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Mollusks comprise a group of soft-bodied animals that includes snails, clams, and sea slugs. The most common characteristic of most mollusks is their shell. One of the largest groups, the snails, is renowned for their shells. Snails are univalves, which means they have one shell. And it is this shell that for many people is the epitome of the ocean. There is perhaps no other ocean treasure that displays more diversity and beauty than the shell. Conchology, the study and collection of shells is a popular hobby the world over. The bivalves, or two-shell mollusks include the clams, scallops, and oysters. It is the oyster that is responsible for producing the most coveted of the ocean's treasures - the pearl. Still other mollusks have lost their shells altogether. The octopus, the squid, and the sea slugs have evolved their own survival strategies to replace their protective armor. Indeed, it is due to the absence of a protective shell the octopus has evolved the largest and most complex brain of all the mollusks. Below is a listing of some of the most common mollusks found on the world's coral reefs.

Yellow Nudibranch (Notodoris minor)
Yellow Nudibranch
(Notodoris minor)

The nudibranch is without a doubt the most colorful of all the mollusks. There are hundreds of species, and are different and exhibit an endless variety of beautiful designs. Nudibranchs are members of the sea slug family and have lost their protective shells. They feed mainly on algae, although some species will eat corals and sponges.

Tridacna Clam (Tridacna maxima)
Tridacna Clam
(Tridacna maxima)

The tridacna clam is one of largest bivalves in the world. Some species can reach 4 feet in length and are commonly known as giant clams. Their bright colors are caused by tiny algae called zooxanthellae. These algae use sunlight to provide nutrients to the clam and receive other nutrients in return. This species if found in the southwest Pacific, and reaches 14 inches in length.

Tridacna Clam (Tridacna crocea)
Tridacna Clam
(Tridacna crocea)

This species of clam is also found in the southwest Pacific Ocean, and is the most colorful member of the tridacna clam family. The colorful spots are the clam's "eyes" and allow it to sense light intensity. If a shadow falls on the clam, it will quickly withdraw into its shell. This species reaches a length of about 9 inches.

Flame Scallop (Lima scabra)
Flame Scallop
(Lima scabra)

This extremely colorful species of scallop is found in the Caribbean, and reaches a size of 3 inches. They have an amusing method of propulsion which involves clapping their shells together to jet propel themselves through the water. They are filter feeders and feed on plankton and other nutrients in the water.

Flamingo Tongue Cowrie (Cyphoma gibbosum)
Flamingo Tongue Cowrie
(Cyphoma gibbosum)

Cowries are a group of snails that are of great interest to shell collectors because of their beautiful shells. The shells are very polished and shiny and feature a wide range of colorful patterns. They are nocturnal and eat algae and dead animals. Some species will feed on both hard and soft corals. The flamingo tongue is not recommended for the aquarium, as it tends not to live long in captivity.

Calf Cowrie (Cypraea vitellus)
Calf Cowrie
(Cypraea vitellus)

The shell of the calf cowrie is not quite as attractive as that of other members of this group. This shell is usually brown and white with spots. Because of the lack of color, the calf cowrie shell does not fetch as high a price on the shell collector market. The muted colors provide this animal with a good means of camouflage.

Common Limpet (Scutus unguis)
Common Limpet
(Scutus unguis)

Limpets are related to snails, and closely resemble them with a few exceptions. Their shell is cone shaped and has a small hole on the tip. Limpets feed on algae and detritus, and then expel waste materials through the hole on top. Because of this, they look like tiny volcanoes as they roam around the rocks.

Common Chiton (Acanthopleura granulata)
Common Chiton
(Acanthopleura granulata)

Chitons are small mollusks that are common in the Caribbean. They are nocturnal and move around the rocks at night feeding on algae. Their strange appearance is reminiscent of the ancient trilobites of prehistoric times. They are commonly introduced into the home aquarium with live rock.

Pacific Octopus (Octopus dofleini)
Pacific Octopus
(Octopus dofleini)

The octopus is probably one of the most well known mollusks. They have lost their hard shell, and have evolved large, complex brains to compensate. The octopus swims by jet propulsion by siphoning water through its body. When threatened, it can release a blinding cloud of black ink. They feed on crustaceans and fish.

Blue Ring Octopus (Hapalochlaena maculosa)
Blue Ring Octopus
(Hapalochlaena maculosa)

The blue ring octopus is a small species found in the waters of Australia. With its bright blue markings it is a beautiful creature, but it is also quite deadly. Although it is only about eight inches in diameter with its tentacles spread, its bite is poisonous enough to kill a person. The blue ring is the only poisonous octopus species known to exist.

Atlantic Squid (Lolliguncula brevis)
Atlantic Squid
(Lolliguncula brevis)

The squid is another member of the mollusk family that has lost its hard shell. Like the octopus, squid have suction cups on their arms that aid in capturing their prey. They feed on fish and crustaceans. This common squid species is found throughout the Atlantic United States coast and grows to a length of 5 inches.

Cuttlefish (Sepia officinalis)
(Sepia officinalis)

Cuttlefish are very closely related to squid. One noticeable difference is that the cuttlefish has noticeably shorter tentacles. These animals also have specialized tentacles that can shoot out quickly to catch their prey. Cuttlefish are masters of disguise. They can change colors in an instant, and have actually been observed to use flashing colors as a means of communication.

Chambered Nautilus (Nautilus macromphalus)
Chambered Nautilus
(Nautilus macromphalus)

For many years the nautilus was thought to be a rare deep water species, but recently they have been discovered in large numbers on Indo-Pacific coral reefs. They are nocturnal animals, swimming around the reef at night in search of small fish and shrimp. Their unique shells are highly prized by shell collectors.