Insights on Chamelea gallina growth dynamics from the Holocene climate optimum in the Northern Adriatic Sea (Italy)

The fossil record documents ecological responses to past climate transitions, providing insights into future scenarios of marine ecosystems and taxa under climate change. In this study, we compared shell growth dynamics, specifically linear extension and net calcification rates, of the bivalve Chamelea gallina between Northern Adriatic Sea (NAS) assemblages from the Holocene Climate Optimum (HCO, 9-5 kyr cal. B.P.) and today. This species is a valuable economic resource, currently threatened by climate warming 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, especially during their early growth stages, at the expense of a linear extension rate, which was lower than modern specimens. This resulted in an average delay of 3-4 months in reaching sexual maturity, which is currently reached after ~ 13-14 months or at a length of ~18 mm. Several environmental factors are probably responsible for these differences between fossil and modern assemblages. Temperature indirectly impacted the geomorphologic evolution of the NAS over the Early-Mid Holocene, shaping the outlet structure of the Po and other NAS rivers. This, in turn, affected environmental factors such as nutrient load, seawater transparency, salinity, and phytoplankton. In addition, recent anthropogenically-derived decreases in natural NAS predators of infaunal bivalves may have reduced the natural predation pressure on modern C. gallina, thus favoring those populations characterized by faster linear extension rates (at the expense of a higher shell density), especially during the initial stages of life, hence facilitating a quicker attainment of sexual maturity.