b).

Feeding and Nutrition

Gastropods have wide range of feeding strategies. Herbivores usually eat small algae that they scrape off rocks with their radula. Some can eat seaweeds, such as kelp. Carnivorous gastropods locate their prey with chemical cues, and eat a variety of prey including cnidarians, echinoderms, and bivalves. Cone snails have a modified harpoon-like radula, which is coated with a toxin and allows them to kill their prey. Other gastropods are scavengers, deposit feeders (e.g., conch), or filter feeders (e.g., sea butterfly, Figure 10.13a and b).

Sea Butterflyby USGS is in the public domain in the United States.116

BIOL 181: Life in the Oceans – Lecture Notes

Clione Limacinaby NOAA is in the public domain in the United States.Figure 10.13a and b. The sea butterfly, a planktonic gastropod mollusk.


Nudibranchs are marine gastropods that lack a shell (Figure 10.14a and b). They are often bright in color, which may be an indication of toxicity. Many have cerata on their back, extensions of the mantle that increase the surface area of the mantle available for gas exchange. Some nudibranchs will feed on cnidaria and retain their nematocysts as protection for themselves.


BIOL 181: Life in the Oceans – Lecture Notes

Berghia Coerulescens Eats Aiptasia Coushii, by Parent Gery, is in the public domain in the United States.

Nudibranch, by Sean Murray, is available under aCreative Commons Attribution 2.0 Generic license.118

BIOL 181: Life in the Oceans – Lecture Notes

Figure 10.14a and b. Nudibranchs showing cerata.

10.4.3. Bivalves (Class Bivalvia)

The body of bivalves is compressed laterally, and they have two valves (shells) attached dorsally by ligaments (seea longitudinal section of a bivalve). The valves are closed by adductor muscles and opened as the muscles relax and the weight of the valves pulls it apart. Bivalves have no head or radula, and their foot is located ventrally and functions in burrowing and locomotion. Their mantle forms inhalant and exhalent openings, and cilia on the gills move water, exchange gases, and filter food particles that are then brought to the mouth.

Bivalves have evolved adaptations to live in a variety of different habitats. Those that burrow in soft bottoms (infauna), such as clams, often have their mantle fused around their inhalant and exhalant openings, creating a siphon that can draw water in from above the sediment. Others live attached to the surface of hard substrates (epifauna), and may attach by cementing one valve to the substrate (e.g., oysters), or by byssus threads, made from a tough protein (e.g., mussels). Unattached surface dwellers such as scallops and file clams can swim by jet propulsion, as they open and close their valves rapidly. Boring bivalves include some boring clams which live in soft rocks, and shipworms, which burrow into wood by swallowing and digesting the wood, with the help of enzymes from their symbiotic bacteria.

10.4.4. Cephalopods (Class Cephalopoda)

The foot of cephalopods is modified into a head-like structure, and they have a ring of tentacles that projects from the anterior end of the head. The tentacles are used to capture prey, for defense, reproduction, and locomotion. With the exception of the nautilus, cephalopods lack a shell or have a small internal shell. Most cephalopods can move by jet propulsion, where water is taken into the mantle, then channeled through a small funnel and expelled. The funnel directs the flow of water and therefore the direction of movement. Some, like octopus, can also crawl on the benthos.

Nautiloid Cephalopods

These cephalopods have a large, coiled shell with multiple chambers filled with gas, which can be used to regulate buoyancy. The animal itself lives in the last chamber. Nautiloids have 60 to 90 tentacles, many of which have chemosensory and tactile functions, while others bring food to the mouth. They commonly come to the surface at night, although they feed on the bottom during the day.


BIOL 181: Life in the Oceans – Lecture Notes

Nautilus Shell Cut in Half, by Chris 73,is available under aNautilus Palau, by Manuae,is available under aCreative Creative Commons Attribution-ShareAlike 3.0 Unported

Commons Attribution-ShareAlike 3.0 Unported licenseFigure 10.15a and b. Nautilus, live (a) and section of shell (b).

Coleoid Cephalopods


Coleoid cephalopods include squid, octopus, and cuttlefish. These cephalopods have the most complex nervous system of all invertebrates, and have highly developed eyes and well- developed tactile senses. They release a cloud of ink when disturbed to distract predators. Squid have a large cylindrical body with a pair of fins derived from the mantle. Squid have an internal shell remnant called a pen, which is made of flexible, hard protein. They have 10 appendages: eight short arms, and two long tentacles. Octopuses have eight arms and no tentacles. Their bodies are sac-like and lack fins, and since they have no shell at all, they are very flexible and can fit into very small openings.


BIOL 181: Life in the Oceans – Lecture Notes

Octopus Vulgaris, by Albert Kok,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.


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Reef Cuttlefish, by Nick Hobgood,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 10.16a and b. Coleoid cephalopods: octopus (a) and cuttlefish (b).

Color and Shape in Cephalopods

Cephalopods have an impressive ability to change color, pattern, and texture, which is used in communication and camouflage. They can achieve this by modifying the distribution of pigments in special cells in their skin called chromatophores.

Feeding and Nutrition in Cephalopods

Most cephalopods are carnivores. They locate their prey visually and capture it with their arms and tentacles. Cephalopods have a radula, but they use their large beak to bite; the salivary gland in octopus and cuttlefish produces a poison. The actual diet depends on the species and its habitat.

Reproduction in Cephalopods


BIOL 181: Life in the Oceans – Lecture Notes

Sexes are usually separate, and mating involves a courtship display. Many species reproduce only once and then die.

10.4.5. Ecological Roles of Mollusks

Mollusks are an important source of food for many marine animals as well as for humans. Sperm whales consume high numbers of squids. Gastropod shells can be an important source of calcium for some seabirds. Shipworms can damage wooden boat and wood pilings.

10.5. Review Questions: Marine Worms, Bryozoans and Mollusks

Why is bilateral symmetry favored in platyhelminthes (flatworms)?What is cephalization?Flatworms reproduce sexually by reciprocal copulation. What does this mean?What phylum do round worms belong to?Give an example of an annelid with toxins in its setae?What are characteristic features of annelids?What are the most common annelids in the marine environment?What type of feeding is characteristic of sedentary polychaetes?Give an example of a sedentary polychaete worm.What part of the mollusk produces the shell?What is the radula of a mollusk?What phylum contains chitons?Do all members of the class Gastropoda have a shell?What is the operculum?Are all gastropods herbivores?Which phylum contains bivalves?What is the role of byssus threads in mollusks?Describe the shell of the nautilus.What are chromatophores?What is the name of the individual animals that make up a colony of bryozoans?What is the name of the crown of tentacles on the head of a bryozoan?


