Co‐occurrence of the cirolanid isopod Eurydice nipponica Bruce & Jones and the ghost shrimp Callianasa japonica Ortmann on an intertidal sand flat

On a moderately protected intertidal sand flat in west Kyushu, Japan, most of the population of the cirolanid isopod Eurydice nipponica Bruce & Jones (89%–100%) was found in the zones occupied by the thalassinidean ghost shrimp Callianassa japonica Ortmann in July, August and December 1980. C. japonica later extensively expanded its habitat, and in July 1984, when almost the whole sand flat had been densely populated by C. japonica , the range of the distribution of E. nipponica was the same as that of C. japonica with a density about 10 times greater than in July 1980. The occurrence of E. nipponica has previously been recorded from several exposed sandy beaches as well as their adjacent subtidal areas of well oxygenated sands along the coast of Kyushu, but not on more protected shores like the present sand flat. It is suggested that C. japonica , through its bioturbating activities, produces sediment characteristics approximating those of the exposed sandy beaches which are the preferred habitat of E. nipponica . It is supposed that E. nipponica is a facultative commensal of C. japonica . In a field experiment to exclude C. japonica and to detect its positive influence on E. nipponica , the densities of E. nipponica were found to be lower in the experimental plots than in the intact plots. Statistically, however, the difference was only weakly significant, and the possible reasons for this are discussed.