Earlier studies demonstrate that blue mussels (Mytilus edulis) from the Baltic population are more sensitive, in terms of physiological response and survival when exposed to toxic substances, compared to mussels from a more marine environment. The question whether this can be explained by environmental factors or genetic differences in the ability to synthesise a common stress-inducible protein (HSP 70) was addressed in two experiments. In the first experiment mussels from the North and Baltic Seas were acclimatised to an intermediate salinity of 15‰ S in the laboratory. The physiological performance was studied when the heavy metal cadmium was added and accumulated by the animals during a one week exposure. Tissue concentration was measured and related to physiological response. The level of expression of HSP 70 was analysed by densitometry on Western blots. North Sea mussels rapidly induced high levels and multiple forms of HSP 70, reached a comparatively low tissue concentration of cadmium, and showed only a minor physiological response and low mortality rates. Baltic Sea mussels had low levels of HSP 70, induced at slow rate, reached higher tissue concentrations of cadmium, and showed a more pronounced physiological response and higher mortality rates. High levels of stress proteins and a rapid induction corresponded well with physiological fitness, and the mussels from the North Sea thus seem to have a more efficient detoxification system, probably including stress-inducible proteins. In the second experiment, juvenile mussels from the Baltic population were acclimatised for a month to North Sea conditions in a field transplant. The mussels were then exposed to low-dose copper in the laboratory and the accumulation, physiological response and synthesis of stress inducible proteins were assessed. The results indicate that the physiological differences described between the populations are to a large extent explained by environmental factors. However, some differences can still be observed between the populations, e.g., a lower rate of induction of a major stress protein in Baltic than in North Sea mussels. It can thus be suggested that a reduced ability to stress protein induction, in their natural low saline habitat, might be a contributing factor to the higher pollution sensitivity earlier demonstrated for Baltic blue mussels.
