When people think of clams, they probably think of a nice hot bowl of clam chowder or other seafood dish. In the United States, clam is the common name for certain mollusks, or shelled animals, that have bivalve, or two-piece, shells. There are over 12,000 clam species found throughout the world in many different habitats.
A clam's oval-shaped shell pieces are hinged by a stretchy ligament. The front of the clam's body features a muscular foot that the clam can extend through the open end of its shell for burrowing into the sand or mud. The clam's body has two siphon-like tubes. Clams feed and breathe by drawing water and food particles into one tube and expel waste-filled water out the other. On some species, the tubes are connected as a single structure called the neck. Clams reproduce by depositing egg and sperm cells into the water. The fertilized egg cell develops into a shell-less larva that grows into an adult over several months.
Some clams are among the longest-lived species in the world. For example, in 2007, scientists discovered a specimen of the ocean quahog that was between 405 and 410 years old. Giant clams live about 150 years while cold seep clams don't even reach maturity until they are 100. However, most species live between three and 10 years.
The giant clam is the world's largest species of mollusk. Found in the South Pacific and Indian Oceans, giant clams can be four feet long and weigh over 500 pounds. They reach these enormous sizes by eating the proteins and sugars produced by the algae that live on their tissues. During the day, the clams keep their shells open, exposing the algae to the sunlight they need for photosynthesis. South Pacific legends claim that giant clams eat people by catching them with their shells. However, scientists believe that the clams shell closes too slowly to catch a human.
Distribution and Habitat
Clams are found all over the world. They live in a variety of habitats including Arctic and Antarctic waters, coastal mud flats, the deep ocean and coral reefs. Most species are found in the ocean, however, two types are found in freshwater. Large freshwater clams, also called freshwater mussels, are a source of mother-of-pearl. Their babies are parasites who live on fish gills. The small freshwater clams are hermaphrodites who keep their fertilized eggs in a pouch and bear young with developed shells.
The soft-shell clam from the coasts of North America is one of the most popular edible clams. However, the oceanic surf clam found on the East Coast of the United States is America's most important commercial species. As of 2011, recent surf clam harvests have yielded between 41 and 63 million pounds of meat. The Pacific Northwest's geoduck clam is the largest clam harvested in the United States. They can weigh around three pounds each and provide over a pound of edible meat.