Growth line deposition and variability in growth of two circumpolar bivalves (Serripes groenlandicus, and Clinocardium ciliatum)

Growth patterns of two common circumpolar bivalves, the Greenland cockle (Serripes groenlandicus), and the hairy cockle (Clinocardium ciliatum) have been used in previous studies to reconstruct environmental conditions in the arctic. To date, there has been no direct determination that growth lines in either species are deposited periodically, and there has been no examination of factors affecting growth. We placed calcein-marked individuals of both species on oceanographic moorings in two fjords (Rijpfjord and Kongsfjord) in the Svalbard archipelago for one and two (Kongsfjord only) years. Growth patterns were compared with concurrent in situ temperature and fluorescence data in order to assess environmental controls on growth. Dark growth lines are evident on the outer shell surface and internally in shell cross section in both S. groenlandicus and C. ciliatum, and both species deposited only one line per year, unequivocally confirming that internal lines are deposited annually. Growth line deposition in both species began in late summer to early fall, before the seasonal decline in temperature. There was no difference in growth of S. groenlandicus between the two fjords despite differences in water temperature (3°C), fluorescence (nearly threefold) and the onset and duration of the winter season. C. ciliatum, however, grew approximately 2.8 times faster in the warmer, more food-rich Kongsfjord than in Rijpfjord. Subannual lines were counted in two individuals of each species from each fjord, but deposition of these lines was not clearly related to number of growing days estimated by temperature and fluorescence.