Amethyst gem clam (Gemma gemma)

Description

The amethyst gem clam (Gemma gemma) is one of the smallest marine clams known, having a maximum length of approximately 5 millimeters (Sellmer 1967). Gem clams have rounded shells with either a smooth surface or fine concentric ridges and a glossy finish. Shell coloration varies from white to pale reddish-violet to dark reddish-purple, giving gem clams their common name.

Background

Distribution. Gem clams are distributed along the Atlantic and Gulf coasts from Labrador, Canada, to Texas and in some locations along the west coast (Sellmer 1967). Previous studies in New York/New Jersey Harbor have found gem clams present in high abundance (Dean 1975), and in this study its distribution was limited to shoreline areas away from shipping channels. The gem clam is a fine-sand species common in bays and estuaries (Franz and Harris 1988, Weiss 1995), and can be found in the intertidal zone to depths of 6 meters (Gosner 1971).

Feeding. Gem clams are filter-feeders and primarily consume diatoms and organic detritus. They are nocturnal feeders, and have been observed actively feeding at times of low light (Sellmer 1967).

Fishery. Gem clams are not commercially harvested.

Life Cycle

Males release spermatozoa into the water column from April to May and from October to November. Females uptake the spermatozoa via the incurrent siphon, and internal fertilization and development occur within the mantle. Females hold approximately 300 developing young until the juveniles hatch and emerge as miniature adults. Newly emerged juveniles immediately take up a benthic lifestyle and can sexually mature in four to five months (2 millimeters in length). If maturity is not reached until after fall, the young clams will mature in the following spring. Most females produce two broods, and the second is often carried through the winter for liberation in the following summer (Sellmer 1967). Gem clams usually live into their second winter, though some have been aged at three years.

Environmental Influences

Salinity. Gem clams have been observed in salinities ranging 13 to 28 practical salinity units (psu; Sellmer 1967).

Predation. Gem clams are a food resource for many waterfowl. Gastropods, crustaceans, and some cnidarians are also known to eat gem clams (Sellmer 1967).

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