Shell morphology and skeletal property variations in the clam Chamelea gallina along a latitudinal gradient in the Adriatic Sea

Phenotype can express different morphologies in response to biotic or abiotic environmental influences. Mollusks are particularly sensitive to different environmental parameters, showing shell morphology variation with several environmental factors. Here, we investigate shell morphology of the commercially important clam Chamelea gallina in relation to natural variations in sea surface temperature (SST) and solar radiation (SR). Specimens of C. gallina were sampled using hydraulic dredges on soft bottoms in the subtidal zone at 3-7 m depth, in six populations along a latitudinal gradient (~400 km) in the Western Adriatic Sea. Only clams of commercial size (25-30 mm) were used in this study. To analyze shell morphology, length, width, height, thickness of the valve and dry shell mass were measured. Volume and shell density parameters were obtained by means of the buoyant weight technique and used to calculate skeletal properties (micro-density, bulk-density and apparent porosity). Compression tests were conducted to check for shell resistance to fracture. Furthermore, to obtain qualitative information about shell mineral composition, X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were performed. C. gallina shells from the warmest and the most irradiated population were about 30% lighter, thinner and more porous. They also showed a reduction in shell stiffness and load to fracture, with an increase in fragility and damage susceptibility. However, no variation was observed in shell polymorphism (100% aragonite), indicating no effects of environmental parameters on the composition of biomineralized shell. Because of the importance of this species as a commercial resource in the Adriatic Sea, the variation of mass and fragility in C. gallina shells with increasing SST and SR could have economic implications along Adriatic coastline. Less resistance to breakage shells could be more damages during the catch with hydraulic dredges, with a bigger amount of clams discarded from the trade and a bigger catch effort for fishermen with a loss in economic yield.