Temporal and spatial distribution of the two closely related Acartia species A. omorii and A. hudsonica (Copepoda,Calanoida) in a small inlet water of Japan

Temporal and spatial distributions of Acartia omorii and Acartia hudsonica were investigated in Maizuru Bay, middle Japan, from 1977 to 1978. In both years, adults of the two species appeared synchronously 17 days after the bottom temperature cell to 20°C in November; the numbers of both species sharply decreased when the bottom temperature rose to 20°C in late June and disappeared in early or mid-July. These facts indicate that a bottom temperature of 20°C is critical for the recruitment of the populations of these species. Acartia hudsonica was more limited to the inner position of the bay than A. omorii; spatial segregation may be explained not only by preference for different hydrographic conditions, but also by interspecific competition between the two species. In temperate waters, co-existing Acartia species belonging to the same subgenus appear to be segregated only in space while those of different subgenera usually exhibit a seasonal segregation. Pacific populations of A. hudsonica appear to be more restricted to inlet waters and estuaries than Atlantic populations, probably owing to the presence of the closely related sympatric species in the Pacific.