Invasion History

First Non-native North American Tidal Record: 1927
First Non-native West Coast Tidal Record: 1927
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Petricolaria pholadiformis is native to the Northwest Atlantic from Chaleur Bay (Quebec-New Brunswick) to Florida and Texas (Bousfield 1960; Abbott 1974). Reports from Uruguay and west Africa are based on confusion with other species (Coan 1997). It burrows in peat, clay, and mud from the mid-intertidal to ~30 m deep, but usually in shallow water at 20-35 PSU (Wass 1972; Castagna and Chanley 1973; Morris 1975; Coan 1997). This clam was introduced to San Francisco Bay in 1927, and was found later in Newport Bay, California (CA); Willapa Bay, Washington (WA); and Boundary Bay, British Columbia (BC) (Carlton 1979; Coan 1997; Gillespie 2007). It has also been introduced to Europe and was introduced to British waters with transplants of Eastern Oysters (Crassostrea virginica) by 1890, and has spread to Germany, Denmark, Norway, and Sweden (Hopkins 2001; Swedish Environmental Protection Agency 2006). In 1994, it was collected in Greek waters, in the Aegean Sea, and is now established there (Zenetos et al. 2009).

North American Invasion History:

Invasion History on the West Coast:

Petricolaria pholadiformis was first found in West Coast waters near Redwood City, California (CA) on South San Francisco Bay in 1927. Populations were reported to be rare and patchy (James Carlton, personal communication, cited by Coan 1997). Dead shells were collected in Lake Merritt, Oakland in 1935 (Carlton 1979). A small population of this bivalve was found ‘in one small area’ in upper Newport Bay, CA in 1972 (Carlton 1979). This population appeared to be extinct, according to Coan (1997). Its present establishment in San Francisco Bay is uncertain (Coan and Valentich-Scott, in Carlton 2007). Petricolaria pholadiformis was found in Willapa Bay, Washington (WA) in 1943 (Eyerdam 1943, cited by Carlton 1979). The False Angelwing is still established and abundant here (Coan 1997; Cohen et al. 2001). A small, established population also occurs in Boundary Bay, British Columbia (Gillespie et al. 1999; Gillespie et al. 2007). Plantings of Eastern Oysters (Crassostrea virginica) is the most likely vector for the introduction of P. pholadiformis to the West Coast, although ballast water is a possible mode of introduction to San Francisco Bay, given its late (1927) appearance there (Carlton 1979; Cohen and Carlton 1995).

Invasion History Elsewhere in the World:

Petricolaria pholadiformis was first collected in Europe in 1890, in the Crouch River Estuary, England, where it was introduced with transplanted Eastern Oysters (Eno et al. 1997). It is present along the southern and eastern coasts of England, with isolated records from Wales and Cornwall (MarLin 2012). It spread rapidly through the North Sea, reaching Belgium by 1899 (Kerckhof et al. 2007), and the west coast of Denmark by 1905 (Jensen and Knudsen 2005). On the east coast of Denmark, the False Angelwing was found in the Kattegat by 1931 and the Belt Sea by 1943 (Jensen and Knudsen 2005). To the north, it colonized the Skagerrak coast of Norway and Sweden by 1955 (Hopkins 2001; Swedish Environmental Protection Agency 2006). A surprising disjunct introduction was discovered in 1994, when P. pholadiformis was found in Evoikos Gulf, Greece, on the Aegean Sea (Zenetos et al. 2003), and later (2007) in the Saronikos Gulf, where it appears to be established. Greek populations are likely to be ballast water introductions (Zenetos et al. 2009).


Petricolaria pholadiformis is a bivalve with an elongated, cylindrical/elliptical shaped shell. The umbo is near the anterior end of the shell, and the posterior is narrowed, with the valves slightly gaping. The shell is chalky-white, and rather fragile, with numerous ribs radiating from the umbo. The 10 anterior-most ribs are larger, with prominent scales. The shell is also marked by concentric growth lines. The umbo is elevated, with a well-defined lunule. The cardinal hinge teeth are long and pointed, with three in the left valve, and two in the right valve. The ligament is external and located just posterior to the beaks. The siphons are separate from each other, almost to their bases, and are large, tubular, and translucent-gray. Adult shells reach ~55 mm. This bivalve bores into peat, mud, stiff clay, or soft rock such as limestone. The larvae are planktotrophic and settle at about 165 to 185 µm (Chanley and Andrews 1971). Description from: Bousfield 1960; Abbott 1974; Morris 1975; Coan et al. 2000.


