Written by Shireen Gonzaga and Marc Airhart

Mollusks have adapted to an amazing range of environments. Clams and snails can be found living on mountains, in lakes and ponds, marshes, bays and estuaries, along sandy seashores, floating on the sea surface and living at the bottom of the sea, near hot springs. There are clams that burrow into wood and limestone rock, that live attached to the undersides of sea urchins and the gills of fish. Snails can be found living in high trees, in the intestines of sea puddings, and within the arms of starfish.

Seashells occur in a great range of shapes, colors and sizes. One of the smallest seashells is the Pythina clam, a tiny, smooth translucent clam the size of a rice grain, that lives attached to the underside of shrimp and crayfish. At the other extreme is the largest known seashell—the giant Tridacna clam of the southwest Pacific. This monster’s shell consists of two attached valves which are four foot long and weigh 500 pounds.

Mollusks first made their appearance 500 million years ago, during the Cambrian period. 100 million years later, the six classes of mollusks living today could be found in the fossil record. The amazing variety of shells—their color, patterns and sculpture—have been shaped by millions of years of environmental changes and genetic mutations.

The largest, most common, and best-known seashells are the univalves or gastropods—conchs, whelks and snails. They have one shell, which is often coiled. Single-shelled animals first appeared in the fossil record 500 million years ago. Some gastropods, such as limpets and abalone have flat saucer-like shells. Snails are the only mollusks to have the distinction of colonizing land as well as freshwater and marine habitats. The Aztecs of ancient Mexico depicted their rain god, Tlaloc, rising from a conch shell. The Greek god Triton, one of Neptune’s trumpeters, was depicted with a large conch shell that he used to summon river deities around their king.

Bivalves, such as oysters, clams, and scallops, are mollusks that have two movable interlocking shell valves attached at one end by a strong muscular hinge. The valves open and close like the front and back cover of a hardback book. By retreating and closing its soft body inside its shell, a bivalve is able to protect itself from most predators. Bivalves such as the blue mussel attach themselves to wharves and rocks with thread-like byssal anchors.

While most gastropods and bivalves are sedentary creatures, the sprightly scallop can be quite lively. An adult scallop, when provoked by a predatory fish, crab or starfish, will suddenly leap up and zigzag away. The snapping action of the valves propels the animal backward. However, the scallop can also move forward by suddenly clamping its valves shut, expelling water from its hinge side.

The third major type of mollusks are cephalopods, such as the squid and octopus. They’re also the most active. They possess a keen intelligence and most have evolved beyond the need for shells. Their fossil record spans over the past 400 million years, when many species then had shells. The only shelled cephalopod that remains today is the chambered nautilus, which was once abundant in ancient seas along with its cousin the ammonite. Ammonites became extinct 65 million years ago, presumably in the same catastrophe that wiped out the dinosaurs. The chambered nautilus is a magnificent animal with a creamy white shell with broad rusty brown stripes. The inside of its shell has an iridescent mother-of-pearl coating. The shell itself is divided into different chambers where volumes of gas and water give the nautilus buoyancy in the water.

Besides gastropods, bivalves and cephalopods, there are three less common types of mollusk. There are the chitons, also known as the armadillos of the sea; the tusk shells, resembling miniature elephant tusks; and the gastroverms—small, fragile and primitive animals that weren’t even considered mollusks until the late 1950s.

sources:
http://www.earthsky.org/article/seashells