Gemma gemma

Invasion History

First Non-native North American Tidal Record: 1893
First Non-native West Coast Tidal Record: 1893
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

Gemma gemma is native to the northwest Atlantic from southern Labrador to Venezuela, including the Bahamas, Cuba, Puerto Rico, and the Virgin Islands (Bousfield 1960; Abbott 1974; Linero Arana and Mata 2003; Florida Museum of Natural History 2012; Rosenberg 2012). Abbott (1974) attributes occurrences on the West Indian Islands to dispersal by migratory birds. Gemma gemma was first found outside its native range in the digestive system of a wild duck sold at a San Francisco market in 1893, and was found to be established in San Francisco Bay (Carlton 1979). It was very likely introduced with plantings of Eastern Oysters (Crassostrea virginica). Reports from Puget Sound and British Columbia resulted from confusion with a native clam Transenella tantilla. Its current range is from Elkhorn Slough, California (CA) to Humboldt Bay, CA (Carlton 1992, Boyd et al. 2002).

North American Invasion History:

Invasion History on the West Coast:

As noted above, Gemma gemma was first found on the West Coast, in the crop of a wild duck, very likely shot in a marsh around San Francisco Bay (Carlton 1979). It was found to be abundant on the ‘East Bay shore’ (Stearns 1899, cited by Carlton 1979). In San Francisco, it is found in Suisun and San Pablo Bays, but is especially abundant on the mudflats of the South Bay, where abundances have exceeded 400,000 m^-3 (Nichols and Thompson 1985; Cohen and Carlton 1995). In their 2004 survey, Cohen et al. (2005) collected it at Coyote Point Marina on the South Bay (Cohen et al. 2005).

Outside San Francisco Bay it was collected in Bolinas Lagoon in 1918, Tomales Bay and Elkhorn Slough in 1965 (Johnson and Juskevice 1965, cited by Carlton 1979; MacDonald 1969) and Bodega Harbor in 1974 (Standing 1975, cited by Carlton 1979). It was initially reported as rare in Bodega Harbor, but became very abundant after 1995, apparently because larger, competing native clams (Nutricola sp.) were preyed upon by the newly introduced Green Crab (Carcinus maenas), as shown in feeding experiments (Grosholz 1995). The range of G. gemma in Bodega Harbor has increased since 1995 from three to all five of a set of long-term sampling stations (Grosholz 2005). This clam was found in Humboldt Bay by 1992, and was abundant and widespread by 2002 (Barnhart et al. 1992, cited by Boyd et al. 2002). Genetic analysis indicates that multiple introductions of G. gemma have occurred in San Francisco Bay, with sorting of genotypes over time. The introductions of this clam to Tomales Bay and Bodega Harbor have probably been separate and independent of its introduction to San Francisco Bay (Hoos et al. 2010).

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Description

Gemma gemma is a small clam, reaching 3-5 mm in length, with a rounded triangular shape which is moderately inflated. Its shell is rather thin and is polished on the exterior with numerous fine concentric furrows. The color is whitish to tan with a purplish wash over the beak and posterior areas. The pallial sinus is variable, but often about the length of the posterior muscle scar, and points upward towards the beak. The inner margin of the shell has fine crenulations, which are visible with a hand lens (Abbott 1974; Gosner 1978; Coan et al. 2000; Coan and Valentich-Scott, in Carlton 2007).

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Taxonomy

Ecology

General:

Gemma gemma is a small burrowing clam (< 5 mm) which inhabits muddy-sandy tidal flats. Sexes are probably separate (Narchi 1971). Eggs and larvae are brooded in the female's gills and are released as miniature non-pelagic clams at 340-390 mm (Chanley and Andrews 1971). Up to 100 eggs are produced (Green and Hobson 1970). In Maine, juveniles were released mostly in July-August, but a few were released as late as February (Bradley and Cooke 1958), while in Venezuela, most breeding took place from September to March (Linero-Arana and Mata 2003). In San Francisco, females were found brooding in March-December, and many produced more than one brood per year (Thompson 1982). The young are dispersed with bottom waves and currents, together with sand and mud particles (termed 'bedload transport') (Thompson 1982). Juveniles can produce byssal threads to anchor themselves to surfaces or sand grains, but this ability is lost in the adults (Narchi 1971).

Gemma gemma inhabits muddy-to-sandy sediments in the intertidal and shallow subtidal zone (Bradley and Cooke 1958; Green and Hobson 1970). They tolerate salinities as low as 5 PSU as adults (with acclimation) and reproduce at salinities as low as 10 PSU (Castagna and Chanley 1973). However, population decreases in Little Narragansett Bay (Rhode Island-Connecticut), were associated with low spring salinities (March-June averages < 30 PSU, Weinberg 1985). This clam is tolerant of a wide range of temperatures, surviving near zero degrees in ice-covered regions at the northern end of its range (Bradley and Cooke 1958). In short-term experiments, G. gemma had an LC50 of 35°C (Kennedy and Mihursky 1971), while populations in a Venezuelan lagoon persisted at summer temperatures of 30°C, but were much more abundant in the cooler months (Linero Arana and Mata 2003). Gemma gemma is a filter-feeder on phytoplankton and other suspended particles. It is normally partially buried in the substrate, with the posterior part of the shell and siphons emerging (Bradley and Cooke 1958; Narchi 1971).

