The shells are smooth, relatively flat, oval or somewhat trigonal in shape, and usually less than 20–30 mm long. The shell color is polymorphic, varying between individuals and between localities. Often most specimens are white, sometimes most are pink, and also yellow and orange shells may occur. Color is best visible in worn-out dead shells and inside the shell. Concentric growth rings indicating the age of the specimen are often clearly visible.
Ecology and habitat
Macoma is an infaunal bivalve, living buried in the mud or silt, and extending its two narrow siphons to the bottom surface. Through the siphons, it feeds on organic matter on the sediment surface or in the water.
Macoma is a euryhaline species, capable of living in a wide range of water salinity, down to 3-4 permille, i.e. 10% of ocean salinity. It usually lives in the intertidal or shallow subtidal. In the brackish Baltic Sea it lives submerged down to depths over 100 m.
Macoma balthica lives in the northern parts of both the Atlantic and Pacific oceans, and also extends to the Subarctic both in North America and in Europe. The European distribution ranges from southern France north to the White Sea and Pechora Sea, and also includes the inner brackish parts of the Baltic Sea. In eastern North America, M. balthica is distributed from the Gulf of St. Lawrence Bay north to Hudson Bay, in the Pacific from Washington to the Beaufort Sea in Alaska, and also on the Russian coast. Along the U.S. Atlantic and Pacific coasts (Oregon to south), M. balthica is replaced by a similar species Macoma petalum.
- Gofas, S. (2010). Macoma balthica (Linnaeus, 1758). In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. World Register of Marine Species (accessed 2010-11-27)
- Baltic macoma Alaska Department of Fish and Game, species fact sheet.
- Strelkov P, Nikula R, Vainola R (2007) Macoma balthica in the White and Barents Seas: properties of a widespread marine hybrid swarm (Mollusca: Bivalvia). - Molecular Ecology, 16, 4110-4127, doi:10.1111/j.1365-294X.2007.03463.x