(1) The genetic variability of the prawn Palaemonetes varians was measured by means of the frequency of alleles and heterozygotes for the enzymes GPI. IDH and PGM. (2) The animals were collected from eight more or less isolated brackish waters in the Delta area of the rivers Rhine, Meuse and Scheldt (S.W. Netherlands) and one station in the Slack estuary (Ambleteuse, N.W. France). Salinities ranged from 1 to 18 %o- (3) The isozymes examined all showed two alleles, the average observed heterozygosity was 0.30, the. expected heterozygosity was 0.28, thus a slight excess of heterozygotes occurred. (4) The frecquency of the GPI alleles showed a significant relationship to salinity; the frequency of the fast allele increased with increasing salinity. For PGM, the excess of heterozygotes showed a significant increase with increasing salinity, especially at salinities above 10 %o-. (5) It is likely that at the higher, unfavourable salinities, animals heterozygotic for PGM and with the fast allele for GPI are better able to survive. (6) For one station, a relatively high genetic distance from all other stations studied was found. We concluded that the prawns at this station are of a different variety (ecotype).
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