After insects, molluscs form the largest group of animals. Molluscs have soft, muscular bodies, often covered by a protective shell. Some, such as snails, move on a muscular foot, which can be withdrawn into the shell for protection. Other, sea-dwelling molluscs, such as squid and scallops, take in water and squirt it out to jet-propel themselves along.
Mollusc is one of the most diverse groups of animals on the planet, with at least 50,000 living species (and more likely around 200,000). It includes such familiar organisms as snails, octopuses, squid, clams, scallops, oysters, and chitons. Mollusca also includes some lesser known groups like the monoplacophorans, a group once thought to be extinct for millions of years until one was found in 1952 in the deep ocean off the coast of Costa Rica.
Molluscs are a clade of organisms that all have soft bodies which typically have a “head” and a “foot” region. Often their bodies are covered by a hard exoskeleton, as in the shells of snails and clams or the plates of chitons.
A part of almost every ecosystem in the world, molluscs is extremely important members of many ecological communities. They range in distribution from terrestrial mountain tops to the hot vents and cold seeps of the deep sea, and range in size from 20-meter-long giant squid to microscopic aplcophorans, a millimeter or less in length, that live between sand grains.
These creatures have been important to humans throughout history as a source of food, jewelry, tools, and even pets. For example, on the Pacific coast of California, Native Americans consumed large quantities of abalone and especially owl limpets. However, the impact of Native Americans on these Molluscan communities pales by comparison to the overharvesting of some molluscan taxa by the United States in the 1960s and 1970s. Species, whose members once numbered in the millions, now teeter on the verge of extinction. For example, fewer than 100 white abalone remain after several million individuals were captured and sold as meat in the 1970s. Besides having yummy soft parts, molluscs often have desirable hard parts. The shells of some molluscs are considered quite beautiful and valuable. Molluscs can also be nuisances, such as the common garden snail; and molluscs make up a major component of fouling communities both on docks and on the hulls of ships.
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