Endemic organisms of ancient lakes have been studied as models to understand processes of speciation and adaptive radiation. However, it remains unclear how ancient lakes play roles in genetic and phenotypic diversity of freshwater mollusks. In the present study, we focus on viviparid freshwater snails in the ancient lakes of East and Southeast Asia (Japan and China) to address this question. Using molecular phylogenetic analyses based on mitochondrial (COI, 16S) and nuclear genes (18S, 28S, H3), we show that patterns of species diversification in viviparid lineages. Colonization to ancient lakes occurred independently in China and Japan at least four times, with subsequent diversification into more than two species within each lake group. Morphological analyses of fossil related viviparids suggest parallel phenotypic evolution occurred in the different lakes and ages. Each lake contained a single lineage which was phenotypically diversified relative to those from other sites. Using genome‐wide SNPs obtained by MIG‐seq, we also examined the genetic structure of three Japanese viviparids, including two endemic species of ancient Lake Biwa. The results suggest that these two species diversified from the population of the third species living in wetlands surrounding the lake. These findings suggest that rapid diversification of lineages and phenotypic divergence can occur in ancient lakes compared to other habitats. Formation of large lakes probably promotes speciation and phenotypic divergence as a result of adaptation into different microhabitats. High numbers of ancient lakes could be a driver of species diversity in Asian viviparid snails.

A new species of viviparid gastropod belonging to the genus Heterogen Annandale, 1921, is described from the Pleistocene Katata Formation of the Kobiwako Group in the southern part of Lake Biwa, central Japan. This new species is continuously recorded from the Nijigaoka Clays (1.2 Ma) to the Hiraen Clays (0.8 Ma) of the Katata Formation. This new species, Heterogen praelongispira sp. nov. is similar to the extant H. longispira in Lake Biwa, but the details of shell morphology are different from those of H. longispira, including fine-to-deep suture, flattened whorl side, and oval shape of embryonic shell.

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Group spawning (1 female and more than 2 males) and streaking behaviour of subordinate males are common in pelagic‐spawning reef fishes such as the temperate tetrarogidid fish Hypodytes rubripinnis. Five microsatellite loci were isolated and characterized for paternity analysis of this species. Four of the five loci were highly polymorphic, having 11–34 alleles with observed heterozygosities between 0.89 and 1.00 in 37 unrelated individuals. These four loci conformed to Hardy–Weinberg equilibrium. These loci should be useful in future studies that assess how fertilization is partitioned among male H. rubripinnis in multimale spawning events.