Single-shelled Mollusks
What are they?
Shells are the hard, outer casings of a large group of water-borne mollusks. Some species are snail-like in having a single, usually spiral-shaped shell, while others - the bivalves - have two shells which can be pulled together to provide a shelter for the soft-bodied mollusk to hide in. After death, only the hard shell survives and often ends up washed on the beach. With dredging and 'beach replenishment', a number of shells of deeper water species can sometimes be found on the beach, as well as the usual range of inshore species. Finding whole specimens of some species is difficult as they often get broken by the surf.
Identification
Single-shelled mollusks include more or less sedentary, limpet-type species that often attach themselves to rocks or other solid objects below the high water mark, and more mobile snail-like species. For identification, the number of spiral turns, texture of the shell and size and shape of the entrance hole to the shell can all be important.
Knobbed Whelk Busycon carica
A large (up to nine inches long) and common species which is commercially fished and sold as conch. These chunky mollusks are predators on other shellfish species.
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Mud Dog Whelk Nassarius obsoletus
A small, dark species of inshore snail which is abundant in muddy waters of the bayside and backbay marshes.
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Lobed Moon Shell Polinices duplicatus
A chunky snail with a shell that may reach up to three inches across. Note how the hole through the center of the shell (umbilicus) is sealed by a piece of shell.
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Northern Moon Shell Polinices duplicatus
A large snail with a shell that may reach up to four inches across. Note how the hole through the center of the shell (umbilicus) is not sealed by a piece of shell.
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Common Slipper Shell Crepidula fornicata
Shells up to an inch and a half long. Living specimens are often found attached firmly to objects washed up on the beach, including living crabs. A bizzare species that is often found arranged in short stacks; the smaller individuals at the top are male, those at the bottom female and those in the middle hermaphrodite. Each individual starts as a male and becomes female as it grows larger!
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