BIOL 181: Life in the Oceans – Lecture Notes

11. Arthropods, Echinoderms, and Invertebrate Chordates

(The majority of the text below originally appeared as chapter 8 ofIntroduction to Marine Biology)11.1. Arthropods (Phylum Arthropoda)

Arthropods are an extremely diverse group; they include terrestrial insects and represent 75 percent of all animal species. Arthropods have an exoskeleton made of chitin, which provides protection and a point of attachment for muscles. However, this exoskeleton does not grow with the animal, and arthropods have to molt periodically.

The body of arthropods is divided into segments, and each usually has a pair of appendages that can be sensory, or used in locomotion or feeding. They have a highly developed nervous system with sophisticated sense organs.

Marine arthropods can be divided into two major groups: the chelicerates and the mandibulates. The subphylum Crustacean makes up the vast majority of mandibulates. The taxonomy of arthropods is still debated by scientists, and the exact grouping of the various taxa changes from textbook to textbook. Table 11.1 shows the classification used in this class for the organisms that will be covered.

Table 11.1. Taxonomic classification of some important groups of marine arthropods.




Common name




Horseshoe crab




Lobsters, crabs, shrimp






Mantis shrimp







11.1.1. Chelicerates (Subphylum Chelicerata)

Chelicerates are a primitive group of arthropods that includes spiders, ticks, and scorpions. Marine chelicerates have bodies composed of three parts; this group includes horseshoe crabs and sea spiders. Chelicerates have six pairs of appendages, one of which, the chelicerae, is an oral appendage modified for feeding. They lack mouthparts for chewing and predigest their food externally before sucking it up in a semi-liquid form.

Horseshoe crabs are chelicerates and therefore more closely related to spiders than to true crabs. They live in shallow coastal waters, and their body comprises three regions: the cephalothorax, which contains most appendages, the abdomen, which contains the gills, and the telson, which is a long spike used in steering and for defense (Figure 11.1). Horseshoe crabs are mostly scavengers and feed on invertebrates and algae.


BIOL 181: Life in the Oceans – Lecture Notes

Tachypleus Tridentatus, by Binh Giang, is in the public domain in the United States.Figure 11.1. A horseshoe crab, subphylum Chelicerata.

11.1.2. Mandibulates

In mandibulates, the feeding appendages are called mandibles and they are used to crush and chew food before it is ingested. Marine mandibulates belong to the subphylum Crustacea.

The body of crustaceans is segmented and can be divided into the head, thorax, and abdomen. In some species (e.g., lobsters), the head and thorax are fused together to form the cephalothorax (Figure 11.2). Each segment has a pair of appendages that have a specific function. All crustaceans have two pairs of sensory antennae, and depending on the species, they may also have walking legs, swimmerets (for swimming), and chelipeds (for defense).


BIOL 181: Life in the Oceans – Lecture Notes

Tachypleus Tridentatus, by Binh Giang, is in the public domain in the United States.Figure 11.2. American lobster,Homarus americanus, subphylum Crustacea.

Crustaceans, like all arthropods, have to molt as they grow. Molting is controlled by hormones and is often initiated by changes in environmental conditions. The frequency of molting decreases with age. Crustaceans are much more vulnerable immediately after molting, and often seek a hiding place until their new exoskeleton hardens.


Decapods are a group of crustaceans with five pairs of walking legs. The first pair is often modified into chelipeds (pincers) used in catching prey and in defense. Decapods include lobsters, crabs, and true shrimp (Figure 11.3a and b).


BIOL 181: Life in the Oceans – Lecture Notes

Southern Rock Lobster, by Stemonitis, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.


BIOL 181: Life in the Oceans – Lecture Notes

Heterocarpus Ensiferby NOAA is in the public domain in the United States.Figure 11.3a and b. Lobster (a) and shrimp (b), order Decapoda.

Many decapods exhibit specialized behaviors. Hermit crabs live in the discarded shells of gastropods and must find larger shells as they molt and grow. Decorator crabs attach other organisms to their shells for camouflage. The last pair of legs of blue crabs has been modified into paddles, allowing them to be very powerful and agile swimmers.

Many decapods are predators, and can use their chelipeds to catch prey. Others are scavengers, deposit feeders, or filter feeders. Food is ground using the mandibles and plates in the stomach.

Mantis Shrimp

Mantis shrimp are highly specialized predators that range in size from 5 cm to 36 cm. Their second pair of thoracic appendages is enlarged, and they have a movable finger that can be extended very rapidly to spear and smash prey and in defense. They can even smash aquarium glass (Figure 11.4).


BIOL 181: Life in the Oceans – Lecture Notes

Mantis Shrimpfrom NSF is in the public domain in the United States.Figure 11.4. Mantis shrimp.


Krill are pelagic, shrimp-like crustaceans that measure 3–6 cm in length. They are filter feeders and eat phytoplankton and other zooplankton. They occur in large swarms and one species,Euphausia superba, is extremely abundant in Antarctica and is the main food source for many species of marine mammals, birds, and fish. One species of krill can molt so quickly it literally jumps out if its skin; this is used as a technique to avoid predation.


BIOL 181: Life in the Oceans – Lecture Notes

A Northern Krill, by Øystein Paulsen, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 11.5. Krill.


Amphipods resemble shrimp and are small and laterally compressed (Figure 11.6). Their posterior three appendages are directed backward and are modified for jumping, burrowing, or swimming. Many are burrowers and tube dwellers, and they can be abundant in the intertidal zone on beaches. They are commonly known as beach fleas.

Figure 11.6. Amphipod.


Amphipod, by Hans Hillewaert

Copepods are the largest group of small crustaceans (Figure 11.7a and b). They are the dominant zooplankton throughout the world’s oceans. Many exhibit daily vertical migrations to feed in the


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Copepodasis in the public domain in the United States.Figure 11.7a and b. Copepods, class Copepoda.


BIOL 181: Life in the Oceans – Lecture Notes

photic zone at night and descend to the aphotic zone during the day (see chapter 19). They are mostly herbivorous suspension feeders.

Copepod, by Uwe Kils,

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Barnacles are the only sessile crustaceans (Figure 11.8a and b). They are found on rocks, shells, hulls of boats, dock pilings, and even on marine mammals. Their shells are made of calcium carbonate and they superficially resemble bivalves. However, their taxonomic relationship to other crustaceans is clear in their larvae and internal anatomy. Upon reaching a suitable settlement site, larvae attach to the substrate by their antennae, which contain adhesive glands. They then extend their cirripeds (legs) into the water to feed and exchange gas. Many are adapted to live in the intertidal zone and can close their shells at low tide to keep water in. Like many other crustaceans, barnacles reproduce sexually through internal fertilization. However, because they are sessile, they have evolved the longest penis relative to body size in the entire animal kingdom.


