Skip to main content

Mollusks: Gastropods, Bivalves, and Cephalopods

The mollusks are a large group of arthropods that have a variety of outward appearances and include animals such as clams, octopuses, and snails. Because of their tremendous range of structures and styles, mollusks are divided into three groups: gastropods, bivalves, and cephalopods. Unlike their plainer terrestrial relatives, marine species of mollusks have extravagant forms that tend toward elaborately shaped shells and bright colors.

Even though the group is diverse, members share some common traits. The group’s name, mollusk, literally means “soft bodied” and describes one of their primary characteristics. In addition to soft bodies, mollusks are also characterized by a foot that is used for locomotion. Internally, mollusk organs are covered with a thin tissue called the mantle. In some species the mantle secretes the shell and one or more defensive chemicals, such as ink, mucus, or acid.
A mollusk feeds with a file-like rod of muscle called the radula. This tongue-like organ is covered in sharp, rasping teeth that enable the animal to scrape up a variety of foods, including algae, animal tissue, or detritus. Most mollusks protect their soft internal parts with a hard shell. Like the body coverings of several types of marine invertebrates, the shells of mollusks are made of calcium carbonate. Mollusks exchange gases with the water through their gills.

Members of all three groups of mollusks live around the reef. Gastropods are common and include snails and nudibranchs. Snails glide over the sandy reef floor on one large muscular foot that is located in the center of their body. Snails’ eyes, which are small, light-sensitive dots on their head, help them find food and keep watch for predators. Many snails have a spiral-shaped shell that contains and protects the internal organs. In some species, an operculum, a flap or door that can close the shell, protects the occupant from danger.

Most gastropods are hermaphrodites, individuals that have both male and female sex organs. Despite the convenience of having both sexes available, sperm are generally exchanged with another individual during mating. Eggs, which may be brooded in the mother’s body, or laid as gelatinous masses or egg cases, hatch into shell-less larvae. Shells are produced by the mantle as the animals mature and are continually added to throughout life.

Several species of snails live on the reef. The stocky cerith (Cerithium literatum), which measures about 1 inch (2.5 cm) long, is white with rows of brown markings. In contrast, the tulip snail (Fasciolaria tulipa) has a smooth, reddish brown to gray shell that is covered with brown spiral lines. The soft tissues of this predatory animal are a dramatic red. One of the most beautiful gastropods on the reef is the flamingo tongue (Cyphoma gibbosum), shown in Figure below. The flamingo tongue has a smooth pink and orange shell that is about 1 inch (2.5 cm) long and is enclosed in the snail’s orange, leopard-spotted mantle tissue. One of the largest snails is the queen conch (Strombus gigas), a white-shelled herbivore measuring up to 12 inches (30 cm) long.
The flamingo tongue snail has a brightly colored shell. (Courtesy of NOAA, Coral Kingdom Collection)
The flamingo tongue snail has a brightly colored shell.
(Courtesy of NOAA, Coral Kingdom Collection)

A group of colorful, shell-less gastropods also live on the reef. Although they have been nicknamed “sea slugs,” these animals are not slugs but nudibranchs, so named because their gills (branchia) are nude (nuda). Most of the 3,000 species of nudibranchs are residents of the coral reef. In many species, gills are conspicuously displayed on their backs as knobby or feathery projections. Nudibranchs dine on an endless variety of foods, including sponges, tunicates (filter feeders related to vertebrates) anemones, corals, worms, crustaceans, and hydroids. A few species are able to consume algae without digesting them. Instead, their bodies store the algal cells just under the skin, keeping them alive and functional. The algae become solar-powered sources of food for the nudibranchs.

The spotted sea hare (Aplysia dactylomela) is a nudibranch that resembles a rabbit hunched and ready to hop. Its olive-drab body is 20 inches (50 cm) long and decorated with black ringlike spots. Structures that look like rabbit ears extend from its head, and a groove that is bordered with two flaps runs down its back. If this peaceful alga eater is disturbed, it emits a purple cloud to distract and confuse its predator.

