Zooplankton of the lakes of the Shiretoko Peninsula, northeastern Hokkaidô

Abstract

A short account is given of the zooplankton of five lakes in the Shiretoko Peninsula, northeastern Hokkaido. In three small lakes located near the coast of the Sea of Okhotsk, the plankton consists of only 6-7 species of animals, among which Acanthodiaptomus pacificus, Diaphanosoma brachyurum, Daphnia rosea and Keratella cochlearis are especially abundant in individuals. Acanthodiaptomus pacificus and Daphnia rosea are also dominant in the plankton of the other two alpine lakes that lie on the central mountain range of the peninsula. The Daphnia populations of these five lakes are different phenotypically from each other, with the exception of the population inhabiting the smaller one of the alpine lakes. The last population appears to be an intermediate form between D. rosea and the other form with a short shell-spine peculiar to Hokkaido and the Kuriles. It becomes manifest that the range of geographical distribution of D. rosea extends from the Kurile Islands to Hokkaido.