Invasion History

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

General Invasion History:

Anadara transversa is native to the Northwest Atlantic, from Cape Cod to Texas. It is found on shallow, sandy bottoms, in polyhaline to euhaline waters (Wass 1972; Morris 1975; Gosner 1978; Miller 2000).

North American Invasion History:

Invasion History on the West Coast:

Dead and worn shells of A. transversa were found on Atlantic oyster beds in San Francisco Bay in 1918 (Packard 1918, cited by Carlton 1979), and again at Coyote Point and Palo Alto Harbor in the bay in 1967 (Wicksten 1976, cited by Carlton 1979). Live animals or dead shells of this bivalve were probably introduced with oysters planted in San Francisco Bay, from Long Island Sound, in the late 1800s (Carlton 1979; Miller 2000). There is no evidence that a breeding population existed in San Francisco Bay.

Invasion History Elsewhere in the World:

Anadara transversa was found in Izmir Bay, Turkey, in the Aegean Sea, in 1972 and given the name A. demiri. It was assumed to be an introduction from the Red Sea, by way of the Suez Canal (Cinar et al. 2005; Albano et al. 2009). It was later recognized as the Northwest Atlantic species A. transversa (Albano et al. 2009), and spread into Greek waters by 2007 (Zenetos 2009). In the Adriatic Sea, A. transversa was first found in 2000 on the coast of Italy and was abundant by 2003 (Morello et al. 2004). It now occurs along most of the Italian Adriatic Coast, from Lecce to the Gulf of Venice (Albano et al. 2009). A specimen has been tentatively identified from the Gulf of Tunis (Antit et al. 2011). This bivalve may have been introduced to the Mediterranean from the East or Gulf Coasts of the US in ballast water or hull fouling (Albano et al. 2009).


Anadara transversa is a marine bivalve with a thick, strongly ribbed shell. The hinge line bears numerous teeth, arranged in a line on both valves. Both valves have prominent beaks which are set close to the anterior end and curved inward. The shell is heart-shaped in a side view. The shell is somewhat oblong, with straight shoulders below the beak. Both shells have about 12 radiating ribs. The shell is white, but it is covered with a brown periostracum. This bivalve does not have a siphon. This shell typically reaches 25 mm. Ark shells occasionally attach to rocks and shells with byssus threads. (Description from: Morris 1975; Gosner 1978).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Mollusca
Class:   Bivalvia
Subclass:   Pteriomorphia
Order:   Arcoida
Family:   Arcidae
Genus:   Anadara
Species:   transversa


Anadara amygdala (Phillipi, 1847)
Anadara demiri (Piani, 1881)
Anadara sulcosa (Van Hyning, 1946)
Arca transversa (Say, 1822)

Potentially Misidentified Species



Anadara transversa , commonly known as the Transverse Ark, is a suspension-feeding bivalve found in sandy and muddy subtidal bottoms, in polyhaline-euhaline salinities (Wass 1972; Morris 1975; Gosner 1978). Arks occasionally attach to rocks and shells with byssus threads. This bivalve has planktonic veliger larvae (Chanely and Andrews 1971; Miller 2000).



Trophic Status:

Suspension Feeder



General HabitatUnstructured BottomNone
General HabitatOyster ReefNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEndobenthicNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Maximum Depth (m)25None
Minimum Salinity (‰)10Experimental, gradual transfer (Castagna and Chanley 1973)
Minimum Length (mm)25Morris 1975; Gosner 1978;
Broad Temperature RangeNoneCold temperate-Warm temperate
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

Anadara transversa is not established in North America and has had no known impacts there. However, the abundance of A. transversa has increased rapidly in the Adriatic Sea. By 2004, it was a numerical dominant at some locations.

Ecological impacts-In the Adriatic Sea, the rapid increase and numerical dominance of A. transversa is suggestive of competition. The attachment of juvenile A. transversa to snails and bivalves may restrict their movement and habitat use (Morello et al. 2004).

