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   Gemma gemma (mollusc)   
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    Taxonomic name: Gemma gemma (Totten,1834)
    Synonyms: Cyrena purpurea (Lea, 1842), Gemma fretensis (Rehder, 1939), Gemma totteni (Simpson, 1860), Parastarte concentica (Dall, 1889), Totteniana gemma (Totten, 1834), Venus gemma (Totten, 1834), Venus manhattensis (Jay, 1852)
    Common names: Amethyst gem clam, gem clam
    Organism type: mollusc
    Gemma gemma, commonly known as the gem clam or amethyst gem clam, is a small benthic organism found in marine, brackish and freshwater environments along the Atlantic coast. The gem clam has been introduced to the California and Washington coasts of the United States. It is not competively aggressive against native populations and has minimal impact, but is oppurtunistic. An extraneous factor, such as increased predation on native fauna, will allow it to competively increase its population.
    The amethyst gem clam, Gemma gemma is a small clam whose size ranges from 350 µm as a juvenile to 5mm as an adult (Selmer, 1956). The shell of the clam is white to pale reddish-purple to dark reddish-purple with a glossy, smooth surface and fine concentric growth rings (Wilber et al. undated).
    Similar Species
    Transennella tantilla

    Occurs in:
    coastland, estuarine habitats, marine habitats
    Habitat description
    G. gemma habitat ranges from estuaries and bays to intertidal and subtidal coastal mudflats, composed of fine-grained silt-sand to muddy-sand sedimentation. The gem clam can survive in a range of salinities from 13- 28 practical salinity units (psu) (Wilber et al. undated). The gem clam is part of benthic communities which are characterized by suspension and deposit feeders (LoCicero et al. 2006). The gem clam dispersion is highly dependent on sediment bedload transport, a passive transport mechanism which can result from many different causes. Another dispersal mechanism can result from high-density pressures leading to passive migration from tidal cycles that disperse juveniles to local low-density areas (Commito et al. 1995).
    General impacts
    The gem clam population in its introduced range has gone from a benign introduction to a dramatic increase in recent years compared to native clam populations. The native clams Nutricola tantilla and Nutricola confusa once had such high population numbers that the gem clam did not pose any serious competitive threat to their populations (Grosholz, 2005). However with the introduction of the European green crab, Carcinus maenas, a highly invasive species, the predation on the native clams has reduced the populations; allowing the gem-clam, an oppurtunistic competitor, to increase in number displacing the native clams (Grosholz, 2005). Since native populations have been reduced the primary mechanism for controlling phytoplankton biomass is through introduced clams, like the gem clam, redirecting the energy flow in these habitats away from native populations into introduced ones (Cohen & Carlton, 1995).

    The gem clam has also shown under a variety of labaratory conditions to increase the larval settlement of another species of clam, Mercenaria mercenaria (Ahn, 1993). Preferential settlement of M. mercenaria increased with increasing density of G. gemma in the sediment, suggesting that the gem clam alters the environment in some way that benefits larval settlement of other species (Ahn, 1993).

    Gemma gemma is not commercially harvested as a food source due to its small size at maturity and is therefore economically unimportant (Wilber et al. undated).
    Geographical range
    Native range: Labrador, Canada, south along the western coast of the Atlantic, on both coasts of Florida, and along the coast of the Gulf of Mexico (Academy of Natural Sciences, 2006).
    Known introduced range: Pacific coast of North America from the Puget Sound in Washington state to San Diego, California (Narchi, 1971). The distribution in California is patchy and localized to a few estuaries and bays (Carlton, 1992).
    Introduction pathways to new locations
    Aquaculture: Accidentally introduced along with Atlantic oysters (Boyd et al. 2002) from the Chesapeake Bay in 1899 (Narchi, 1971).
    Live food trade: First Pacific coast report was in 1893, with the introduction occuring from the crop of a duck that was brought to San Fransico Bay (Boyd et al. 2002).

    Local dispersal methods
    Transportation of habitat material (local): Both the juvenile and adult stages of Gemma gemma are sedentary and therefore rely on passive transport mechanisms. These include tidal cycles, sediment bedload transport, storm deposition, and other local disturbances via human influence or though means of other local fauna (Commito et al. 1995).
    Management information
    Management of the gem clam is not a high priority since it does not negatively affect native clam populations unless native clams have been placed under predation selection pressure, in which case the population of the gem-clam can increase to highly abundant levels (Nichols, 1985).

    Preventive measures: Preventive measures include a focused approach on estuarine habitats in California, and not on entire coastal sections of the state. This recommendation comes from a study that cataloged the comparative numbers of exotic species in open coast habitats to estuarine habitats and found that the greatest exotic species diversity and minimal native species diveristy was found in estaurine sites (Wasson, 2005). Some ways for prevention of further introduction is minimizing human impacts in estuarine ecosystems by reducing the number and frequency of artificial land formations, like jetties, docks, gravel bars, etc. (Wasson, 2005). Other preventive options include stronger regualtions on mariculture operations like oyster farming, and the introduction of new species that could potentially transmit other invasive species. Antifoulling and ballast water regulations need to be strictly enforced (Wasson, 2005).

    Cultural: Increased education and research on the biology and habitat of the gem-clam and its effects in marine and freshwater ecosystems is needed, to better understand the dynamics of these ecosystems in order to manage them in ways that is beneficial to the environment and native populations (Wasson, 2005).

    Integrated management: A possible effective way of stabilizing gem clam populations so that they do not overwhlem native populations is through control of introduced invasive predators like the European green crab. If the green crab population was kept low enough to ward off some predation on the native species, then native populations may return to normal status (Grosholz, 2005).

    The gem clam is a benthic organism that feeds on diatoms, detritus and phytoplankton filtered from the water through the use of siphons (Driscoll, 2006).
    The amethyst gem clam is dioecious. The male clam releases its gametes into the environment during two different points in the year, from April-May and from October-November (Wilber et al. undated). The female takes up the gametes through its incurrent siphon for internal fertilization. The female clam can hold up to three hundred eggs in its mantle. The female retains juveniles in its mantle until mature enough for release. The female has one to two broods per year, with the first brood released near the end of the summer. If the female has a second brood in the fall, she will carry the eggs through the winter for release in the spring (Driscoll, 2006). The juveniles can sexually mature within 4-5 months (Wilber et al. undated).
    Lifecycle stages
    The gem clam does not have a planktonic or larval stage that is free-floating. Instead miniature adults are retained in the mantle of the mother clam until they are mature enough to survive independently in the environment. Both the juvenile stage and adult stage are sedentary, unless transported by passive mechanisms, like tides, water currents, or natural disturbances, erosion, or sediment deposition. Once juveniles are released they form a byssus thread that helps to anchor them to the coastal floor and assists them in burrowing into fine-grained sands and sediments (Narchi, 1971). The typical lifespan is two years, but some reports have indicated up to three years (Wilber et al. undated).
    Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
    Last Modified: Monday, 14 May 2007

ISSG Landcare Research NBII IUCN University of Auckland