BIOL 181: Life in the Oceans – Lecture Notes

Cirripediais in the public domain in the United States.

Figure 11.8a and b. Barnacles.

11.1.3. Ecological Roles of Arthropods

Entenmuscheln im EcoMare, by M. Buschmann,license.

Decapods such as crabs, lobsters, and shrimp are an important source of food for humans. Copepods and krill are an important link in pelagic food chains, feeding on small phytoplankton and in turn being food for larger organisms (such as baleen whales and penguins). Many, such as cleaning shrimp, act in symbiosis with other organisms, removing their parasites. Barnacles are important fouling organisms that can create problems for boats, sometimes reducing a ship's speed by 30 percent.

11.2. Echinoderms (Phylum Echinodermata)

Echinodermata means ―spiny skin.‖ This is a strictly marine group that includes animals such as urchins, sea stars, and sea cucumbers. They have a complete digestive tract, and though their larvae show bilateral symmetry. The adults exhibit modified radial symmetry, usually five-part radial symmetry, which allows these slow-moving organisms to respond to stimuli from all directions. They are particularly common in the deep sea but are also abundant in shallow ecosystems. Echinoderms have great powers of regeneration, giving them the ability to reproduce a new individual asexually from a part of the body if it is lost. They can also


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BIOL 181: Life in the Oceans – Lecture Notes

reproduce sexually through external fertilization, and they release their gametes into the water column.

11.2.1. Structure of Echinoderms

Echinoderms have an endoskeleton made of calcareous ossicles (plates) held together by connective tissue (Figure 11.9). The ossicles may be fused together to form a hard test (e.g., urchins) or far apart forming a softer covering (e.g., sea cucumbers). Spines may project outward from the ossicles. Echinoderms have a water vascular system which is used in locomotion (through the tube feet), feeding and gas exchange. Water enters the water vascular system through the madreporite, and is pumped into the tube feet from the ampullae when it contracts, causing the feet to project. Then when the muscles in the tube feet contract, water is forced back into the ampullae, and the feet shorten.

Sea Star at Ship Harbor, by Aldaron, is available under aCreative Commons Attribution-ShareAlike 2.5 Generic license.

Figure 11.9. Sea star, phylum Echinodermata.

11.2.2. Sea Stars (Class Asteroidea)

Sea stars typically have a central disk and five arms that radiate from it. Their mouth is on their underside, along with the tube feet used in locomotion, which are found in the ambulacral


BIOL 181: Life in the Oceans – Lecture Notes

grooves. The top surface is often rough or spiny. Most sea stars are carnivores or scavengers, and they can evert their stomach to digest prey outside their body (Figure 11.10a). Sea stars, like most echinoderms, have great power of regeneration, and can regenerate lost arms (Figure 11.10b); extra arms are often produced in the process. If the lost arm includes a portion of the central disk, it can also regenerate a new individual. Some species reproduce this way through division of the central disk, and each half recreates an entire individual.

Sea Star Eating a Mussell, by Brocken Inaglory,

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BIOL 181: Life in the Oceans – Lecture Notes

Sea Star Regenerating Legs, by Brocken Inaglory,

Figure 11.10a and b. Sea star eating a mussel (a) and in the process or regenerating arms (b).

11.2.3. Brittle Stars (Class Ophiuroidea)

Brittle stars also have five arms, but they are more slender than those of sea stars, and are distinct from the central disk (Figure 11.11a and b). They have no suckers on their tube feet, and can move their arms for locomotion. They tend to hide in crevices in the day and come out at night to feed. Brittle stars have a variety of feeding strategies, and can be carnivores, scavengers, deposit feeders, or suspension feeders. For example, some species string mucous between the spines on adjacent arms to form a net. Brittle stars can reproduce asexually through division of the central disk, like sea stars. If caught by predators, they can autotomize and cast off an arm, which undulates wildly to distract the predator, and they can later regenerate.

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BIOL 181: Life in the Oceans – Lecture Notes

Brittle Starsis in the public domain in the United States.


BIOL 181: Life in the Oceans – Lecture Notes

A Green Brittle Star, by Neil,

Figure 11.11a and b. Brittle stars.

11.2.4. Sea Urchins and Their Relatives


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The body of echinoids is enclosed by a calcareous test. Spines project from the test and function in defense, containing venom in some species (e.g.,Diademasp.). Spines also dissipate the energy of waves for urchins living in the intertidal zone.

In regular echinoids (sea urchins), the body is roughly spherical and armed with moveable spines (Figure 11.12a and b). The mouth is on the underside and the anus on top. They typically live on hard substrates and graze on algae with their five-part mouth parts called the Aristotle’s lantern (Figure 11.13). They are important herbivores on coral reefs, and the negative effect of their absence is evident in Jamaica and many other Caribbean reefs where macroalgae now dominate.

Irregular echinoids (heart urchins and sand dollars), on the other hand, have adapted for burrowing in the sand. They are flattened and their test is covered in very small spines which function in locomotion and keeping sediment off their body (Figure 11.13). They are selective deposit feeders and consume organic material from the sediments.


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Sand Dollar, by Gerhard H,is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.Figure 11.12a and b. Regular (a) and irregular (b) echinoids.


BIOL 181: Life in the Oceans – Lecture Notes

EchinideaFigure 11.13. Echinoids. Aristotle’s lantern, used in feeding in regular echinoids, is depicted at

the bottom center.

11.2.5. Sea Cucumbers (Holothurians)

Sea cucumbers have an elongated body and lie on their sides. Superficially, they may resemble worms, but the five-part radial symmetry characteristic of echinoderms is still evident, even if only internally in some species. The body wall is leathery as they have few ossicles. Like sea stars, they move with their tube feet, which are only present on their underside. Gas exchange occurs through the respiratory tree located in their anus. Sea cucumbers are deposit feeders or suspension feeders, and their oral tentacles used in catching food are modified tube feet coated in mucous. Sea cucumbers (Figure 11.14) are slow-moving and do not have spines or an external test; therefore, they have evolved interesting adaptations to avoid predation. Some species release sticky Cuverian tubules from the anus, which immobilize crustaceans and are distasteful


BIOL 181: Life in the Oceans – Lecture Notes

to fish. Others can eviscerate (release their internal organs) through the anus or mouth. Both tactics can distract the predator while the animal escapes.

Sea Cucumber, by Maxim Gavrilyuk,

Figure 11.14. Sea cucumber.

