Scarponi, D. | bioRxiv Science

bioRxiv2025

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.

paleontology↗

bioRxiv2025

Spatial patterns of trematode-induced pits on bivalve skeletons: Challenges and prospects for research on parasite-host dynamics

Interactions between the parasitic larvae of digenean trematodes (mainly gymnophallids) and bivalves often result in characteristic shell malformations, i.e., pit-like traces. Tracking these traces through the Holocene and modern marine death assemblages has made studying parasite-host responses to natural and anthropogenic environmental change possible. Despite major breakthroughs, empirical explorations of parasite-host dynamics in the geological record are primarily based on trace occurrence data, overlooking that trace spatial patterns on the host skeleton could carry ecological information and potentially document different aspects of the parasite-host interactions (e.g., infective behavior, association with specific host anatomy, spatial relationships of traces with different qualitative properties such as size class, etc.). The Spatial Point Pattern Analysis of Traces (SPPAT) has been increasingly employed to overcome similar challenges in studying predatory traces on bivalve prey. Although this approach holds considerable promise for research on trematode-host dynamics, several assumptions and caveats need to be considered (e.g., the number of traces required to capture the parasite-host dynamics accurately, the reliability of point patterns constructed from multiple host skeletons in describing parasite interactions). Here, we introduce a spatially explicit framework for extracting information from spatial patterns of trematode-induced pits on bivalve shells using SPPAT, address methodological questions involved in assembling a point pattern of traces from multiple host specimens, and discuss critical issues related to drawing inferences from pooled point data. We illustrate our approach using a case study on late Holocene samples of the commercially relevant bivalve Chamelea gallina from the northern Adriatic of Italy. This species holds high value in the seafood industry and is increasingly used in climate change research. Our results reveal that trematode-induced malformations on bivalve shells are not random; they show an aggregated pattern for metacercaria traces of the same size classes, while an independent pattern arises when examining traces of two distinct size classes. This study highlights the potential value of spatial information from parasite-induced traces in enhancing our understanding of parasite-host dynamics over time.

paleontology↗