Seasonal changes in the shell microstructure of the bloody clam, Scapharca broughtonii (Mollusca: Bivalvia: Arcidae)

Abstract

In this study, seasonally controlled changes in shell microstructures in Scapharca broughtonii (S. broughtonii, Mollusca: Bivalvia) were demonstrated. We observed shell microstructures and analyzed stable oxygen isotope ratios to reveal the factors controlling cyclical microstructural changes in S. broughtonii. The specimens examined were collected alive from three localities in Japan. The outer layer of S. broughtonii was subdivided into a composite prismatic structure on the exterior side and a crossed lamellar structure on the interior side. Relative thickness of these two structures in the outer layer changed cyclically with ontogeny. Major growth breaks were formed immediately after thickening of the crossed lamellar layer in most specimens. Growth breaks were also marked by thickening of the composite prismatic structure. Fluctuations in the relative thickness of the two microstructures were synchronized with those of the shell oxygen isotope ratios that indicated the seasonality of water temperature. The crossed lamellar structure thickened at high water temperatures in summer. Shell oxygen isotope records indicated that the clams can form their shells at temperature higher than approximately 12°C with no record of lower winter temperatures. Growth breaks observed after the peaks of high water temperatures may have formed as a result of spawning, because the breaks corresponded to the spawning season. The proportion of the thickness of the shell microstructures can directly indicate fluctuations in the water temperature. The methods used in this study can contribute to age determination, the characterization of seasonal shell growth, and an understanding paleotemperature changes in coastal regions.