Temperature dependence of SERCA activity in thermally acclimated freshwater mussels Anodonta anatina and Unio tumidus (Bivalvia: Unionidae)

Similar to other aquatic ectotherms, freshwater mussels are prone to the predicted global warming of surface waters. Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) has a crucial role in regulating intracellular Ca2+ concentration in muscle cells and potentially involved in setting thermal preferences and behaviors of animals. To this end, we examined temperature effects on SERCA activity of adductor and foot muscles in two freshwater mussel species, Anodonta anatina and Unio tumidus. Mussels were acclimated to three different temperatures (+4, +14, +24 degrees C) and SERCA activity was measured at those temperatures for each acclimation group. SERCA activity of both tissues was much higher in U. tumidus than A. anatina. SERCA activity in the adductor muscle was higher than that in the muscular foot in both species at all test temperatures, and in all acclimation groups. Coldacclimated (+4 degrees C) animals of both species tend to conserve energy by turning off SERCA activity in the cold, as 73% of A. anatina and 55% of the U. tumidus individuals did not have measurable SERCA activity in the muscular foot at +4 degrees C. Partial temperature compensation of SERCA activity occurred only in the adductor muscle of A. anatina as shown by the higher activity in the +4 degrees C-acclimated animals than in the +14 and +24 degrees C acclimated animals. Temperature dependence of SERCA was strong (Q10 between 3.05 and 13.14), whereby SERCA activity responds strongly to temperature changes. Strong temperature-dependent up- or down-regulation of muscle SERCA activity in mussels is probably associated with species-specific behaviors including valve gaping, locomotion and reproduction under variable environmental conditions. Importantly, SERCA activity can potentially provide a mechanistic explanation for these behaviors and has likely implications on freshwater mussel aquaculture and conservation.