Malate dehydrogenase (MDH) activity in freshwater shrimps, Paratya compressa, Caridina leucosticta and Neocaridina denticulata (Decapoda: Atyidae), was examined by starch gel electrophoresis. In accordance with the previous results, most populations of P. compressa had only one activity zone (MDH-1). The Biwa Lake population, however, was found to have two activity zones (MDH-1 and MDH-2). Since both had a triple-banded phenotype and since densitometric measurements showed the expected activity pattern (1:2:1), it is most likely that they are dimeric enzymes and are encoded by two independent loci (Mdh-1 and Mdh-2). C. leucosticta and N. denticulata examined had the two activity zones. But some individuals of both species showed a null phenotype in either of the two zones but not in both. Possibility that in most populations of P. compressa null allele at the Mdh-2 locus was fixed was discussed.
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