Food:

Phtyoplankton; Detritus

Consumers:

Waterfowl, fishes, crabs

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Unstructured Bottom	None
General Habitat	Oyster Reef	None
Salinity Range	Mesohaline	5-18 PSU
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Euhaline	30-40 PSU
Tidal Range	Subtidal	None
Tidal Range	Low Intertidal	None
Vertical Habitat	Endobenthic	None

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	-2	Based on geographical range (Bousfield 1960)
Maximum Temperature (ºC)	37	24 hr LC 50, experimental, 30 C acclimation (Kennedy and Mihursky 1971)
Minimum Salinity (‰)	5	Experimental (Chanley and Castagna 1978)
Maximum Salinity (‰)	35	Field data (Miller 2000)
Minimum Length (mm)	1.8	Minimum reprducitive size, Green and Hobson 1978
Maximum Length (mm)	5	Abbott 1974, Gosner 1978
Broad Temperature Range	None	Cold temperate-Subtropical
Broad Salinity Range	None	Mesohaline-Euhaline

General Impacts

Gemma gemma is a potential competitor with other small bivalves and with the juveniles of large bivalves. In Maine, native Gemma populations were studied as possible competitors to the Soft-Shell Clam (Mya arenaria), but field data and sediment cores indicated that Gemma was adversely affected by M. arenaria (Bradley and Cooke 1958).

On the West Coast, G. gemma appears to compete with the small native clam Nutricola spp. (=Transennella tantilla) (Purple Dwarf-Venus) on the Pacific coast. Narchi (1971) and Carlton (1979) noted that G. gemma and Nutricola rarely co-occurred in San Francisco and Tomales Bays, although both could be very abundant. In Bodega Harbor, Nutricola was the superior competitor in experiments, but predation by the invasive Green Crab (Carcinus maenas) gave the smaller, faster-reproducing G. gemma an advantage over the somewhat larger (6-12 mm) native clam (Grosholz 2005).

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Regional Impacts

P090	San Francisco Bay		Ecological Impact	Competition
Competition is suspected with Nutricola tantilla (=Transenella tantilla), rarely found together in the same samples (Carlton 1979).
P110	Tomales Bay		Ecological Impact	Competition
Competition is suspected with Nutricola tantilla (=Transennella tantilla), rarely found together in the same samples (Narchi 1971; Carlton 1979).
P112	_CDA_P112 (Bodega Bay)		Ecological Impact	Competition
In Bodega Harbor, G. gemma was rare, until the arrival of the invasive Green Crab (Carcinus maenas), which preferred the larger Nutricola spp. In experiments, Gemma grew slowly in the presence of abundant Nutricola, but grew and reproduced much faster when Nutricola was rare (Grosholz 2005).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Competition
In San Francisco and Tomales Bays, competition was suspected with Nutricola tantilla (=Transennella tantilla). The two species were rarely found together in the same samples (Narchi 1971; Carlton 1979). In Bodega Harbor, G. gemma was rare, until the arrival of the invasive Green Crab (Carcinus maenas), which preferred the larger Nutricola spp. In experiments, Gemma grew slowly in the presence of abundant Nutricola, but grew and reproduced much faster when Nutricola was rare (Grosholz 2005).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Food/Prey
Gemma gemma comprised 55% of food items of the shorebirds Avocet (Recurvirostra americana and Red Knot (Calidris canutus) surveyed in Palo Alto, San Francisco Bay (Recher 1963).
CA	California		Ecological Impact	Competition
In San Francisco and Tomales Bays, competition was suspected with Nutricola tantilla (=Transennella tantilla). The two species were rarely found together in the same samples (Narchi 1971; Carlton 1979). In Bodega Harbor, G. gemma was rare, until the arrival of the invasive Green Crab (Carcinus maenas), which preferred the larger Nutricola spp. In experiments, Gemma grew slowly in the presence of abundant Nutricola, but grew and reproduced much faster when Nutricola was rare (Grosholz 2005)., Competition is suspected with Nutricola tantilla (=Transenella tantilla), rarely found together in the same samples (Carlton 1979)., Competition is suspected with Nutricola tantilla (=Transennella tantilla), rarely found together in the same samples (Narchi 1971; Carlton 1979)., In Bodega Harbor, G. gemma was rare, until the arrival of the invasive Green Crab (Carcinus maenas), which preferred the larger Nutricola spp. In experiments, Gemma grew slowly in the presence of abundant Nutricola, but grew and reproduced much faster when Nutricola was rare (Grosholz 2005).
CA	California		Ecological Impact	Food/Prey
Gemma gemma comprised 55% of food items of the shorebirds Avocet (Recurvirostra americana and Red Knot (Calidris canutus) surveyed in Palo Alto, San Francisco Bay (Recher 1963).

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