Evaluating the effects of ocean acidification on the mechanics and structures of southern tanner crab mineralized tissue

Abstract

Increasing levels of atmospheric carbon dioxide have led to a reduction in the pH of ocean waters (i.e. ocean acidification, OA) and an associated drop in calcium carbonate saturation states. In many taxa, these changes in seawater chemistry affect the ability of calcifying organisms to build and maintain mineralized tissue. To date, few studies have assessed the effect of OA on mineralized tissue in crustaceans. This study examines the effects of decreased pH on mechanical and structural properties of the mineralized cuticle in southern Tanner crabs, Chionoecetes bairdi. Mature multiparous female southern Tanner crabs were exposed to one of three pH levels (8.1 (ambient), 7.8, and 7.5) for two years. Following exposures, the carapace and chelae were dissected, sectioned longitudinally, embedded in epoxy resin and polished. Sectioning exposed the two distinct mineralized layers of the cuticle, the inner endocuticle and outer exocuticle, enabling separate analyses in each region. For the carapace, microhardness (a measure of a material's resistance to deformation) was significantly reduced at pH 7.5 in the endocuticle but not the exocuticle. Likewise, thickness of the endocuticle in crabs exposed to pH 7.5 was significantly reduced, whereas thickness of the exocuticle was not affected by pH. Initial microhardness testing of chelae revealed a trend toward reduced microhardness in the exocuticle but not the endocuticle in lower pH treatments. Results suggest that OA negatively affects the ability of southern Tanner crabs to maintain calcified tissue, potentially compromising structural integrity of the crab cuticle.