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Genetic variability of a selected population of Dreissena polymorpha, an invasive species playing a significant role in aquatic ecosystems, was studied. Starch gel electrophoresis was used to analyse 8 enzymatic loci in 200 individuals collected from 20 sites in a lake. The population was found to have 75.0% of polymorphic loci, 2.8 alleles per locus, 3.5 alleles per polymorphic locus, 0.393 coefficient of expected heterozygosity, and 149 genotypes. Zebra mussel clumps were strongly polymorphic; almost every individual had a different genotype. The high polymorphism observed in the D. polymorpha clumps had most likely resulted from external cross-fertilisation and the presence of free-swimming veliger larvae as well as from a considerable heterozygosity of individual bivalves. Genetic variability of the population studied was found to be similar to that of populations inhabiting other Western Pomeranian lakes, including both highly polluted ones and those formed as recently as about 40 years ago. This provides evidence for a mass colonisation of freshwater reservoirs effected by very polymorphic parent populations of D. polymorpha. The literature data on North American zebra mussel populations which invaded that continent about 10 years ago show them to be polymorphic, too, but not as much as European ones.
Mytilopsis leucophaeata (Conrad’s false mussel), an invasive American bivalve, has been found for the first time in the Gulf of Gdańsk on hard substrata (PVC panels) deployed at depths from 3.5 to 6.0 m.
Variation of some structural and developmental characteristics of ctenidia was studied in bivalves representing the families Mytilidae, Unionidae, Dreissenidae, Sphaeriidae and Cardiidae. Fingernail and pill clams were investigated most thouroughly. Significant differences in the outer demibranch position were found in Sphaeriidae (in the genus Pisidium) and Cardiidae. In the first family, correlation between the outer demibranch position and the time of its ontogenetic origin is observed. Some taxonomic interpretations of the described differences are discussed. For Unio and Anodonta, the differences in the outer demibranch growth rate are demonstrated.
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