Unlike the gastropods that have one shell, or are lacking a shell altogether, bivalves are animals that have two shells, or valves. The valves, which hinge together on one side and are opened and closed by strong muscles, provide these animals with protection from predators. Clams and oysters are some of the bivalves that live on the reef.

The foot of a bivalve helps it to either attach to a substrate or burrow into the sand. The foot is a large muscle that can be extended between the open shells. Although some gastropods scrape up food with a radula, bivalves use their gills to filter food from the water. A bivalve’s gills, which are located in the mantle cavity, are covered with hairlike cilia and mucus. As water moves over the gills, tiny bits of food become trapped there. The cilia sweep the food into the bivalve’s mouth.

Giant clams (Tridacna maxima) are extraordinary reef bivalves. Long ago mislabeled as “killer clams” because of their behemoth size, the largest individuals of this species measure 3 feet (1 m) wide and weigh a half ton. Giant clams, like all bivalves, are gentle filter feeders.

The much smaller file clam (Lima) is also found on the reef, measuring 1 to 3 inches (2.5 to 7.5 cm) long. Brown ridged shells protect the orange animals that live inside. Spiny, or thorny, oysters (Spondylus americanus) are reef residents that have round shells from which long spines stick in all directions. Below the spines, the shell surface is white, yellow, brown, purple, or red. The winged pearl oyster (Pteria colymbus) is a brownish purple animal that can be recognized by the long, thin extensions of its shell.

Popular posts from this blog

Advantages and Disadvantages of an Exoskeleton

More than 80 percent of the animal species are equipped with a hard, outer covering called an exoskeleton. The functions of exoskeletons are similar to those of other types of skeletal systems. Like the internal skeletons (endoskeletons) of amphibians, reptiles, birds, and mammals, exoskeletons support the tissues and give shape to the bodies of invertebrates. Exoskeletons offer some other advantages. Serving as a suit of armor, they are excellent protection against predators. Also, because they completely cover an animal’s tissues, exoskeletons prevent them from drying out. In addition, exoskeletons serve as points of attachment for muscles, providing animals with more leverage and mechanical advantage than an endoskeleton can offer. That is why a tiny shrimp can cut a fish in half with its claw or lift an object 50 times heavier than its own body.
Despite all their good points, exoskeletons have some drawbacks. They are heavy, so the only animals that have been successful with them …

Differences in Terrestrial and Aquatic Plants

Even though plants that live in water look dramatically different from terrestrial plants, the two groups have a lot in common. Both types of plants capture the Sun’s energy and use it to make food from raw materials. In each case, the raw materials required include carbon dioxide, water, and minerals. The differences in these two types of plants are adaptations to their specific environments.
Land plants are highly specialized for their lifestyles. They get their nutrients from two sources: soil and air. It is the job of roots to absorb water and minerals from the soil, as well as hold the plant in place. Essential materials are transported to cells in leaves by a system of tubes called vascular tissue. Leaves are in charge of taking in carbon dioxide gas from the atmosphere for photosynthesis. Once photosynthesis is complete, a second set of vascular tissue carries the food made by the leaves to the rest of the plant. Land plants are also equipped with woody stems and branches that …

Prokaryotic Cell Structure

Prokaryotic cells are about 10 times smaller than eukaryotic cells. A typical E. coli cell is about 1 μm wide and 2 to 3μm long. Structurally, prokaryotes are very simple cells when compared with eukaryotic cells, and yet they are able to perform the necessary processes of life. Reproduction of prokaryotic cells is by binary fission—the simple division of one cell into two cells, after DNA replication and the formation of a separating membrane and cell wall. All bacteria are prokaryotes, as are the archaea.

Embedded within the cytoplasm of prokaryotic cells are a chromosome, ribosomes, and other cytoplasmic particles (Fig. 1). Unlike eukaryotic cells, the cytoplasm of prokaryotic cells is not filled with internal membranes. The cytoplasm is surrounded by a cell membrane, a cell wall (usually), and sometimes a capsule or slime layer. These latter three structures make up the bacterial cell envelope. Depending on the particular species of bacterium, flagella, pili (description follows)…