Gemma gemma (Totten, 1834)
Amethyst gemclam (English)
The Amethyst Gem Clam (Gemma gemma) is a tiny, somewhat triangular clam (usually less than 3mm) with a smooth, glossy shell. The inner shell has a finely scalloped margin (visible with a hand lens). Shell color is whitish or gray, often tinted with purple. (Morris 1973; Gosner 1978; Kozloff 1993) This clam is an Atlantic species introduced into Pacific waters. It lives in soft mud and is therefore found on muddy beaches. The short siphons never extend more than a few millimeters beyond the end of the shell. (Narchi 1971)
Kozloff (1993) notes that on the Pacific coast Gemma gemma can be confused with tiny clams in the genus Transennella, but Transennella lacks the fine scalloping on the inner margin of the shell.
Detailed accounts of the internal anatomy of Gemma gemma can be found in Sellmer (1967) and Narchi (1971). The shell of Gemma gemma is small, subtrigonal, moderately inflated and rather thin. The exterior is polished and has numerous fine concentric riblets. There are three teeth in the left valve and two teeth in the right (this seems to have been reversed in error in Sellmer 1967). Color is whitish to tan, with purplish over the beak and posterior areas. The animals may be so abundant along the coast of New England that the beaches appear purple, due to the many shells. The shell may reach 5.0 mm long, 4.5 mm high, and 3.0 mm wide, but is usually half this size. Several authors have noted the alga Enteromorpha [now in Ulva: Hayden et al. 2003] attached to the posterior region of the shell. Algal growth is confined to the siphonal region because the animal is buried so that the posterior part of the shell and siphons are exposed. Near the tips of the siphons, irregular patches and streaks of reddish brown pigment occur on the outer surface. Elsewhere the siphons are smooth creamy white, due to opaque white patches on the inner surfaces, which shine through the tissues. The siphons are fused along as much as half their lengths and their tentacles are well developed. There is a ring of simple tentacles surrounding the inhalant aperture. The tentacles are of two alternating sizes. The inhalant siphon bears 8 to 12 tentacles which are loosely interdigitated over the opening when the animal is pumping water. The exhalent siphon in both species has a thin semitransparent sleeve, the valvular membrane, which is an extension of the siphon. The foot and siphons may be withdrawn quickly when the animals are disturbed. When this occurs, the valvular membrane is drawn into the inside of the base of the exhalent siphon. When the animal is undisturbed, the siphons protrude slowly without showing the valvular membrane; then suddenly with explosive force the valvular membrane is extruded. The foot is well developed, wedge-shaped, and adapted to burrowing. It can be extended to a distance approximately equal to the length of the shell. The tip has considerable mobility, moving readily and quickly. When placed flat on the surface of the sand, the animal disappears into the substratum in a minute or less. (Narchi 1971 and references therein).
The Amethyst Gem Clam (Gemma gemma) is found in muddy or sandy tidal flats or subtidally at shallow depths in salinities as low as 5 parts per thousand (Gosner 1978; Rehder 1981) (salinity in the Atlantic Ocean is typically around 35 parts per thousand).
These tiny clams live in the intertidal zone buried in a depth of two centimeters in sand or muddy sand (Narchi 1971).
The Amethyst Gem Clam (Gemma gemma) is found from Nova Scotia to Texas and is often common, although easily overlooked due to its tiny size (Morris 1973; Gosner 1978). It was introduced from the Atlantic to the Pacific coast, where it is now common in muddy situations in a number of bays (Abbott 1968; Kozloff 1993). Carlton (1992) reported that on the Pacific coast this clam was restricted to 5 bays in central California (Bodega Harbor (not Bodega Bay), Tomales Bay, Bolinas Lagoon, San Francisco Bay, and Elkhom Slough). Grosholz (2005) documented a dramatic increase in the distribution and abundance of Gemma gemma in Bodega Harbor, where it has been present since at least the 1960s. This clam was first introduced to the western U.S. in the late 1800s by means of the oyster trade (Carlton 1999, cited in Grosholz 2005).
Gemma gemma is an Atlantic species, occurring from Nova Scotia to Florida, Texas, and the Bahamas. It occurs on the west coast of the United States as well, from Puget Sound to San Diego, including the shores of San Francisco Bay, having been introduced from Chesapeake Bay (Maryland, U.S.A.) around1899. (Narchi 1971 and references therein)
The northern limit of the range of G. gemma is Labrador or Nova Scotia, but the southern limit is less certain as a result of taxonomic confusion with G. purpurea. Gemma gemma certainly occurs as far south as Cape Hatteras, where it is predominantly subtidal. In Chesapeake Bay (Maryland) and Raritan Bay (New Jersey), the species is partly subtidal and partly intertidal, while on the north side of Cape Cod and in Sagadahoc Bay (Maine), it is entirely intertidal. High summer temperatures are likely an important factor limiting this clam's distribution. (Green and Hobson 1970 and references therein)
Gemma gemma often reaches densities of 105 per square meter in relatively protected, shallow, sandy sediments along the Atlantic coast of North America (Green and Hobson 1970). See Green and Hobson (1970) for a thorough study of the population structure and dynamics of Gemma gemma around Barnstable, Massachusetts (U.S.A.).
Amethyst Gem Clam sexes are separate. The fertilized eggs develop within the female's mantle (the fleshy outer structure covering a mollusk's vital organs and usually containing glands that secrete the shell). The young hatch and leave the mother's shell to burrow into mud or sand. (Green and Hudson 1970; Rehder 1981). In California, Narchi (1971) found animals brooding juveniles in the gills in February, March, and April.
Grosholz (2005) provided evidence that the rapid increase in G. gemma in recent decades in Bodega Harbor, California, is the result of the introduction of the European Green Crab (Carcinus maenas), which is believed to have been first introduced to the western U.S. in San Francisco Bay around 1989. It became established outside of San Francisco Bay in nearby estuaries, including Bodega Harbor, by 1994 and is now common in bays and estuaries from Monterey Bay, California, to Gray’s Harbor, Washington. The positive impact of this introduced crab on the introduced Gemma gemma seems to be mediated by the crab's disproportionate negative impact on native Nutricola clams, which apparently previously suppressed Gemma gemma through competitive interactions. Grosholz presents this interaction as an example of how a new invader can transform an older invader into a serious new management problem by means of positive indirect interactions that may produce an invasional meltdown (Grosholz 2005).
Reported predators of Gemma gemma on the Atlantic coast include the horseshoe crab Limulus polyphemus and the sand shrimp Crangon septumspinosa (Green and Hobson 1970 and references therein).
- Gemma purpurea Lea, 1842 (synonym)