Unexpected Covariances between Shell Growth Parameters are Controlling the Range of Interspecific Variations of Shell shape in the Deep-Sea Clams Genus Calyptogena (Bivalvia: Veneroida)

Abstract

In Bivalves, shell shape descriptors (in particular, elongation E, dissymmetry D, ventral convexity K) are considered functionally-relevant parameters, each of them likely being exposed to specific selective pressures, according to environmental conditions. Indeed, previous investigations, have repeatedly confirmed that different types of constraints do apply to the respective ranges of interspecific variations of E, D, K: in particular, a trend for a substantially extended range of interspecific variation of shell-elongation E (likely attributed to niche differentiation) and, on the contrary, a trend for a severely restricted range of interspecific variation of ventral-convexity K (closely centered at an optimum for mechanical resistance of valves). In turn, due to rigid geometrical relationships linking shell-shape to shell-growth, this pattern of constraints on shell-shape variations implies, mathematically, the occurrence of specifically associated covariances between shell-growth parameters, the occurrence of which were, indeed, repeatedly observed within all groups of shallow-waters clams examined so far. Yet, the fairly different environmental conditions prevailing at bathyal / abyssal depths invite addressing comparatively some typically deep-sea clams; hereafter the genus Calyptogena. Contrary to expectation, the same types of covariances between shell-growth parameters are reported in deep-sea Calyptogena as those reported in shallow-waters clams, thus suggesting less dissimilar patterns of constraints than expected between shallow waters and deep-sea conditions.