Thermal dependency of shell growth, microstructure, and stable isotopes in laboratory-reared Scapharca broughtonii (Mollusca: Bivalvia)

We experimentally examined the growth, microstructure, and chemistry of shells of the bloody clam, Scapharca broughtonii (Mollusca: Bivalvia), reared at five temperatures (13, 17, 21, 25, and 29°C) with a constant pCO₂ condition (∼450 μatm). In this species, the exterior side of the shell is characterized by a composite prismatic structure; on the interior side, it has a crossed lamellar structure on the interior surface. We previously found a negative correlation between temperature and the relative thickness of the composite prismatic structure in field-collected specimens. In the reared specimens, the relationship curve between temperature and the growth increment of the composite prismatic structure was humped shaped, with a maximum at 17°C, which was compatible with the results obtained in the field-collected specimens. In contrast, the thickness of the crossed lamellar structure was constant over the temperature range tested. These results suggest that the composite prismatic structure principally accounts for the thermal dependency of shell growth, and this inference was supported by the finding that shell growth rates were significantly correlated with the thickness of the composite prismatic structure. We also found a negative relationship between the rearing temperature and δ¹⁸O of the shell margin, in close quantitative agreement with previous reports. The findings presented here will contribute to the improved age determination of fossil and recent clams based on seasonal microstructural records. Key Points: Thermal plasticity of shell microstructural formation was examined Relative volume of composite prismatic structure was greatest at cooler temperature Growth rates were correlated with volume of composite prismatic structure