Callista chione – geochemical archive of δ18O and δ13C data

The Smooth clam Callista chione is a commercially important venerid bivalve. It is widely distributed in the eastern Atlantic Ocean and the Mediterranean Sea and inhabits sandy sediments in coastal waters at depths down to 180 m. With specimens that can reach 10 cm of shell length and with a lifespan of more than four decades, C. chione represents an interesting archive for sclerochronological research. The aim of this study was to analyse possible variations in δ¹⁸O_shell and δ¹³C_shell values between C. chione specimens collected in different parts of the Mediterranean Sea.

Callista chione shells were collected alive from three localities: (1) Caleta de Vélez in the north region of the Alborán Sea, Spain; (2) Gulf of Venice, Italy, North Adriatic, and (3) west coast of the Istrian peninsula, Croatia, also in the North Adriatic. At the first two localities, specimens were obtained from catch of the commercial fishing vessels, while in Istria they were collected by SCUBA diving. Prior to analysis, the external shell surface was physically cleaned by grinding. Shell powder for δ¹⁸O_shell and δ¹³C_shell analysis was then collected by milling narrow sample swaths in the outer shell surface and processed at Mainz University using a GasBench II - IRMS.

Modelled daily temperature and salinity values were obtained for each locality and used for calculating the predicted δ¹⁸O_shell values. For Caleta de Vélez, daily temperature and salinity values were obtained by MEDSEA model; for the Venetian region by the AdriSC climate model, and for Istria by the 3D numerical model ROMS. Temporal alignment of measured δ¹⁸O_shell values was conducted manually in Excel by best-fitting measured isotope data to predicted δ¹⁸O_shell curves.

Seasonal δ¹⁸O_shell cycles were observed in all studied specimens. Temporal alignment of measured and modelled δ¹⁸O_shell values clearly showed that C. chione grew fast during the warm part of the year, while slower growth occurred during the winter months. Samples collected in Caleta de Vélez had the narrowest range of δ¹⁸O_shell values (-0.43 to +1.73 ‰), while δ¹⁸O_shell values in C. chione from Istria showed the largest amplitudes (-1.61 to +2.67 ‰). Growth patterns varied between sampling localities.

The δ¹³C_shell values varied strongly between localities and specimens. Highest δ¹³C_shell values were obtained for C. chione shells from Caleta de Vélez (0.51 ± 0.03 ‰, range -0.19 to +1.06 ‰). The broadest range of δ¹³C_shell values (-3.37 to -0.08 ‰) were measured in shells from the Venetian region. These samples also had the lowest mean (-1.42 ± 0.14 ‰). Shells from Istria had δ¹³C_shell values ranging from -1.57 to +0.38 ‰ (mean: -0.42 ± 0.28 ‰). Observed differences between localities are statistically significant (Kruskal Wallis H = 150.4, p < 0.001).  Isotope data obtained for this study were compared with data from a previous study on the same species in the Eastern Adriatic.

Research was supported by the Croatian Science Foundation, research project BivACME.