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:

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).


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).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Mollusca
Class:   Bivalvia
Subclass:   Heterodonta
Order:   Veneroida
Superfamily:   Veneroidea
Family:   Veneridae
Genus:   Gemma
Species:   gemma


Gemma manhatensis (Prine, 1862)
Gemma pupurea (Lea, 1842)

Potentially Misidentified Species

Nutricola sp.
This native northeast Pacific clam (as Transennella tantilla) was often confused with Gemma gemma (Narchi 1971; Carlton 1979).



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).


Phtyoplankton; Detritus


Waterfowl, fishes, crabs

Trophic Status:

Suspension Feeder



General HabitatUnstructured BottomNone
General HabitatOyster ReefNone
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Tidal RangeLow IntertidalNone
Vertical HabitatEndobenthicNone

Tolerances and Life History Parameters

Minimum Temperature (ºC)-2Based on geographical range (Bousfield 1960)
Maximum Temperature (ºC)3724 hr LC 50, experimental, 30 C acclimation (Kennedy and Mihursky 1971)
Minimum Salinity (‰)5Experimental (Chanley and Castagna 1978)
Maximum Salinity (‰)35Field data (Miller 2000)
Minimum Length (mm)1.8Minimum reprducitive size, Green and Hobson 1978
Maximum Length (mm)5Abbott 1974, Gosner 1978
Broad Temperature RangeNoneCold temperate-Subtropical
Broad Salinity RangeNoneMesohaline-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).

