Comparative study of the physiology and its probable effect on size in Blue Mussels ( Mytilus Edulis L.) from the North Sea and the Northern Baltic Proper

It is a well-known fact that the Baltic Sea blue mussels are very abundant but do not grow as large and fast as Mytilus edulis from the North Sea. This is often attributed to physiological differences due to the decline in salinities, although the mechanisms behind this are not yet fully understood. In order to elucidate this, the metabolism of mussels from the northern Baltic proper (Askö) and the North Sea (Tjärnö) was compared in the laboratory at ambient salinities of about 6.5 and 28.8 ‰, respectively, together with the magnitude of response to acute changes in the salinity. At 13°C and ambient salinities, respiration was higher for North Sea mussels in a non-reproductive stage, while after spawning respiration was higher for Baltic mussels. NH4-N excretion was always about two times higher for Baltic than for North Sea mussels at ambient salinities, and very sensitive to changes in salinity due to changes in the metabolism of free amino acids. While lowered salinities increased excretion, these decreased with raised salinities. The resulting O:N ratios (calculated from respiration and excretion) at ambient salinities were consistently lower for Baltic mussels, indicating that these have a less favourable energy metabolism, which may contribute to their smaller size.