Growth patterns within the shell of the ocean quahog, Arctica islandica : A review and recent observations

A number of studies reported within the literature over the past decade have suggested that certain structural patterns within the entirely argonitic shell of the ocean quahog, Arctica islandica , may reflect annual cycles of growth. Recent analyses of radially-sectioned shells of known-age specimens of A. islandica : 1) spawned under laboratory conditions in September 1980; 2) subsequently reared in mesh containers suspended from fixed and floating platforms in Gulf of Maine waters; 3) and sacrificed between Jul 1988 and Aug 1989 did not reveal an unambiguous one-to-one correspondence between the number of years elapsed since spawning and the number of simple prismatic or granular microstructural regions in either the outer or inner shell layer. It is suggested that caution should be exercised in utilizing microstructural patterns within the shell of A. islandica for obtaining unambiguous age estimates of specimens of this species from at least certain environments.