Regional Impacts

P090San Francisco BayEcological ImpactCompetition
Competition is suspected with Nutricola tantilla (=Transenella tantilla), rarely found together in the same samples (Carlton 1979).
P110Tomales BayEcological ImpactCompetition
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 ImpactCompetition
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-VNorthern California to Mid Channel IslandsEcological ImpactCompetition
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-VNorthern California to Mid Channel IslandsEcological ImpactFood/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).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NA-ET3 Cape Cod to Cape Hatteras 0 Native Estab
CAR-VII Cape Hatteras to Mid-East Florida 0 Native Estab
CAR-I Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida 0 Native Estab
CAR-V None 0 Native Estab
NA-ET2 Bay of Fundy to Cape Cod 0 Native Estab
NA-ET1 Gulf of St. Lawrence to Bay of Fundy 0 Native Estab
NEP-V Northern California to Mid Channel Islands 1893 Def Estab
NEP-IV Puget Sound to Northern California 1992 Def Estab
P130 Humboldt Bay 1992 Def Estab
P090 San Francisco Bay 1893 Def Estab
P080 Monterey Bay 1965 Def Estab
P110 Tomales Bay 1965 Def Estab
P112 _CDA_P112 (Bodega Bay) 1974 Def Estab
P095 _CDA_P095 (Tomales-Drakes Bay) 1918 Def Estab
P093 _CDA_P093 (San Pablo Bay) 1899 Def Estab
NA-S3 None 0 Native Estab
CAR-IV None 1952 Native Estab
CAR-III None 0 Native Estab
CAR-II None 0 Native Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
5132 Green and Hobson 1978 1966 1966-11-08 None Native 41.7167 -70.2667
5133 Commito et al. 1995 1985 1985-11-09 Assateague Island Native 37.8833 -75.3333
5134 Green and Hobson 1978 1967 1967-03-01 None Native 40.4464 -74.1786
5135 Green and Hobson 1970 1970 9999-01-01 Cape Hatteras Native 35.2244 -75.5306
5136 Green and Hobson 1978 1967 1967-03-01 Raritan Bay Native 40.4742 -74.1811
5137 Casu et al. 2005 2002 2008-07-02 Assateague Island Native 37.8833 -75.3333
5138 Casu et al. 2005 2002 2008-07-02 Lubec Native 44.8500 -66.9833
5139 Weinberg 1989; Weinberg 1985 1978 1978-01-01 Sandy Point Native 41.3192 -71.8658
5140 Abbott 1974 1974 9999-01-01 Port Joli Native 43.9000 -64.8000
5141 Barnhart et al. 1992, cited by Boyd et al. 2002 1992 1992-01-01 None Def 40.7500 -124.2083
5142 Boyd et al. 2002 2002 2000-01-01 None Def 40.7500 -124.2500
5145 Carlton 1992 1965 1965-01-01 None Def 36.8058 -121.7892
5148 Stearns 1899, cited by Carlton 1979 1899 1899-01-01 None Def 37.7000 -122.3000
5149 Nichols and Thompson 1985 1985 9999-01-01 None Def 38.0667 -122.3833
5150 Nichols and Thompson 1985 1985 9999-01-01 None Def 37.6750 -122.3803
5152 Johnson and Juskevice 1965, cited by Carlton 1979 1965 9999-01-01 None Def 38.1697 -122.9089
5153 Cohen and Carlton 1995 1918 1918-01-01 None Def 37.9183 -122.6800
5154 Standing 1975, cited by Carlton 1979 1974 9999-01-01 None Def 38.3236 -123.0467
5155 USNM 878357 1967 1967-11-02 Pine Island Native 28.5700 -82.6600
5156 USNM 1028615 1955 1955-05-01 Cape Cod Native 41.6000 -70.4000
5157 USNM 1037688 1962 1962-09-08 Cape Cod Native 42.0000 -70.7000
5158 USNM 1049103 1964 1964-06-12 None Native 30.3000 -80.5000
5159 USNM 1050213 1964 1964-06-27 None Native 35.3000 -75.0000
5160 USNM 1055634 1964 1964-07-29 None Native 39.2000 -75.2000
5224 USNM 1034996 1960 1960-07-08 None Native 41.4000 -70.9000
5225 USNM 1060206 1965 1965-05-11 None Native 35.8000 -75.7000
5227 Bradley and Cook 1958 1950 9999-01-01 Georgetown Island Native 43.7628 -69.7572
5228 Botton 1984 1979 1979-07-01 Cape May Native 39.0500 -75.1500
5238 Totten 1834 1834 9999-01-01 Cape Cod Native 42.0333 -70.4167
5239 Totten 1834, cited by Rosenberg 2012 1834 9999-01-01 Newport Harbor Native 41.4825 -71.3261
5240 Rehder 1939 1939 9999-01-01 Crab Meadow State Park Native 40.9100 -73.3000
5241 Ekdale 1974 1974 9999-01-01 Quintana Roo Native 20.0000 -87.0000
5242 Zingmark 1978 1978 9999-01-01 Hilton Head Native 32.2200 -80.7000
5245 Bousfield 1959 1959 9999-01-01 None Native 44.8500 -65.4000
5246 Pearse 1936 1934 9999-01-01 Beaufort Native 34.7772 -76.6861
5247 Prezant et al. 2002 1998 9999-01-01 South Beach, St. Catherines Island Native 31.6567 -81.1517
5248 Prezant et al. 2002b 1996 9999-01-01 Assateague Native 38.3244 -75.0908
5249 Prezant et al. 2002b 1996 9999-01-01 Assateague Native 38.1170 -75.2500
5755 Museum of Comparative Zoology 2008 1943 1943-09-21 None Native 46.6000 -63.8667
5756 Museum of Comparative Zoology 2008 1950 1950-01-01 Hawk Island Native 45.6000 -60.9825
5757 Museum of Comparative Zoology 2008 1935 1935-08-04 Plum Point Native 38.6204 -76.5136
5758 Searchers of Brazoria County 2003 2003 2003-01-01 Stedman Island Native 27.8892 -97.1125
5759 Museum of Comparative Zoology 2008 None 9999-01-01 South Thomaston Native 44.0515 -69.1278
5760 Museum of Comparative Zoology 2008 1971 1971-04-01 None Native 44.4390 -68.3700
7765 Linero Arana and Mata 2003 None 9999-01-01 Chacopata lagoon Native 10.5692 -63.9272
7766 lorida Musuem of Natural History 2012 1961 9999-01-01 Judiths Fancy, St. Croix Native 17.7444 -64.7500
7767 Florida Museum of Natural History 2012 1996 1996-01-01 Punta Checuzam, Ca. 13 km W of San Felipe Native 21.5667 -88.2167


Abbott, R. Tucker (1974) <missing title>, Van Nostrand Reinhold, New York. Pp. <missing location>

