Genotypic and phenotypic differences between Baltic and North Sea populations of Mytilus edulis evaluated through reciprocal transplantations. III. Physiology

Abstract

Mussels Mytilus edulis L., aged 1 yr, were transplanted from a low saline regime (6 to 7 %o S, northern Baltic proper) to a high saline regime (20 to 30 %O 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, 0 / N ratios, and clearance 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 NHi-N excretion and lower 0/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.

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

“…Enzyme analysis by Johannesson et al (1990) showed that the high initial mortality of North Sea mussels transplanted to the Baltic was selective in some enzyme loci and that only North Sea mussels of Baltic Sea genotype survived. Also the sudden mortality of Baltic mussels in late summer 1 yr after transplantation to the North Sea seems to be due to genetic factors, as it was very selective against Baltic genotypes (Johannesson et al 1990).…”

“…Thus we conclude that the main reason for the much lower growth rate is likely to be a salinity-dependent change in amino acid metabolism and nitrogen excretion resulting in a less favorable energy balance in low saline areas (Tedengren & Kautsky 1986) (cf. also di.scussion in Tedengren et al 1990).…”

“…Some 200 mussels survived and continued growing until August 1988 when 74 of them were sampled for genetic analysis (cf. Johannesson et al 1990). Some 100 mussels of the 1986 transplants were still living in August 1989, but as mortality was very North Sea (TJ) populations.…”

“…Fig. 1) selective with mainly North Sea genotypes surviving (Johannesson et al 1990), these mussels were not used for further experiments. The proportion of biomass allocated to shell, meat and adductor muscle in the 2 populations is given in Table 1.…”

“…In this paper we compare growth rate and morphology in reciprocally transplanted mussels from Baltic a n d North Sea populations with that of their native populations. Enzyme variability and physiological differences between the 2 populations are discussed in Parts I1 and I11 of this series (Johannesson et al 1990, Tedengren et al 1990). …”