Genotypic and phenotypic differences between Baltic and North Sea populations of Mytilus edulis evaluated through reciprocal transplantations. I Growth and morphology

Growth rate and maximum size of Mytilus edulis is much lower in the Baltic Sea (7 %o S) than in the North Sea (28 %O S).Reciprocally transplanted mussels grew at rates similar to those of native mussels at each site, indicating that this variation can largely be explained by physiological differences due to environmental salinity Initial sunrival of Baltic mussels was very high after transfer to the ma.rin.e environment both in 1984 and 1986, but was followed by very high mortality in the follow~ng late summer In contrast immediate mortality was over 90 O/O among North Sea mussels transferred to the Baltic Sea with only some of the smallest mussels surviving.These, however, suffered no further excessive mortality.These different survival rates probably have mainly genetic causes.Genetic factors also seem to explain morphological differences, since these were to a large extent maintained even more than 1 yr after transplantation.Native North Sea mussels had 24 % larger volume and 9 % greater shell hei9h.tthan transplanted Baltic mussels of the same length (48 mm), while shell width differed little.Total dry weight and shell weight were about 2 times larger in North Sea than in transplanted mussels, and weights of meat and posterior adductor muscle were 1.6 times and 3.3 times larger, respectively.However, in mussels of Baltic origin more biomass was allocated to meat (14.7 O/o of total dry wt) relative to shell and adductor muscle, than in North Sea mussels (11.2 %).Predation experiments with Asterias rubens showed that Baltic mussels grown for 12 mo in the North Sea were more readily attacked and more easily opened than equal-sized North Sea mussels.We suggest that the lack of thck shells and large adductor muscles is caused by low predation pressure in the Baltic which does not select for such protective characters.Relatively more of the available energy instead seems to be allocated to reproduction.