Distribution and zoogeography of the marine cladoceran Podon schmackeri in the northwestern Pacific

Distribution and zoogeography of Podon schmackeri in the northwestern Pacific was studied on the basis of materials collected during the Cooperative Study of the Kuroshio and Adjacent Regions, 1965 to 1974. P. schmackeri was mainly distributed in waters along the coast of the Asian continent from the south of Vietnam to northern waters adjacent to Japan and the Kuroshio Extension. The Kuroshio Current seems to be important in transporting this species from Japanese coastal regions to the offshore waters of the Kuroshio Extension, whereas the Oyashio front forms the northern limit of its distribution. A few individuals found in the Oyashio region might be those transported by detached warm-core rings. This species occurred from April to August in the southern waters of Vietnam, whereas it appeared during the later period, June to September, in the northern waters of the Kuroshio Extension. Temperature and salinity ranged from 19.71° to 30.39°C and from 29.37 to 34.27‰, respectively. Maximum abundance of 79.2 individuals m-3 was recorded in the Japanese coastal region with 23.98°C and 33.50‰ water. Later occurrence in northern waters might be due to the delay in raising of the water temperature. Geographical variations in size were observed; individuals were smaller in standard length (0.34 to 0.50 mm, mean 0.42 mm) in southern waters of Kyushu, Japan, with a gradual increase toward northern waters of the Sea of Japan where size range was 0.49 to 0.65 mm (mean 0.57 mm). Geographical variations in fecundity in terms of mean number of embryos per batch was also detected. Fecundity was higher in southern waters of Kyushu and the Sea of Japan, 5.20 and 5.36, and lower in waters of southern Honshu and the Korea Strait, 2.35 and 3.90. These variations seemed to be derived from a compound effect of environmental conditions such as temperature, salinity and food availability.