Shell isotope data of the variegated scallop (Mimachlamys varia) and queen scallop (Aequipecten opercularis) provide information on marine environmental conditions and the timing and rate of seasonal growth

The variegated scallop, Mimachlamys varia , is a relatively small-sized scallop whose growth patterns are not well studied. It occurs as fossils in sedimentary deposits and is of potential commercial interest. In this study, we analysed oxygen (δ 18 O calcite ) and carbon (δ 13 C calcite ) stable isotope values in M. varia specimens from the Adriatic Sea. Three M. varia specimens were collected in the North Adriatic using a commercial fishing vessel in December of 2024, and three were collected from the aquaculture protection net at a tuna aquaculture site near the island of Brač, middle Adriatic Sea on February 2025. Shell carbonate samples for the stable isotope analysis were collected from the external shell surface of these specimens and compared the records from two Aequipecten opercularis specimens collected in December 2024 in the northern Adriatic Sea. Shell stable oxygen isotopes were manually aligned along predicted δ 18 O calcite curves, which in turn were computed from local temperature and salinity values (modelled using the ROMS numerical model). Results indicate differences in isotopic signatures between these two sites, with shells from Brač having lower δ 18 O calcite and δ 13 C calcite values. According to the temporal alignment of the shell record, A. opercularis shells from the northern Adriatic slowed down growth during summer, M. varia shells from the northern Adriatic slowed their growth both during the winter and summer, while M. varia from Brač only slowed their growth during the cold season. Results were compared to previously published data for other scallop species from the Adriatic Sea, including A. opercularis , Pecten jacobaeus and Flexopecten glaber .