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Mollusca
Class:   Bivalvia
Subclass:   Heterodonta
Order:   Veneroida
Superfamily:   Veneroidea
Family:   Petricolidae
Genus:   Petricolaria
Species:   pholadiformis


Gastrionella tumida (Verrill, 1872)
Petricola carolinensis (Conrad, 1863)
Petricola dactylus (Sowerby, 1823)
Petricola flagellata (Say, 1834)
Petricola fornicata (Say, 1822)
Petricola pholadiformis lata (Dall, 1925)
Petricola rogersi (McGavock, 1944)

Potentially Misidentified Species

Petricola calforniensis
Shell less elongated, native to southern California, may reach central California in warm years (Coan and Valentic-Scott, in Carlton 2007)



Petricolaria pholadiformis is a bivalve with separate sexes. Males and females release eggs and sperm freely into the water column. Females in Long Island Sound became gravid at sizes larger than 27 mm. An average female produces ~325,000 eggs (Brousseau 1981). Fertilized eggs develop into trochophore larvae, and then planktonic veligers, which settle at about 165 to 185 µm (Chanley and Andrews 1971).

Petricolaria pholadiformis burrows in mud, peat, stiff clay, or soft rock. The adults live completely buried in the substrate, feeding and breathing through their two siphons. As they grow, they burrow deeper into the sediment (Abbott 1974; Gosner 1978). It is common at salinities of 20-35 PSU, but can tolerate salinities as low as 7.5-10 PSU for brief periods (Wass 1972; Castagna and Chanley 1973). This bivalve often occurs where salinity is somewhat reduced (Zenetos et al. 2009). Adults are filter-feeders, ingesting phytoplankton and other particles through their long siphons. Their burrowing habits reduce the risk of predation, but when exposed by a predator, or by erosion, their fragile shells make them vulnerable.


Phytoplankton, detritus

Trophic Status:

Suspension Feeder



General HabitatUnstructured BottomNone
General HabitatOyster ReefNone
General HabitatSalt-brackish marshNone
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 Salinity (‰)1090% survival in 3 experiments of 52-92 days (Castagna and Chanley 1973)
Maximum Salinity (‰)35Based on field distribution
Minimum Length (mm)11.8Brousseau 1979
Maximum Length (mm)55Bousfield 1960; Abbott 1974; Morris 1975
Broad Temperature RangeNoneCold temperate-Subtropical
Broad Salinity RangeNoneMesohaline-Euhaline

General Impacts

Petricolaria pholadiformis appears to have few or no impacts on the West Coast (Cohen and Carlton 1995; Coan 1997) and in most European waters. At one time, it was reported to have replaced the native Barnea candida (White Piddock) in Belgian waters (ICES 1972, cited by Eno et al. 1997; Swedish Environmental Protection Agency 2006), but B. candida is now abundant in Belgium, while P. pholadiformis is rare. Petricolaria pholadiformis is also rare in Danish, German, Norwegian, and Swedish waters (Jensen 2010).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NA-S3 None 0 Native Estab
NA-ET2 Bay of Fundy to Cape Cod 0 Native Estab
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
NEP-V Northern California to Mid Channel Islands 1927 Def Unk
NEP-VI Pt. Conception to Southern Baja California 1972 Def Extinct
NEP-IV Puget Sound to Northern California 1943 Def Estab
NEA-II None 1890 Def Estab
B-I None 1955 Def Estab
B-II None 1931 Def Estab
MED-VI None 1985 Def Estab
P090 San Francisco Bay 1927 Def Unk
P270 Willapa Bay 1943 Def Estab
P040 Newport Bay 1972 Def Extinct
NEA-III None 0 Def Unk
B-III None 1943 Def Estab
NEP-III Alaskan panhandle to N. of Puget Sound 1995 Def Estab
CAR-V None 0 Prb Estab
AR-V None 1987 Def Estab
P050 San Pedro Bay 2017 Def Estab
NEP-VI Pt. Conception to Southern Baja California 2021 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


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

Boets, Pieter; Brosens, Dimitri; Lock, Koen; Adriaens, Tim; Aelterman, Bart; Mertens, Joost; Goethals, Peter L.M. (2016) Alien macroinvertebrates in Flanders (Belgium), Aquatic Invasions 11: In press

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

Breton, Gerard; Girard, Annie; Lagardere, Jean-Paul (1995) Especes animales benthiques des bassins du port du Havre (Normandie, France) rares, peu connues ou nouvelles pour la region., Bulletin Trimestrial de la Societe geologique de Normandie 82(2): 7-28

Brousseau, Diane J. (1981) Spawning cycle and fecundity in a populaiton of Petricolaria pholadiformis (Pelecypoda: Petricolidae) from Milford, Connecticut, Veliger 24(1): 56-61

Buschbaum, Christian; Lackschewitz, Dagmar; Reise, Karsten (2012) Nonnative macrobenthos in the Wadden Sea ecosystem, Journal of Ocean Management 68: 89-101

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. (Ed.) (2007) The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon Fourth Edition, Completely Revised and Expanded, 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

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

Coan, Eugene V. (1997) Recent species of the genus Petricola in the Eastern Pacific, Veliger 40(4): 298-340

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) Bivalve Seashells of Western North Ameira, Santa Barbara Museum of Natural history, Santa Barbara CA. Pp. <missing location>

Cohen, Andrew N. and 22 authors (2001) <missing title>, Washington State Department of Natural Resources, Olympia. 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>

Eno, N. Clare; Clark, Robin A.; Sanderson, William G. (1997) <missing title>, Joint Nature Conservation Committee, Peterborough. Pp. <missing location>