11.2.6. Crinoids

Crinoids are the most primitive of echinoderms. The majority of present-day crinoids are feather stars, which cling to the bottom with their grasping cirri and extend their arms into the water at night to feed. They are able to swim by undulating their arms, as an escape response. They are suspension feeders and like most echinoderms have high powers of regeneration.

11.2.7. Ecological Roles of Echinoderms

Because of their spines, echinoderms do not have many predators, but some sea otters, mollusks, crabs, and fish eat sea stars and sea urchins. In some parts of the world, humans eat the gonads of urchins and sea cucumbers, believed to be an aphrodisiac.


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BIOL 181: Life in the Oceans – Lecture Notes

Many echinoderms are important herbivores or predators and sometimes play a disproportionately important role in ecosystems. The crown-of-thorn sea star is a major predator of corals in the Indo-Pacific, and its population explosions, which may be caused by overfishing of their predators, threaten many Pacific reefs. The sea urchinDiadema antillarumis an important herbivore on Caribbean reefs, and when its population suffered from a mass mortality in the early 1980s, the resulting overgrowth of macroalgae on reefs resulted in a massive decline in coral cover. Sea urchins in temperate zones (Figure 11.15) can also be very important in shaping the ecosystem; in the Northwest Atlantic, the decline in sea otters allowed for an increase in the population of urchins (Strongylocentrotussp.) and consequently a decline in kelps. Some sea cucumbers produce a poison that can be used to poison tide pools and suppress growth of tumors, due to its effects on nerves and muscles.

Black Sea Urchin, by Johnmartindavies,Figure 11.15. The long-spined sea urchin,Diadema antillarum.

11.3. Tunicates (Phylum Chordata, Subphylum Urochordata)


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BIOL 181: Life in the Oceans – Lecture Notes

Tunicates are mostly sessile animals named after their body covering of tunic, largely similar to cellulose. Tunicates are part of the phylum Chordata, which also includes all vertebrates. Their larvae share many features with vertebrates, including a notochord and gill clefts. However, they lose those features as adults.

Sea squirts are sessile tunicates, with a round or cylindrical body (Figure 11.16a, b, and c). They are filter feeders with an incurrent and excurrent siphon, and the food gets trapped in a mucus net in the pharynx. Sea squirts can be solitary or colonial.

Salps and larvaceans are pelagic tunicates often found in the open ocean. Their incurrent and excurrent siphons are located at opposite ends of the body. Larvaceans produce a mucous net to trap food particles; this is frequently shed every four hours when clogged. This mucus net sinks, becoming marine snow, and carries a substantial portion of the upper ocean's productivity to the deep seabed as a source of food for deep-sea animals.

Polycarpa Aurata, by Nick Hobgood,

Pelagic Colonial Salp, by Nick Hobgood, is

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available under aCreative Commons


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Figure 11.16 a, b, and c. Benthic tunicate (a) and pelagic tunicates (b and c).

11.4. Review Questions: Arthropods, Echinoderms, and Invertebrate Chordates

4. Which subphylum lacks mouthparts for chewing food and must suck up its food in a semi-

7. Give two examples of important zooplankton in pelagic food chains from the phylum

8. Are barnacles a mollusk or crustacean?


BIOL 181: Life in the Oceans – Lecture Notes

9. What does the term echinoderm mean?

What is the advantage of radial symmetry shown by members of the phylum


What is the system unique to echinoderms that is used in locomotion, feeding, and gas


Give an example of a member of the phylum Echinodermata that has venom in its spines.What is the name of the five-toothed feeding structure in sea urchins?Give an example of a method of defense in sea cucumbers.Give an example of a free-living tunicate.Why are tunicates members of the phylum Chordata?


BIOL 181: Life in the Oceans – Lecture Notes

12. Marine Fish

(The majority of the text below originally appeared as chapter 9 ofIntroduction to Marine Biology)

All fish are vertebrates and have a series of bone or cartilage that supports the spinal cord and provides attachment sites for muscles. Marine fish can be found from the surface waters to the deepest trenches; from the highly diverse coral reef communities to the almost barren open oceans. Their species outnumber all other vertebrate species in the ocean combined, and they display an amazing array of adaptations enabling them to thrive in almost every niche available. Fish are a valuable resource to humans, being commercially harvested as food for humans and animals, fertilizers, and many other products.

Like many other taxonomic groups, classification of fish has been shifting as more molecular data become available. Our current understanding of fish phylogeny recognizes five main groups (Figure 12.1). The two most primitive groups, the hagfish and lampreys, lack jaws and paired fins. The cartilaginous fish have a skeleton made of cartilage and include sharks, rays, and relatives. The ray-finned fish have a bony skeleton and are by far the most diverse group of fish. The lobe-finned fish have a bony skeleton, with skeletal extension into some of their fins. They are the precursor to land vertebrates.

Phlyogeny, by Zebra.element, is in the public domain in the United States.

Figure 12.1. Phylogenetic trees showing relationships among the Deuterostomia.


BIOL 181: Life in the Oceans – Lecture Notes

12.1 Jawless Fish

The first fish to evolve lacked both paired fins and jaws and probably spent their time scavenging for food in the bottom sediments of the early seas. Modern jawless fish include the hagfish and lampreys, which both still lack jaws and paired appendages. Their skeletons are entirely composed of cartilage and their bodies lack scales. Although superficially similar, hagfish and lampreys are morphologically and behaviorally very different.

12.1.1 Hagfish (Also Known as Slime Eels)

Hagfish are deep-sea bottom-dwelling fish found throughout the world. They inhabit depths of more than 600 m, often in the tropics, although they are sometimes found in the shallower seas. Hagfish feed using two dental plates, containing horny cups, with which they grab their prey and draw it into the mouth. They feed primarily on small invertebrates but may also be scavengers on larger carcasses found on the sea floor. Hagfish can produce large amounts of milky gelatinous slime when disturbed. This slime is thought to be used for protection as it coats the gills of predatory fish, either suffocating them or at least discouraging them. To remove the copious amount of slime that can build up on the body, hagfish have the ability to tie themselves in a knot, then move the knot along their bodies and scrape off excess slime (Figure 12.2). This property is also used to gain leverage to tear flesh from large carcasses, such as whales.

Pacific Hagfishby NOAA is in the public domain in the United States.Figure 12.2. Hagfish.

12.1.2 Lampreys

Lampreys can inhabit both salt and freshwater. They have a bony vertebral column. Their mouth parts consist of an oral disk and rasping tongue covered with tooth-like plates of keratin (Figure 12.3). Several species use these plates to grasp prey, rasp a hole in the victim, and suck out the tissue and fluids. Marine lamprey species spend their adult life in the open oceans but have been found to migrate to freshwater to spawn, where they die shortly afterwards. They have very large nerves, making them a great subject for neurobiological research.


