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.