Derivation of populations with different-sized eggs in the palaemonid prawn Macrobrachium nipponense

Population-genetic and morphometric investigations were undertaken in 28 po pulations of the palaemonid prawn Macrobrachium nipponense with different-s ized eggs which occur in different habitats from estuaries to inland waters of the Japanese islands, in order to elucidate the process of their differ entiation. In estuarine populations with small eggs, allele frequencies in the Pgm locus changed with a dine along the Pacific coast. This appears to have resulted from infrequent unidirectional gene flow by sea current-media ted larval dispersal. Estuarine populations along the coast of the Sea of J apan, on the other band, were genetically homogeneous, suggesting more freq uent gene flow in them. Populations with medium and large eggs, which are o bserved in brackish- or fresh-water limnetic systems of sea-relict lake ori gin on both sides of the Japanese islands, are considered to have different iated from estuarine populations after the Holocene marine transgression. S ome lacustrine populations were significantly different in their genetic co ntent and morphometric traits, implying their independent derivation.