BIOL 181: Life in the Oceans – Lecture Notes

Figure 12.3. The oral disk of a lamprey.

12.2 Cartilaginous Fish

Petromyzon Marinus (Lamprey) Mouth

Modern cartilaginous fish include sharks, skates, rays, and chimaeras, which possess both jaws and paired fins. Their skeletons are composed entirely of cartilage, although this is often strengthened by calcium salts. The bodies of cartilaginous fish are covered in placoid scales (dermal teeth; Figure 12.4), which are modified to form rows of teeth on their jaws. There are two major groups of cartilaginous fish; the elasmobranchs (sharks, skates and rays) and the holocephalans (chimaeras and ratfish).

12.2.1 Sharks

Sharks typically have streamlined bodies and are excellent swimmers, using their strong body in a sideways sweeping motion. Their caudal fin is heterocercal in shape, meaning the dorsal (upper) lobe is longer than the ventral (lower) one, which gives the shark lift as it swims (Figure 12.4). The paired pelvic fins of males are modified into claspers, which transfer sperm to the


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Carcharhinus Amblyhynchos Detail of Tail

Figure 12.4. The heterocercal caudal fin of sharks.


BIOL 181: Life in the Oceans – Lecture Notes

females when mating (Figure 12.5). Sharks do not possess swim bladders, and so will sink if they stop swimming. To counteract this buoyancy problem, their livers contain large quantities of an oily substance called squalene, which helps to offset the shark’s high density.

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BIOL 181: Life in the Oceans – Lecture Notes

Physical Characteristics of a General Shark, by Chris_huh, is in the public domain in the United States.Figure 12.5. The anatomy of sharks.

12.2.2 Skates and Rays

Skates and rays differ from sharks by having flattened bodies with greatly enlarged pectoral fins, and reduced dorsal and caudal fins. They lack anal fins. Their eyes and spiracles (openings for the passage of water) are located on the top of their heads while their gill slits are on the ventral side, allowing debris-free water to enter through the spiracles and be passed out over the gills (Figure 12.6a and b). Most skates and rays are adapted to live a benthic lifestyle where they feed on invertebrates (e.g., crustaceans and mollusks) using their specialized crushing teeth. However, a few species, including manta rays (Manta birostris) and eagle rays, glide through the water column feeding on plankton (Figure 12.7).


BIOL 181: Life in the Oceans – Lecture Notes

Common Torpedo


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Batoideais available under aCreative Commons Attribution 2.0 Generic license. © Dirk Vorderstrabe.Figure 12.6a and b. Most skates and rays have adapted to a benthic lifestyle with eyes and

spiracles on their upper surface and mouth and gill slits on their lower surface.


BIOL 181: Life in the Oceans – Lecture Notes

Manta Rayis available under aCreative Commons Attribution 2.0 Generic license. © John Hanson.Figure 12.7. Manta rays are pelagic and feed on plankton.

Although they may be visually very similar, skates and rays have very different morphology and behavior. Skates swim by creating sinusoidal ripples, which travel along their pectoral fins, as opposed to rays, which move their fins up and down. Skates also have small fins on their tails; these are not present in rays, which may have venomous barbs instead. Rays often grow larger than skates (manta rays can be up to 7 m wide) and reproduce in a ovoviviparous strategy as opposed to skates reproducing oviparously producing little egg-sacks knows as mermaid’s purses.

Skates and rays have evolved various defensive mechanisms to protect themselves from predators. Electric rays can deliver up to 220 volts from organs in their heads that may also help them navigate and stun prey. Stingrays use hollow barbs on the base of their tails, which may be lined with venom, although these tend to be used primarily for defensive purposes. Sawfish and guitarfish have a series of barbs running along their pointed rostrums, which can be used to inflict damage on either predator or potential prey as they shake their heads.

12.2.3 Chimaeras


BIOL 181: Life in the Oceans – Lecture Notes

Chimaeras are generally bottom dwellers inhabiting habitats from the shallows to deep waters (Figure 12.8). They contain species such as the ratfish, rabbitfish, and spookfish, named because of their pointed heads and long slender tails. Unlike other cartilaginous fish, their gills are covered with an operculum, and water is taken in through the nostrils before being expelled over the gills. Chimeras are oviparous, producing large eggs in leathery cases. They feed on a variety of prey, including crustaceans, mollusks and fish, crushing them between oral plates (instead of teeth).

Chimoera Callorynchus, by William Smyth, is in the public domain in the United States.Figure 12.8. Chimera.

12.3 Bony Fish

There are approximately 25,000 species of modern bony fish, with most being characterized by the presence of a swim bladder, bony skeleton, bony scales, and rays constituting part of their fins. They can be divided into two major lineages: the lobefins and ray-finned fish.

12.3.1 Lobefin Fish

Coelacanths (Figure 12.9) are living fossils, in that they have changed very little in the last 300 million years. Biologists knew of the coelacanth first from the fossil record, and they were thought to be extinct, until a living specimen was discovered in 1938. Several other coelacanths have been seen and caught since then. Their skeletons consist of both bone and cartilage with a reduced skeleton and a fat-filled swim-bladder that allows them to remain neutrally buoyant.


BIOL 181: Life in the Oceans – Lecture Notes

Coelacanths, like sharks, maintain high concentrations of urea within their body fluids to remain relatively isotonic to the surrounding environment. Coelacanths, among lungfish, are known as lobefins because of rod-shaped bones that extend into their pelvic and pectoral fins. Lobefin fish represent the evolutionary line that gave rise to the terrestrial tetrapods.

, by Rept0n1x, is available under a

Figure 12.9. The primitive lobe-finned fish coelacanth, the sister group of the line that gave rise to tetrapods.

12.3.2. Ray-Finned Fish

The ray-finned fish are the most numerous group of vertebrates in the ocean. They are typically covered in scales, and their fins are attached to their bodies by fin rays. They have a homocercal caudal fin, where the dorsal and ventral lobes are relatively similar. The caudal fin is typically used for propulsion. They maneuver with paired pectoral and pelvic fins, which give increased stability (Figure 12.10). In some fish, the pectoral fins have been modified for flying (e.g., flying fish) or walking (e.g., mudskipper). Wrasse and parrotfish use their pectoral fins as their main force for propulsion.



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BIOL 181: Life in the Oceans – Lecture Notes

Fish Anatomy Diagramby USDA is in the public domain in the United States.Figure 12.10. The anatomy of a ray-finned fish.

