Disturbed by pH? Nacre tablet thickness of freshwater pearl mussels (Margaritifera margaritifera) is a poor temperature proxy

Abstract

Context The ultrastructure of marine bivalve shells, specifically the thickness of nacre tablets has recently been identified to be related to water temperature. Aims This study investigates whether the nacre tablet thickness (NTT) of freshwater pearl mussel shells can serve as a proxy for stream water temperature. Methods Laboratory-grown juvenile and adult Margaritifera margaritifera from three streams were analysed using scanning electron microscopy to determine the relationship between NTT and water temperature. Key results Neither laboratory-grown juvenile nor adult specimens from the field revealed consistent NTT trends with temperature. Conclusion The NTT of M. margaritifera is largely uncoupled from stream water temperature. Presumably, fluctuations in stream water pH caused physiological stress and forced the animal to allocate more energy to homeostasis than to shell growth. The absence of a coupling between NTT of juvenile mussels and temperature is likely to be due to stronger physiological control on nacre tablet morphology during early stages of ontogeny, favouring thinner tablets with a larger proportion of organics v. CaCO3, which increases the fracture resistance of the shell. Implications Therefore, only if ambient pH remains stable, NTT of freshwater mussels beyond the age of five may serve as a temperature proxy, albeit being associated with a large error.