In the studies of reptiles, I started the study of phylogeny and biogeography of the scincid genus, Plestiodon (formerly Eumeces) in the East Asian Islands by cladistic analysis and neighbor joining method based on morphological data. Then our laboratory has continued to study this group genetically using allozymes and DNA data. We revealed the process of the speciation of the genus in Japanese and Ryukyu Archipelagos. During the studying this group, we described five cryptic species. While studying the taxonomy and phylogeny of reptiles, I found that the cladistic taxonomy has several problems. I proposed the gradistic classification system showing cladistic relationship by putting a tag of the descendant grade within the ancestral grade.

Ostracods are small aquatic crustaceans with a calcified, dorsally hinged bivalve-like carapace that can enclose the entire body. They are distributed almost every aquatic environment, such as shallow to deep pelagic environments, freshwater, brackish-water, and also subsurface-water. With an abundant and nearly continuous fossil record since the Ordovician, over 33,000 species of recent and extinct ostracods are currently known. Thus, ostracods are suitable materials for evolutionary biology. However, genomic information of this taxon is far from abundant. Starting in the Middle Jurassic, cypridoideans appear to have repeatedly invaded continental habitats and rapidly radiated. This explosive diver­sification leading to the “sudden appearance” of extant cypridoidean linages in the fossil record has hampered reconstruction of relationships in this group. To clarify the relationships among cypridoidean ostracods, I conducted the molecular phylogenetic analyses. In the results, relationships of superfamilies in Podocopida were almost consistent with previous classification systems. However, there was a problem with the internal systematics of the Cyprididae. Considering their ecology and morphology, I proposed to divide the family Candonidae into three different families. After receiving my doctoral degree while continuing research on ostracods, I am also conducting several researches in population genetics. Here I give an overview of these studies as well.

Nematodes of the genus Phasmarhabditis Andrássy, 1976 (Secernentea: Rhabditida: Rhabditidae) are terrestrial gastropod parasites and mainly target land slugs. In this study, land slugs were surveyed at 14 locations in seven prefectures of Honshu Island, Japan, to determine Phasmarhabditis spp. infection. Juvenile nematodes of unknown species were found in Meghimatium bilineatum Benson, 1842, at five of the 14 locations. The prevalence and mean intensities ranged from 4.5% to 93.3% and from 4.7 to 22.5 nematodes per host, respectively. A total of 881 juveniles were incubated with slug tissues for 2–10 days, and subsequently developed into adult stage showing the diagnostic characteristics of Phasmarhabditis spp. No nematodes were found from slugs of the genus Lehmannia sampled where M. bilineatum were infected, indicating a difference among host species in their sensitivity to the nematodes. Twenty M. bilineatum from Meguro in Tokyo, Japan, where the prevalence in the host population was >90%, were maintained under laboratory conditions. After 23 days, 11 of the 20 slugs died and the cadavers were infected with numerous nematodes. Since three species of Phasmarhabditis nematodes are known to be lethal to terrestrial gastropods, the nematodes we sampled possibly have a lethal effect on the host slugs.