Chamelea gallina growth declined in the Northern Adriatic Sea during the Holocene Climate Optimum.

The fossil record of past climate transitions offers insights into future biotic responses to climate change. Here, we compare shell growth dynamics, specifically linear extension and net calcification rates, of the bivalve Chamelea gallina between Northern Adriatic Sea assemblages from the Holocene Climate Optimum (HCO, 9 - 5 cal. kyr B.P.) and today. This species is a valuable economic resource, currently threatened by climate change and numerous anthropogenic stressors. During the HCO, regional sea surface temperatures were warmer than today, making it a potential analog for exploring ecological responses to increasing seawater temperatures predicted in the coming decades. By combining standard aging methods with reconstructed sea surface temperatures, we observed a significant reduction in linear extension and net calcification rates in warmer HCO assemblages. During the HCO, immature C. gallina specimens developed a denser shell at the expense of a linear extension rate, which was significantly lower than modern specimens. This resulted in an average delay of 3 months in reaching sexual maturity, which is currently reached after 13-14 months or at a length of ~ 18 mm. This study sheds light on the natural range of variability of C. gallina over longer time scales and its potential responses to near-future global warming.