Ray-finned bony fish are represented by many different body shapes, which are correlated with the lifestyle and characteristics of their habitats (Figure 12.11). Fish that are active swimmers, such as tuna and marlin, have a very streamlined shape, to move through the water with great efficiency. Fish that live in seagrass meadows or coral reefs, such as butterflyfish (Chaetodonspp.) or angelfish (Pomacanthusspp), typically have laterally compressed bodies for increased maneuverability in these complex habitats. Some bottom-dwellers, such as flounders, have dorso-ventrally depressed bodies adapted to their benthic lifestyle, whereas others, such as scorpionfish (Scorpaena) or anglerfish (Antennarius), have evolved globular bodies, ideal for a sedentary lifestyle. Others, which may burrow or hide in tight crevices, such as moray eels (Gymnothorax), have long, thin snake-like (attenuated) bodies with reduced pelvic and pectoral fins.


BIOL 181: Life in the Oceans – Lecture Notes

Chromys Lapidifer, Clinus Fasciatus, Malthea Notata, and Acanthurus Coeruleus

Figure 12.11. The shape of bony fish is determined by behavior and habitat.

The shape of the caudal fin (Figure 12.12) also varies and depends on the swimming behavior of the fish. Fast-cruising pelagic fish, such as tuna and marlin, have a lunate caudal fin with a small caudal peduncle (the base of the caudal fin), which is rigid and very efficient at high speed propelling for long durations, but not very efficient at slow speeds or maneuvering. A forked fin is commonly seen on fast swimmers, although it also functions well at slower speeds, too, e.g., the yellowtail snapper. A more maneuverable fin shape is the truncate tail; this provides strength but slower speeds, and is seen for example in salmon and herring. The most flexible caudal fin is


BIOL 181: Life in the Oceans – Lecture Notes

the rounded fin, very efficient for maneuvering at low speed. This is seen on many reef fish, such as the angelfish.

Fin Shapesby the USDA is in the public domain in the United States.Figure 12.12. Caudal fin shapes: lunate, forked, truncate, and rounded.

12.4. The Biology of Fish

12.4.1. Fish Coloration and Patterns

Most bony fish use vision as their primary sensory stimuli for finding food and mates, and for communication. Color is therefore extremely important for both species recognition and concealment in the marine environment. Color is derived from specialized cells called chromatophores, which can alter their color by moving pigments around within the cells.

Specialized chromatophores, called iridophores, produce the mirror-like silver of many pelagic fish or the iridescent gleam in many reef fish. Fish that live in the open ocean, such as billfish and tuna, display a coloration pattern called obliterative countershading, where fish have a dark dorsal surface and a lighter silvery ventral surface. Therefore, when viewed from above, the dark dorsum blends in with the dark surrounding waters, and yet when viewed from below, the white belly blends in with the brightly lit surface waters, thus offering these fish camouflage in the open ocean. Many coral reef species (for example, butterflyfish) may exhibit disruptive coloration, in which the background color of the body is interrupted by vertical lines, often passing through the eye, making the fish more difficult to see against the reef. Some fish may also exhibit false eyespots on the posterior part of the body near the caudal peduncle. This extra eyespot may deceive predators into thinking that the fish may be larger than it actually is and that it is facing the other way, allowing the potential prey to escape. Cryptic coloration allows the fish to be camouflaged into its environment, which is important for reef species. Fish such as scorpionfish rely on their irregularly shaped bodies and coloration to blend almost perfectly into their environment while waiting for their prey to swim within attacking distance. Many fish associated with coral reef environments however, exhibit poster colors—bright, showy color patterns that may advertise terrestrial ownership, aid in sexual displays, or simply warn of toxic defenses.

12.4.2. Locomotion


BIOL 181: Life in the Oceans – Lecture Notes

Water is 1,000 times denser than air, making movement more difficult. For this reason, fish must be streamlined to achieve maximum efficiency of movement. Fish rely on their trunk muscles, arranged in a series of bands along each side of their bodies, to propel themselves through the water column. These bands contract alternatively from one side to the other, originating at the anterior end and pushing the fish along. Elongated fish, such as eels, undulate their entire bodies while swifter swimmers, such as jacks, flex only the posterior portion of their bodies (Figure 12.13a and b). Other fish such as wrasses or parrotfish use only their pectoral fins, and triggerfish their dorsal and anal fin, to propel themselves about the reef. The highest recorded speeds have been set by swordfish and marlin, reaching 75mph in bursts.

Fish Fillet, by Brucke-Osteuropa, is available under aCreative Commons 1.0 Universal Public Domain Dedication license.

Swimming of the Fishis in the public domain in the United States.

Figure 12.13a and b. Fillet of shark showing segmentally arranged trunk muscles (a) and swimming in fish (b).

12.4.3. Respiration and Osmoregulation

Bony fish use their gills to extract oxygen (O2) from the water and eliminate carbon dioxide (CO2) from the bloodstream. The operculum is a hard, bony plate that covers the gills. Gills are composed of thin, highly vascular rod-like structures called gill filaments attached to a gill arch. The gill filaments are highly folded to increase the surface area for gas exchange, and unfolded can be 10 times the size of the fish. In these structures, the blood flows in the opposite direction to the incoming water, creating a countercurrent multiplier system, in which water constantly meets blood with lower O2and higher CO2concentration, maintaining a stable gradient that favors the diffusion of gasses across the membrane (Figure 12.14a and b). This mechanism is very efficient, removing almost 80 percent of O2from the incoming water.


BIOL 181: Life in the Oceans – Lecture Notes

Countercurrent Flow for the Exchange Across a Gradient of a Material Property, by Joe, is in the public domain in the United States.

Gill Arches of a Northern Pike, by GFDK, is available under aCreative Commons Attribution-ShareAlike 3.0 Unported license.


BIOL 181: Life in the Oceans – Lecture Notes

Figure 12.14a and b. A countercurrent exchange system transfers almost all of the oxygen in the surrounding water to the bloodstream of the fish. Percentages represent percent oxygen saturation (a). Gill arches of a northern pike bearing gills (b).

The gills can be ventilated by actively pumping water through the gills, and in active fish by continuous swimming with an open mouth (ram ventilation). Fish can ―suffocate‖ if their gill arches collapse (e.g., if removed from water and lose the buoyant force provided by it) or if the oxygen levels fall too low (e.g., in a red tide).

Salt concentration in the blood of most fish is about a third of that in the marine environment. For that reason, marine fish then to loose water across their membranes to the surrounding environment through osmosis. Different groups have different solutions to this problem. Hagfish are osmoconformers, their internal osmotic concentration is the same as the surroundings. Coelacanths and cartilaginous fish retain urea in their blood and body fluids to increase their internal osmolarity, which ends up being the same or higher than the surrounding water. They have unique physiological adaptations to tolerate high levels of urea, which are toxic to most vertebrates.