Bousfield, E. L. (1960) Canadian Atlantic Sea Shells, In: (Eds.) . , Ottawa. Pp. <missing location>

Bousfield, E. L., Leim, A. H. (1959) The fauna of Minas Basin and Minas Channel, National Museum of Canada Bulletin 166(61): <missing location>

Boyd, Milton J.; Mulligan, Tim J; Shaughnessy, Frank J. (2002) <missing title>, California Department of Fish and Game, Sacramento. Pp. 1-118

Bradley, W. H.; Cooke, Peter (1958) Living and ancient populations of the clam Gemma gemma in a Maine coast tidal flat, Fishery Bulletin 137: 305-334

Carlton, James T. (1979) History, biogeography, and ecology of the introduced marine and estuarine invertebrates of the Pacific Coast of North America., Ph.D. dissertation, University of California, Davis. Pp. 1-904

Carlton, James T. (1979) Introduced invertebrates of San Francisco Bay, In: Conomos, T. J.(Eds.) San Francisco Bay: The Urbanized Estuary. , San Francisco. Pp. 427-444

Carlton, James T. (1992) Introduced marine and estuarine mollusks of North America: An end-of-the-20th-century perspective., Journal of Shellfish Research 11(2): 489-505

Carlton, James T. (Ed.) (2007) <missing title>, University of California Press, Berkeley. Pp. <missing location>

Castagna, M.; Chanley, P. (1973) Salinity tolerance of some marine bivalves from inshore and estuarine environments in Virginia waters on the western mid-Atlantic coast., Malacologia 12(1): 47-96

Casu, Marco and 6 authors (2005) Fine-grained spatial genetic structure in the bivalve Gemma gemma from Maine and Virginia (USA), as revealed by Inter-Simple Sequence Repeat markers, Journal of Experimental Marine Biology and Ecology 325: 46-54

Chanley, Paul; Andrews, J. D. (1971) Aids for identification of bivalve larvae of Virginia, Malacologia 11(1): 45-119

Coan, Eugene V.; Valentich-Scott, Paul (2007) The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon, University of California Press, Berkeley CA. Pp. 807-859

Coan, Eugene V.; Valentich-Scott, Paul; Bernard, Frank R. (2000) <missing title>, Santa Barbara Museum of Natural history, Santa Barbara CA. Pp. <missing location>

Cohen, Andrew N. and 10 authors (2005) <missing title>, San Francisco Estuary Institute, Oakland CA. Pp. <missing location>

Cohen, Andrew N.; Carlton, James T. (1995) Nonindigenous aquatic species in a United States estuary: a case study of the biological invasions of the San Francisco Bay and Delta, U.S. Fish and Wildlife Service and National Sea Grant College Program (Connecticut Sea Grant), Washington DC, Silver Spring MD.. Pp. <missing location>

Commito, John A.; Currier, Carol A.; Kane, Laura R.; Reinsel, Kathleen A.; Ulm, Irene M. (1995) Dispersal dynamics of the bivalve Gemma gemma in a patchy environment, Ecological Monographs 65(1): 1-20

Ekdale, Allan A. (1974) Marine molluscs from shallow-water environments (0 to 60 meters) off the northeast Yucatan coast, Mexico, Bulletin of Marine Science 24(3): 638-668

2009-2013 Invertebrate Zoology Master Database.

Gosner, Kenneth L. (1978) A field guide to the Atlantic seashore., In: (Eds.) . , Boston. Pp. <missing location>

Green, Roger H.; Hobson, Katharine D. (1970) Spatial and temporal structure in a temperate intertidal community, with special emphasis on Gemma gemma (Pelecypoda: Mollusca), Ecology 51(6): 999-1011

Grosholz, Edwin D. (2005) Recent biological invasion may hasten invasion meltdown by accelerating historical introductions., Proceedings of the National Academy of Sciences of the U.S.A. 102(4): 1088-1091

2008-2021 Museum of Comparative Zoology Collections database- Malacology Collection.

