Relation between expanding range of bivalve-inhabiting hydrozoans and water temperature

Abstract

After the first find, in 2002, of polyps of Eugymnanthea japonica Kubota and Eutima japonica Uchida on the Sea of Japan coast of western Japan, colonization patterns of these two hydrozoans were studied at six sites in this region between October, 2003, and October, 2004. The prevalence of polyps in bivalves was analyzed with respect to the annual fluctuation in seawater temperature. Presence of polyps was confirmed at four of the sites, and their prevalence was higher in October, 2004, than in October, 2003. This indicates that settlement of these polyps is progressing. Overwintering populations were observed at three of the four sites, but not at least at the two eastern sites. Their absence there in winter was evidently due not only to the low water temperature in that season, but also to the distance from presumptive source populations at Tsushima Island and Iki Island. At Tsushima Island and Hamada, polyps of both species coexisted in the same mussel, and there was a clear difference in the peak time of sexual reproduction of the two hydrozoan species, a case of allochronic reproduction. At the four sites where polyps were observed, a time-lagged correlation existed between the prevalence of polyps and water temperature fluctuations. The distributional range expansion of these hydrozoans in the Sea of Japan may be the result of the reproductive success of a settled population, supported by the recent climatic regime shift.