Mollusk Habitats

by Dr. Gary Rosenberg
Many people, when they think of collecting shells, picture doing so on beautiful, white, sandy beaches with the waves rolling in. However, sandy beaches, especially those with heavy surf, are not productive habitats for mollusks. Only a few agile burrowers such as Donax, Hastula and Bullia can survive in the shifting sands of the surf zone. When a sandy beach has large numbers of shells cast up in the drift line, this indicates the presence of more productive habitats nearby.

The stable substrata of intertidal mud and sand flats have much more diverse communities of mollusks. They are home to various surface-living bivalves such as oysters and burrowing bivalves such as soft shell clams, razor clams, surf clams, Venus clams, cockles, lucines, tellins, and angel wings. These are preyed upon by carnivorous gastropods including moon snails, murex, whelks and chanks. Other inhabitants of mud and sand flats include herbivorous conchs, scavenging mud snails and deposit feeding ceriths. Mangroves on mud flats are populated by oysters, periwinkles, ceriths, and ellobiids.

Many of the inhabitants of intertidal sand and mud flats also occur subtidally, where they are joined by tusk shells, awning clams, bittersweets, pen shells, and gastropods such as tons, helmets, harps, olives, volutes, cones and terebras, Sandy areas can have varying amounts of rubble mixed in, and often grade into reef flats. Many of the mollusks found on sand flats also occur in sandy pockets within the reef, along with miters and ceriths.

Within reefs and rocky areas live many mollusks that prefer hard substrata. Only a few kinds of mollusks, such as ovulids (allied cowries) and coralliophilas, live in direct association with corals, but the coral rubble around living reefs is rich in mollusks because it supports luxuriant growth of algae and encrusting invertebrates on which many gastropods and polyplacophorans feed. Typical reef-dwelling gastropods include cones, whelks, frog shells, tritons, cowries, abalones, murex, dove shells, top shells and turrids. Bivalves include arks, jewel boxes, limas, mussels, scallops, thorny oysters and giant clams.

Rocky intertidal areas typically support a rich molluscan fauna. A variety of limpets are usually present, along with chitons, nerites, top shells and periwinkles. They feed on vegetation and sessile invertebrates encrusting the substratum. Muricid snails often occur on intertidal rocks, where they feed on mussels and barnacles.

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The above material has been adapted from Dr. Rosenberg's The Encyclopedia of Seashells, published by Robert Halt, Ltd., London, 1992. Dr. Rosenberg is Associate Curator of Malacology at the Academy of Natural Sciences of Philadelphia