Hoos, Phillip M.; Miller, A. Whitman; Ruiz, Gregory M.; Vrijenhoek, Robert C.; Geller, Jonathan B. (2010) Genetic and historical evidence disagree on likely sources of the Atlantic amethyst gem clam Gemma gemma (Totten, 1834) in California, Diversity and Distributions 16: 582-592

Hopkins, Dale R. (1986) Atlas of the distributions and abundances of common benthic species in San Francisco Bay, California, US Geological Survey Water Resources Investigations Report 86-4003: 1-16+ 25+228

Kennedy, V. S.; Mihursky, J. A. (1971) Upper temperature tolerances of some estuarine bivalves., Chesapeake Science 12(4): 193-204

Lee, Henry and 8 authors (2008) Integrated monitoring and information systems for managing aquatic invasive species in a changing climate., Conservation Biology 22(3): 575-584

Lee, Henry II; Thompson, Bruce; Lowe, Sarah (2003) Estuarine and scalar patterns of invasion in the soft-bottom benthic communities of the San Francisco estuary., Biological Invasions 5: 85-102

Linero Arana, Ildefonso; Mata, Marelis (2003) [Biological aspects of Gemma gemma (Totten 1894) (Pelecypods: Veneridae) on Chocpata Lagoon, Venezuela], Iberus 21(2): 133-139

Macdonald, Keith B. (1969) Quantitative studies of salt marsh mollusc faunas from the North American Pacific coast., Ecological Monographs 39(1): 33-60

Miller, A. Whitman; Ruiz, Gregory M.; Minton, Mark S.; Ambrose, Richard F. (2007) Differentiating successful and failed molluscan invaders in estuarine ecosystems., Marine Ecology Progress Series 332: 41-51

Miller, Alexander Whitman (2000) <missing title>, University of California at Los Angeles, Los Angeles. Pp. <missing location>

Narchi, Walter (1971) Structure and adaptation in Transennella tantilla (Gould) and Gemma gemma (Totten) (Bivalvia: Veneridae), Bulletin of Marine Science 21(4): 866-885

Neira, Carlos; Grosholz, Lisa; Levin, Lisa A.; Blake, Rachael (2006) Mechanisms generating modification of benthos following tidal flat invasion by a Spartina hybrid., Ecological Applications 16(4): 1391-1404

Nichols, Frederic H.; Thompson, Janet K. (1985b) Persistence of an introduced mudflat community in South San Francisco Bay, California, Marine Ecology Progress Series 24: 83-97.

Pearse, A. S. (1936) Estuarine animals at Beaufort, North Carolina, Journal of the Elisha Mitchell Scientific Society 52(2): 174-224

Prezant, Robert S.; Toll, Ronald B.; Rollins, Harold B.; Chapman, Eric J. (2002) Marine macroinvertebrate diversity of St. Catherines Island, Georgia., American Museum Novitates 3367: 1-31

Prezant, Robert; Counts, Clement L.; Chapman, Eric J. (2002) Mollusca of Assateague Island, Maryland and Virginia: additions to the fauna, range extensions, and gigantism., Veliger 45: 337-355

Recher, Harry F. (1966) Some aspects of the ecology of migrant shorebirds, Ecology 47(3): 393-407

1995-2015 Malacolog.

Thompson, Janet K. (1982) Population structure of Gemma gemma (Bivalvia: Veneridae) in South San Francisco Bay, with a comparison to some northeastern United States estuarine populations, Veliger 24(3): 281-290

2002-2021 Invertebrate Zoology Collections Database. <missing description>

2003-2022 Nonindigenous Aquatic Species Database. Gainesville, FL.

Wasson, Kerstin; Zabin, C. J.; Bedinger, L.; Diaz, M. C.; Pearse J. S. (2001) Biological invasions of estuaries without international shipping: the importance of intraregional transport, Biological Conservation 102: 143-153

Weinberg, J. R. (1985) Factors regulating population dynamics of the marine bivalve Gemma gemma: population dynamics and salinity, Marine Biology 86: 173-182

Zhang, Haibin; Geller, Jonathan B.; Vrijenhoek, Robert C. (2014) Genetic diversity in native and introduced populations of the amethyst gem clam Gemma gemma (Totten, 1834) from the U.S. east and west coasts, Biological Invasions 16(12): 2725-2735

Zingmark, Richard G. (Ed.) (1978) <missing title>, Belle W. Baruch Institute for Marine Biology and Coastal Research, Columbia SC. Pp. <missing location>