Ray-finned fish do not retain urea, and therefore constantly lose water to the environment. To compensate, they drink considerable amounts of seawater, up to 25 percent of their body weight a day. They also remove and excrete the extra salts, primarily through specialized chloride cells within the gills. Salt is also eliminated through the kidneys and digestive tracts and marine fish produce negligible amounts of urine because of their need to retain water (Figure 12.15).


BIOL 181: Life in the Oceans – Lecture Notes

The Movement of Water and Ions in Saltwater Fish, by Kare Kare, modified by Biezl, translation improved by smartse, is used under the terms of theCreative Commons Attribution-ShareAlike 3.0 Unported license.

Figure 12.15. Osmoregulation in ray-finned fishes.

12.4.4. Cardiovascular System

The cardiovascular system of fish is relatively similar to that of terrestrial vertebrates, containing a heart, arteries, veins, and capillaries. Active swimmers, such as tuna, use a countercurrent arrangement in their blood vessels to warm up blood coming from their body surfaces and maintaining a body temperature at 2° to 10° C above that of the surrounding environment (Figure 12.16). In this way, they increase the efficiency of their swimming muscles and nerve signals, and maintain a higher internal temperature. They can reach speeds of 70 mph.

Thunnus OrientalisCreative Commons Attribution-NonCommercial-ShareAlike 2.5 Generic license.


BIOL 181: Life in the Oceans – Lecture Notes

Figure 12.16. The bluefin tuna, an active long-distance swimmer, has evolved a countercurrent heat exchange system that can keep its body temperature 20° C warmer than that of the surrounding water.

12.4.5. Buoyancy Regulation

Most bony fish, with the exception of some pelagic species, bottom-dwellers and deep-sea fish, use a gas-filled sac located in their abdomen called a swim bladder to help them maintain buoyancy in the water column. By adjusting the amount of gas in the swim bladder, a fish can maintain its position without any unnecessary expenditure of energy for swimming. Two mechanisms have evolved for adjusting the volume of gas in the bladder. First, an open swim bladder can be filled by gulping air from the surface, for example in herring and eels. Second, in a closed swim bladder, gas can be added through a specialized gas gland which fills the bladder from gases dissolved in the blood. This is a derived condition found in deeper water and more advanced fish, such as percids and cod.

Pelagic fish which are active swimmers do not have swim bladders because the response time is too slow to respond to the rapid changes in pressure; therefore, they must continue swimming to maintain their position in the water column. Benthic dwellers also have no swim bladder, since they have no need to maintain buoyancy as they rest on the bottom. Another method of maintaining buoyancy is to incorporate a greater proportion of low density oils and fats into muscles, internal organs, and body cavities.

The swim bladder of bony fish also functions in additional roles, for example, as an accessory breathing organ to supplement breathing (e.g., tarpon), or as a sound-producing organ amplifying sounds (e.g., drums and grunts).

12.4.6. Nervous System and Senses

Olfaction and Taste

Olfaction (smell) receptors are highly developed in sharks and are located in pits surrounding the mouth. Almost two-thirds of a shark’s brain is devoted to processing olfactory information. Olfaction in ray-finned fish occurs through specialized olfactory pits located in nares (nostrils) on the snout. These are open to the external environment, but there is no connection to the throat. Taste receptors are located on the surface of the head, jaws, mouth, tongue, and barbells (in goatfish and catfish).

Lateral Line System and Hearing

The lateral line is a series of canals running the length of the body and over the head. At regular intervals, the canals are opened to the outside, where there is free movement of water in and out of them. Within the canals are neuromasts that can detect vibrations and are used to locate prey and potential predators (Figure 12.17).


BIOL 181: Life in the Oceans – Lecture Notes

The lateral line of bony fish is similar to that of sharks, and helps detect movement on the water. Neuromasts in fluid-filled canals detect vibrations in the water when the projecting hairs within it are moved. The ears of bony fish are internal and composed of three bones called otoliths, which can detect a smaller range of frequencies than human ears. There is no external opening or ear drum; instead, the sound waves travel through the soft tissues to the internal ear, as the tissue has the same acoustic density as water. In some fish, the swim bladder lies against the ear and acts as an amplifier to enhance sound detection.

The Lateral Line Sensory Organ, by Thomas.haslwanter,

Figure 12.17. The lateral line system.


Sharks are the top predators of the oceans and have evolved highly adapted sensory systems. Their eyes are able to see color yet lack eyelids, instead having a clear nictitating membrane which covers the eye and protects it from damage. Behind the retina of their eyes is a reflective layer, called a tepetum lucidum, which reflects light back through the retina, allowing sharks to see in very low light levels.

The eyes of bony fish lack eyelids; instead, they have tough corneas. Bony fish do not need to regulate the size of their pupils, as the amount of light in water is generally low. Just as a camera, the entire lens moves if needed to adjust for distance. It is thought that fish generally have monocular vision, with each eye seeing its own independent field, and shallow water species may perceive color. Some fish, however, have no eyes; for example, the blind cave fish, which relies more heavily on its other sense.

Ampullae of Lorenzini

Sharks can also detect electrical current in the water using a specialized organ called the ampullae of Lorenzini. These organs are scattered over the head and are thought to detect the


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BIOL 181: Life in the Oceans – Lecture Notes

electrical fields of alternating current created by the muscles of prey. It is also suspected that this organ may detect the earth’s magnetic field, assisting the shark in navigation.

12.4.7. Feeding Types

Sharks exhibit a range of feeding strategies, from the predatory great white shark to the whale shark, which strains plankton from the water. The teeth of sharks are composed of modified placoid scales, which form several rows of blade-like triangular teeth. These teeth are used for grasping and tearing flesh and are continually lost and replaced in a conveyer belt system. Sharks are unable to move their jaws back and forward to chew, and must shake their heads to tear off flesh or devour their prey whole.

The great diversity of bony fish is reflected in their ability to exploit virtually every food resource available. Most bony fish are carnivores and usually seize and swallow whole prey, since chewing would block the flow of water over their gills. Examples of carnivores include groupers and pufferfish. Butterflyfish are also carnivorous, picking out coral polyps with their tiny mouths. Herbivores feed on a variety of plants and algae and are important in maintaining the delicate balance of algal abundance on the reef. The key herbivores on coral reefs are surgeonfish and parrotfish. Herbivorous fish tend to require longer guts with a greater surface area for digestion and absorption. Filter-feeders such as anchovies rely primarily on plankton. They often travel in large schools and typically use gill rakers to filter out plankton and detritus from the water column.

