Bioenergetics of calcification in mytilid bivalves along the Baltic Sea salinity gradient
The Baltic Sea salinity gradient provides an excellent system to investigate how seawater ion availability impacts calcification rates and the energetic costs of calcification. Additionally, the gradual changes in environmental conditions along this gradient also provide an excellent system to investigate how environmental salinity drives natural selection and local adaptation. In this thesis the effects of salinity on calcification rates and costs were investigated in mytilid mussels using laboratory experiments. In addition, the extent of local adaptation and salinity driven selection in Baltic mytilid larvae were also investigated as well as the mechanisms behind potential local adaptations. Findings suggest that calcification rates at low salinity are limited by calcium availability and the costs of calcification are higher at low salinity compared to high salinity. Costs of calcification in mussels were also found to constitute up to 58 % of available energy, considerably higher than previously estimated. Low salinity populations of Baltic mytilid mussels exhibit local adaptation to low salinities observed by better growth, lower mortality and higher settlement success at low salinities than high salinity adapted populations. The mechanisms behind these adaptations appear to be adaptive changes in intracellular osmolytes with low salinity populations containing higher concentrations of cations than high salinity populations. Overall, these findings suggest calcification is a costly process at low salinities and may be responsible for slow growth rates of mussels at low salinity. Additionally, salinity is a powerful selective force driving local adaptation and genetic divergence in Baltic Sea mussels and the mechanisms behind this adaptation appear to be changes in intracellular osmolytes and potentially, the cellular processes of calcification.