Macoma petalum (Valenciennes, 1821)
Macoma petalum was formerly treated as a synonym of M. bathica by many authors, and known as the 'Baltic Clam'. It is now recognized as a member of a circumboreal species complex of estuarine clams (Meehan and Carlton 1988; Vainola 2003; Nikula et al. 2007). Macoma petalum is an oval shaped, laterally compressed clam with small umbones. A strong brown ligament is located behind the umbones, linking the two valves. The hinge lacks lateral teeth. The pallial sinus is larger in one valve than the other. The shell is usually dingy white, with a chalky texture, but sometimes has a rosy flush. Adults range from about 10 to 38 mm (Abbott 1974, as M. balthica; Gosner 1978). Definite identification of M. petalum from Atlantic or North Pacific M. balthica requires molecular methods (Meehan and Carlton 1988; Meehan et al. 1989; Vainola 2003; Nikula et al. 2007). However, some statistical differences have been found in comparing American East coast populations (M. petalum) and European (M. balthica) populations. Macoma petalum shells have a higher frequency of red or yellow in the interior of their shells than European M. balthica, and they have fewer white shells, a lower weight-to length ratio, and a higher siphon weight for their size (Beukema and Meehan 1985; Kamermans et al. 1999).
NE Pacific, Bodega Bay-San Diego (Abbott 1974; Coan and Valentich-Scott, in Carlton 2007)
'Macoma balthica' was formerly regarded as a circumpolar species, but now appears to be part of a complex of at least two sibling species. Temperate Northwestern Atlantic populations, based on enzyme allele frequencies, are genetically distinct from East Atlantic and East Pacifc populations (Meehan et al. 1988; Meehan et al. 1989). Meehan has applied the name M. petalum (Vaenciennes 1821) to Northwest Atlantic populations. Populations in San Francisco Bay are M. petalum, and distinct from populations in Alsea Bay and Coos Bay, Oregon (OR), which resemble European M. balthica (Meehan et al. 1988; Meehan et al. 1989).
NE Pacific, silty bays (Coan and Valentich-Scott, in Carlton 2007)
NE Pacific, Los Angeles-Alaska (Abbott 1974; Coan and Valentich-Scott, in Carlton 2007)
NE Pacific, Mexico-Alaska (Abbott 1974; Coan and Valentich-Scott, in Carlton 2007)
NE Pacific, Mexico-British Columbia (Abbott 1974; Coan and Valentich-Scott, in Carlton 2007)