12.4.8 Adaptations to Avoid Predation

As most species of fish are predatory in nature, specialized adaptations have evolved to avoid becoming prey themselves. Porcupinefish and pufferfish can inflate themselves by swallowing large amounts of water, and this also causes their spines to stick out, making them too large for most fish to swallow. Flying fish (Cypselurusspp.) can leap out of the water and glide for long distances using their enlarged pectoral fins. Some species of parrotfish secrete a mucous cocoon at night to mask their scent from predators while they rest. Surgeonfish have sharp scalpel-like spines located on their caudal peduncle, which may be used in defense. Scorpion and stonefish are highly camouflaged and have venomous dorsal spines to avoid predation.

12.4.9. Reproduction

Sharks reproduce through internal fertilization, with male sharks transferring sperm to the female through modified pelvic fins called claspers. There are three different reproductive strategies seen in sharks: oviparity, ovoviviparity, and viviparity. Oviparity is the most primitive method, with eggs being laid outside the female, and development occurs in a protective casing. The offspring usually hatch relatively small due to the limited availability of nutrients in the eggs. This is exhibited in whale sharks. Ovoviviparity is the most common method, with eggs hatching inside the mother's uterus, yet no placental connection is made. The developing young are nourished through yolk stored in the egg, but are able to develop within the protection of the mother. This is seen for example in basking sharks and thresher sharks. Viviparity is the most evolutionary advanced method where the offspring are attached to the mother’s uterus. The


BIOL 181: Life in the Oceans – Lecture Notes

young are fed directly from the mother and are able to be born relatively large and independent. This is exhibited in hammerhead sharks.

Bony fish have developed a variety of reproductive strategies, although most of them are oviparous. Some species are pelagic spawners, e.g., cod, tuna, sardines, parrotfish, and wrasse, releasing large amounts of eggs and sperm in the water column. In this strategy, there is no parental care and there is an extremely high mortality rate. In benthic spawning, nonbuoyant eggs and sperm are usually spread over large areas of vegetation or other substrate; there may still be a high mortality rate, as there is no parental care. Certain species may hide their eggs in some way, e.g., the grunion (Leuresthes tenuis) buries its eggs in the sand at high tide. Guarders, such as damselfish, blennies, and gobies, exhibit strong parental care, typically from the males, who guard the eggs until they hatch, and thus fewer eggs are produced. In a rarer strategy, fish such as jawfish (Ophistognathus macrognathus) or sea horses (Hippocampus), which are known as bearers, the males will actually carry the eggs until they hatch.

Certain bony fish exhibit hermaphroditism in which individuals have both testes and ovaries for at least some part of their lives. Hermaphroditism may be synchronous, with an individual possessing functional gonads of both sexes at one time (for example, hamlets, salmon and deep- sea fish), or sequential, changing from one sex to another throughout their life. Sequential hermaphroditism may be protogynous, where females change into males (e.g., wrasses and parrotfish) or protandrous, where males later change to females (e.g., anemonefish).

12.4.10. Migrations

There are many instances of migrations in fish. These migrations are due to physical and biological factors such as temperature variations, following food or mating, and spawning requirements. Daily migrations occur to avoid predators or to obtain food; for example, grunts migrate from the reefs in the daytime to feed in the seagrass at night. Vertical migration through the water column is exhibited in species such as the lanternfish and is primarily in response to migrating prey species. Seasonal migrations occur primarily associated with mating and spawning. Some fish, such as salmon, spend most of their lives in saltwater and move to freshwater to spawn, and are known as anadromous. The reverse, found in freshwater eels which spend their lives in freshwater yet migrate to saltwater to spawn, is termed catadromous.

12.4. Review Questions: Fish

3. Which fish is able to tie its body in knots to remove excess slime and gain leverage to tear

8. Name three mechanisms sharks use to remain neutrally buoyant. 9. How do sharks achieve internal fertilization?


BIOL 181: Life in the Oceans – Lecture Notes

What do the ampullae of Lorenzini detect?Are all sharks predators?Describe the three different reproductive strategies seen in sharks.Why are the eyes and spiracles of a ray on the top side?Give four functions of the pectoral fins in ray-finned fish.Which caudal fin shape is best for high speeds?Which caudal fin shape is best for maneuvering at low speeds?What is the body shape of a reef fish, such as the butterfly fish? How is it adapted to its


What is the typical body shape of a bottom-dwelling fish?What are chromatophores?What is an iridophore?What is the advantage of countershading?Give an example of a fish showing disruptive coloration.Which fins do parrotfish and wrasses use to swim?What is the operculum?Explain the countercurrent exchange system in the gills, and its importance.How do marine fish regulate their internal body fluid concentration (osmoregulation)?Why is it advantageous for active swimmers to have a countercurrent arrangement of

arteries and veins?

What is a swim bladder, and what is its main role?How does a fish with a closed swim bladder control its volume?Why do active swimmers not have a swim bladder, and how do they achieve buoyancy


In addition to providing buoyancy, what other roles does the swim bladder have?What do neuromasts of the lateral line system detect?How are fish ears different to ours?What role does the swim bladder play in hearing?How does the lens in a fish eye adjust for distance?Why does a carnivorous fish swallow its prey whole?Give three adaptations to avoid predation seen in fish.Give an example of a simultaneous/synchronous hermaphroditic fish.Give an example of a protogynous sequential hermaphroditic fish.Describe the reproductive strategy of a protandrous sequential hermaphroditic fish.


BIOL 181: Life in the Oceans – Lecture Notes

13. Marine Reptiles and Birds

(The majority of the text below originally appeared as chapter 10 ofIntroduction to Marine Biology)13.1. General Characteristics and Adaptations

Reptiles and birds are vertebrates that inhabit both terrestrial and marine environments. They are typically large organisms that are high in the food chain, at the tertiary level or higher. They feed on a variety of organisms and have relatively few predators. They are adapted to move freely and have highly developed senses. They have proportionally large brains and a high capacity for learning.

The ancestors of marine reptiles and birds were terrestrial, and the reinvasion of the sea by these organisms is reflected in their body formation, locomotion, insulation, osmotic balance, and feeding. Marine reptiles and birds excrete excess salts, and their nitrogenous wastes are converted to uric acid, which allows for very little water loss with their wastes.

13.2. The Amniotic Egg

The evolutionary success of reptiles and birds in due in large part to the development of the amniotic egg, which evolved 340 million years ago and allowed the egg to develop for a longer period of time within a protective covering (Figure 13.1). This eliminated the need for a free- swimming larval stage and decreased early mortality. It also allowed the egg to be laid in a dry area away from aquatic predators. However, the egg must be fertilized before the shell is added, which required the evolution of copulatory organs for internal fertilization.