2020 年 AA2020 巻 p. AA2020-4-
Nymphonella tapetis is a sea spider showing a larval endoparasitism on Manila clam (Asari) and some other bivalves. A large-scaled outbreak of the species occurred in Tokyo Bay in 2007 and caused serious damage to the Asari fishery. The species has been recorded afterward in several places in Japan and seemed to be expanding its distribution. In the present study, we conducted a population genetic analysis of N. tapetis using mtDNA COI sequences from the populations of five localities in Japan: Tokyo Bay (Chiba Prefecture), Mikawa Bay (Aichi Pref.), Matsukawaura (Fukushima Pref.), Tateyama (Chiba Pref.), and Misaki (Kanagawa Pref.). Phylogenetic tree from a partial COI sequence (562 bp) robustly supported a monophyly of N. tapetis of these localities. Parsimony haplotype network analysis showed the two genetic lineages among these populations: the Tokyo Bay (Tokyo Bay, Mikawa Bay and Matsukawaura) and the Tateyama (Tateyama and Misaki) lineages. The lower average nucleotide sequence divergence of the Tokyo Bay lineage suggests its younger history than another lineage. As the localities of the Tokyo Bay lineage are remotely separated each other and the N. tapetis larvae lack the planktonic stages, they are considered sharing a common source such as a recent commercial circulation of Manila clam. Although Tateyama and Misaki are geographically much closer to Tokyo Bay compared to Mikawa Bay and Matsukawaura, the migration between the Tateyama + Misaki population and the Tokyo Bay population seems to be an unlikely event.