Regional Impacts

MED-VIINoneEcological ImpactCompetition
Anadara transversa has achieved numerical dominance at some locations, suggesting successful competition (Morello et al. 2004)
MED-VIINoneEcological ImpactHabitat Change
Juvenile Anadara transversa frequently attach, by byssus threads to the native gastropod Aporrhais pespelecani, the native bivalve Chamelea gallina, and the introduced Anadara inaequivalvis, interfering with their movement. However, the shells of A. transversa also tend to be heavily infested with boring polychaetes (Polydora sp.) (Morello et al. 2004).

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
NEP-V Northern California to Mid Channel Islands 1918 Def Failed
P090 San Francisco Bay 1918 Def Failed
MED-VI None 1972 Def Estab
MED-VII None 1995 Def Estab
MED-III None 2010 Def Estab
MED-IV None 0 Def Unk

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Mrcelic, Jelic Gorana; Nerlovic, Dogan, Vedrana Alper (2023) Sustainable Management of High-Impact Non-Native Molluscs and Their Potential Commercial Importance in the Eastern Adriatic Sea, Sustainabliity 15(1134): <missing location> su151411384

Orth, Donald J. (2010) Socrates Opens a Pandora’s Box of Northern Snakehead Issues, American Fisheries Society Symposiums 89: 203-221

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

Albano, Paolo G.; Rinaldi, Emidio; Evangelisti, Francesca; Kuan, Michela; Sabelli, Bruno (2009) On the identity and origin of Anadara demiri(Bivalvia: Arcidae), Journal of the Marine Biological Association of the United Kingdom 89(6): 1289-1298

Albayrak, Serhat (2011) Alien marine bivalve species reported from Turkish seas, Cahiers de Biologie Marine 52: 107-118

Amor, Kounofi-Ben; Rifi, M.; Ghanem, R.; Draief, I.; Zouali, J.; Souissi, J. Ben (2016) Update of alien fauna and new records of Tunisian marine fauna, Mediterranean Marine Science 17(1): 124-143

Antit, M.; Gofas, S.; Salas, C.; Azzouna, A. (2011) One hundred years after Pinctada: an update on alien Mollusca in Tunisia, Mediterranean Marine Science 12(1): 53-73

Campbell, Matthew D. and 9 authors (2022) Rapid spatial expansion and population increase of invasive lionfish (Pterois spp.) observed on natural habitats in the northern Gulf of Mexico, Biological Invasions 24: 93-105

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

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

Çinar, M. E.; Noglu, M. Bilece; Özturk, B.; Katagan, T. ; Aysel, V. (2005) Alien species on the coasts of Turkey, Mediterranean Marine Science 6/2: 119-146

Çinar, Melih Ertan and 7 authors (2021) Current status (as of end of 2020) of marine alien species in Turkey, PLOS ONE 16: Published online

Crocetta, Fabio (2011) Marine alien Mollusca in the Gulf of Trieste and neighbouring areas: a critical review and state of knowledge (updated in 2011), Acta Adriatica 52(2): 247 - 260,

Crocetta, Fabio (2012) Marine alien Mollusca in Italy: a critical review and state of the knowledge, Journal of the Marine Biological Association of the United Kingdom 92(6): 1357-1365

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>

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

Lipej, L.; Mavric, B.; Orlando-Bonaca, M.; Malej, A. (2012) State of the art of the marine non-indigenous flora and fauna in Slovenia, Mediterranean Marine Science 13(2): 243-249

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

Morello, Elisabetta B.; Solustri, Cristiano ; Froglia, Carlo (2004) The alien bivalve Anadara demiri (Arcidae): a new invader of the Adriatic Sea, Italy, Journal of the Marine Biological Association of the United Kingdom 84: 1057-1064

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

Pecarevic, M.; Mikus, J.; Cetinic, A. Bratos; Dulcic, J.; Calic, M. (2013) Introduced marine species in Croatian waters (Eastern Adriatic Sea), Mediterranean Marine Science 14(1): 224-237

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

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 and 6 authors (2009) Aquatic alien species in Greece (2009): tracking sources, patterns and effects on the ecosystem, Journal of Biological Research-Thessaloniki 12: 135-172