Gargan, Laura M.; Brooks , Paul R; ; Vye, Siobhan R.; . Joseph E. Ironside . Jenkins, Stuart R.; Crowe, Tasman P.;. Carlsson,Jens (2021) The use of environmental DNA metabarcoding and quantitative PCR for molecular detection of marine invasive non-native species associated with artificialstructures, Biological Invasions Published online: <missing location>

Gillespie, G. E.; Parker, M.; Merrilees, W. (1999) Distribution, abundance, and fisheries potential of the exotic varnish clam Nuttallia obscurata in British Columbia., Canadian Stock Assessment Directorate Research Document 99/3: 1-40

Gillespie, Graham E. (2007) Distribution of non-indigenous intertidal species on the Pacific Coast of Canada, Nippon Suisan Gakkaishi 73(6): 1133-1137

Gillespie, Graham E.; Phillips, Antan C.; Paltzat, Debbie L.; Therriault, Tom W. 2007 Distribution of nonindigenous intertidal species on the Pacific Coast of Canada. <missing URL>

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

Harvard Museum of Comparative Zoology 2008-2021 Museum of Comparative Zoology Collections database- Malacology Collection. <missing URL>

Hopkins, Christopher C. E. 2001 Actual and potential effects of introduced marine organisms in Norwegian waters, including Svalbard.. <missing URL>

Jensen, Kathe R. 2010 Invasive Alien Species Fact Sheet: <i>Petricola pholadiformis</i> From: Identification key to marine invasive species in Nordic waters. <missing URL>

Jensen, Kathe R.; Knudsen, Jorgen (2005) A summary of alien marine benthic invertebrates in Danish waters., Oceanological and Hydrobiological Studies 34 (suppl. 1): 137-161

Kerckhof, Francis; Haelters, Jan; Gollasch, Stephan G. (2007) Alien species in the marine and brackish ecosystem: the situation in Belgian waters., Aquatic Invasions 2(3): 243-257

MarLin- Marine Life Information Network 2006-2024 MarLin- Marine Life Information Network. <missing URL>

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

Morris, Percy A. (1975) A field guide to shells of the Atlantic, Houghton-Mifflin, Boston. Pp. <missing location>

Nehring, Stefan (2006) Four arguments why so many alien species settle into estuaries, with special reference to German River Elbe., Helgoland Marine Research 60: 127-134

Nikolaev, I. N. (1951) [On new additions to the fauna and flora of the North sea and Baltic from distant regions], Zoologicheskii Zhurnal 30(8): 556-561

Norris, James N. (2010) Marine Algae of the northern Gulf of California: Chlorophyta and Phaeophyceae, Smithsonian Contributions to Botany 94: 1276

Reise, K.; Gollasch, S.; Wolff, W.J. (1999) Introduced marine species of the North Sea coasts., Helgoländer Meeresuntersuchungen 52: 219-234

Rosenberg, Gary 1995-2023 Malacolog 4.1.

Ruiz, Gregory M.; Geller, Jonathan (2018) Spatial and temporal analysis of marine invasions in California, Part II: Humboldt Bay, Marina del Re, Port Hueneme, and San Francisco Bay, Smithsonian Environmental Research Center & Moss Landing Laboratories, Edgewater MD, Moss Landing CA. Pp. <missing location>

Ruiz, Gregory; Geller, Jonathan (2021) Spatial and temporal analysis of marine invasions: supplemental studies to evaluate detection through quantitative and molecular methodologies, Marine Invasive Species Program, California Department of Fish and Wildlife, Sacramento CA. Pp. 153 ppl.

Swedish Environmental Protection Agency 2006-2012 [Alien species in the Swedish Seas]. <missing URL>

Thomsen, Mads S. and 6 authors (2008) Annual changes in abundance of non-indigenous marine benthos on a very large spatial scale., Aquatic Invasions 3(2): 133-140

U.S. National Museum of Natural History 2002-2021 Invertebrate Zoology Collections Database.

Wass, Melvin L. (1972) A checklist of the biota of lower Chesapeake Bay, Special Scientific Report, Virginia Institute of Marine Science 65: 1-290

Wolff, W. J. (1973) The estuary as a habitat: An analysis of data on the soft-bottom macrofauna of the estuarine area of the rivers Rhine, Meuse, and Scheldt, Zoologische Verhandelingen 126: 4-242

Wolff, W. J. (2005) Non-indigenous marine and estuarine species in the Netherlands., Zoologische Verhandelingen 79(1): 1-116

Zenetos, A.; Koutsoubas, D.; Vardala-Theodorou, E. (2005) Origin and vectors of introduction of exotic mollusks in Greek waters., Belgian Journal of Zoology 135(2): 279-286

Zenetos, Argyro; Ovalis, Panayotis; Vardala-Theodorou, Evi (2009) The American piddock Petricola pholadiformis Lamarck, 1818 spreading in the Mediterranean Sea, Aquatic Invasions 4(2): 385-387