Mollusca, meaning “soft-bodied,” is one of the largest phylums in the animal kingdom. The word mollusc (or mollusk) derives from the Latin word “mollis,” which means “soft.” There are an estimated 200,000 species of mollusk worldwide accounting for nearly a quarter of all marine life. Familiar phylum species include snails, squid, octopuses, clams and oysters.
According to fossil records, the first mollusks appeared on earth some 600 million years ago. After another 100 million years, primitive forms of six of the seven currently recognized classes of mollusks existed. It took another 100 million years before mollusks first appeared in and near freshwater lakes and streams. It is only within the last 100 million years that mollusks have fully evolved to their present states, populating diverse land, freshwater and marine habitats worldwide.
Species of mollusk vary widely; however, most mollusks have a few basic features in common with one another. All mollusks have soft, unsegmented bodies. Most have a single muscular foot used for moving from place to place or digging into the sand or mud of their habitat. In the cephalopod class of mollusk, tentacles take the place of a foot. A mantle that secrets an exoskeleton or shell further distinguishes mollusks from other animals.
At least 60,000 species make up the largest class of mollusks, the gastropods. These mollusks populate land, freshwater and marine environments. Snails and slugs are common examples of gastropods. Land gastropods have lungs, while their marine counterparts breathe through gills. Most gastropods feed on plant matter, and many land snails and slugs are a nuisance to farmers and gardeners.
The 600 to 650 species of cephalopod inhabit the sea, including squid, octopuses, and cuttlefish. Most lack the shell of other mollusca, but have tentacles that propel them through the water using suction. All cephalopods are scavengers and predators. Their advanced nervous system makes them the most intelligent of all invertebrates. Cephalopods also are the largest mollusks -- a giant squid can be as much as 65 feet long.
Other Marine Classes
Five other classes of mollusks live solely in the ocean; most on the ocean floor. The 900 species of chitons, or polyplacophora, are distinguished by a shell of eight plates and graze on algae. Around 250 species of worm-like aplacophora live in and on the ocean floor. These mollusks feed on detritus, though some are predators. Scaphopoda mollusks, known as “tusk shells” for the shape of their tubular shells, are also detritus feeders. Bivalves inhabit both oceanic and freshwater habitats worldwide. These filter feeders can grow to over four feet in length. Monoplacophora mollusks are the fewest in number, and mostly known from fossil records. A few species still exist today, consuming algae and microorganisms on the ocean floor.
Importance to Humans
Mollusks have been important to humans throughout history. Their shells have been used for tools and as jewelry, many of which are considered beautiful and high in value. Some ancient groups used mollusk shells as a form of currency. Sepia, an artist’s dye, is made from the ink of the cuttlefish, a cephalopod. Mollusks have long been a staple of the human diet worldwide, particularly gastropods and cephalopods such as clams, mussels, scallops, oysters and squid.
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