Glycymeris bimaculata (Poli, 1795) - A new sclerochronological archive for the Mediterranean?

Glycymeris bimaculata is one of the largest (up to ~115mm) bivalves in the Mediterranean Sea, yet there is a paucity of information about the biology, ecology and growth of the species. Maximum longevity, growth and periodicity of internal shell growth line formation were assessed in individuals collected from Pag Bay in the Adriatic Sea. We validated annual periodicity of line formation using stable isotope analysis of shell calcium carbonate to construct standardized growth indices (SGIs) in live collected specimens obtained in 2008 and 2012 and developed a statistically robust master chronology from the growth increment series to explore shell growth responses to environmental indices. The estimated age of 55 shells (shell length 49.1 to 109.5mm) ranged from 6 to 57 years and shell growth is described by the von Bertalanffy growth equation: L_t=90.85-(e^{-0.10(t+3.13)}). The dark growth lines visible in acetate peel replicates of polished and etched shell cross-sections were deposited annually at the beginning of spring. Growth increment data for the period between 1991 and 2007, derived from 9 individuals (age between 17 and 34years), were used in the construction of a master chronology. The SGIs had pronounced inter-annual variability with the period between 1997 and 1999 characterized by slower growth, the period between 2000 and 2005 by moderate growth, while the widest growth increments were deposited in 2006 and 2007. Differences in growth between these periods were related to differences in surface seawater temperatures and salinity.