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

Abstract

Mussels Mytilus edulis L., aged 1 yr, were transplanted from a low saline regime (6 to 7.permill.S, northern Baltic proper) to a high saline regime (20 to 30.permill.S, North Sea). Physiological responses of these mussels, after 1 mo and 1 yr of acclimatisation, were compared to those of native North Sea mussels and allele frequencies for 2 polymorphic loci (Pgi and Pgm) in the populations were scored. Oxygen consumption, ammonia excretion, O/N ratios, and clearances rate of transplanted Baltic mussels approached those of native North Sea mussels, indicating that these metabolic parameters are to a major extent regulated by physiological adaptations to environmental differences between the Baltic and North Seas. However, after 1 yr, significantly higher NH4-N excretion and lower O/N ratios were observed in mussels of Baltic origin. A lower clearance rate and consequently lower scope for growth was observed in Baltic mussels. This may be due to a smaller gill area, resulting from a genetically induced more elongate shell shape, typical for Baltic mussels, that still remained after 1 yr of acclimatisation. The genetic composition in Pgi and Pgm, 2 loci coding for enzymes which are central to the metabolic rate of cellular processes, differed between the native populations, probably due to differential selection, and part of the observed physiological differences between mussels from the 2 seas may be determined by genotypic factors acting directly on metabolic pathways.