11.0.5 11.7: Most mollusks live in shells.

Just because they are related, that doesn’t mean organisms will have much of a physical resemblance. Consider these animals. A colossal predatory squid, more than 40 feet long, with eyes the size of beach balls and a razor-sharp beak. A small snail—escargot in French—one of the more than 700 million consumed in France each year. An oyster, with its nearly 2,000-year-old reputation as an aphrodisiac (unsupported by any scientific proof, it turns out). All of these are mollusks, among the most diverse groups of animals and occupying an impressive and impressively varied position within the human imagination.

More than 100,000 mollusk species have been named (and many more have not yet been named). The members of this large phylum (Mollusca) live in the ocean, in fresh water, and on land and include many familiar creatures in addition to those mentioned above, including clams, scallops, mussels, and octopuses. They are so diverse that it is difficult to describe any single defining characteristic. The position of mollusks within the animal phylogeny, however, reflects several important characteristics that they all share: they have defined tissues, are bilaterally symmetrical, and are protostomes. Further, all mollusks grow by adding tissue rather than by molting.

Several additional features are also common to many mollusks. Some groups of mollusks have a shell that protects the soft body, a mantle (the tissue that secretes calcium carbonate to form the shell), and a sandpaper-like tongue structure, called the radula, that is used during feeding.

Here, we examine three of the major groups of mollusks. The animals in these groups share most of the features common to mollusks, but with very different body plans: gastropods, bivalve mollusks, and cephalopods (FIGURE 11-13).

Gastropods Snails and slugs are gastropod mollusks. Gastropod means “belly foot,” and these mollusks get their name from the expanded foot on the bottom of their body, which allows them to climb a vertical surface as easily as they glide across a horizontal one. Snails have a one-piece, curled shell, and slugs are snails that have just a tiny remnant of a shell that is not even visible because it is covered by the mantle. Found in both aquatic and terrestrial environments, snails and slugs account for three-quarters of all mollusks.

The snail’s shell is its primary protection against predators, but for terrestrial slugs and sea slugs, which have very little shell material, other defense mechanisms are necessary. A terrestrial slug relies on slime for defense: when a slug is attacked, it secretes slime that sticks to the predator. Worse still, anything that touches the slime coating the predator sticks to it, so a bird that attacks a slug quickly finds that its beak and face are covered by pieces of dead leaves, clods of soil, and twigs. Sea slugs have other methods of defense. Some species synthesize toxic chemicals, while those that feed on sponges can sometimes recycle sponge toxins into their own slime. One of the most remarkable uses of another animal’s defense equipment is found among sea slugs that eat sea anemones. Anemones are protected by stinging cells, and when eaten, some of these cells are transferred to the sea slug’s own skin, enabling it to sting other creatures.

Bivalve Mollusks The bivalve mollusks are soft-bodied animals protected by a pair of shells hinged together by a ligament. Clams, scallops, oysters, and mussels are examples of bivalves. There are about 8,000 bivalve species and most of them live in the ocean, although there are some freshwater species. Clams spend their lives buried in mud or sand, scallops live on the seafloor, and oysters and mussels fasten themselves to underwater objects, such as rocks, the pilings that support piers, and the hulls of boats. All bivalves are filter-feeders that draw a current of water in through a tube called the “incurrent siphon,” across their gills, where tiny food particles are captured, and out through the “excurrent siphon.”

When a grain of sand is trapped in the shell of a bivalve, the mantle may secrete layer after layer of a shell-like protein material that covers the sand grain and forms the iridescent gem called a pearl. Oysters are the best known source of pearls, but clams and other bivalve mollusks also form pearls.

Cephalopods The third major group of mollusks is the cephalopods (FIGURE 11-14). It includes 6 nautilus species and more than 600 species of squids and octopuses. The nautilus has an external shell, squids have very small shells that are covered by the mantle, and octopuses have lost the shell entirely. In all these species, the tentacles appear to grow directly from the head, which explains the name: cephalopod translates as “head-footed.”

The most obvious feature of cephalopods may be their tentacles, which they use to walk and swim and to capture prey. Squids—many of which are ferocious predators—have eight short tentacles called arms and two long, sucker-bearing tentacles that are used to capture prey.

After a slow approach to its unsuspecting prey, a squid propels its tentacles forward with astonishing speed—accelerations of more than 800 feet (about 250 m) per second per second have been measured, the equivalent of your car accelerating from 0 to 6,000 mph in 10 seconds! The suckers on the tentacles adhere to the prey and draw it back toward the squid, where the arms take over, turning and manipulating the prey as it is bitten by the squid’s sharp beak and pulled into the mouth by the tongue-like radula.

Squids are often more than 3 feet (about 1 m) long, and members of some species may exceed 40 feet (about 13 m). When a squid is in a hurry, it swims tail-end first, using jet propulsion. Water is expelled through the siphon at the tentacle end of the body, shooting the animal backward.

Octopuses, which can have tentacles that spread 12 feet (about 4 m), are bottom-dwellers that live in coral reefs and on rocky coasts. Because they have no shell at all, octopuses can squeeze through astonishingly small openings, and their ability to escape from even carefully covered tanks is a perpetual challenge for the keepers of zoos and aquaria. Keepers are accustomed to arriving in the morning and finding an octopus wandering around the room.

In the next section we further explore the cephalopods and consider the question of whether their skills of manual dexterity make them the smartest of the invertebrates.