SPATIAL DISTRIBUTIONS AND POPULATION STRUCTURES IN SCOPIMERA GLOBOSA AND ILYOPLAX PUSILLUS (DECAPODA : OCYPODIDAE)

Abstract

Spatial distributions and population structures of Scopimera globosa De Haan and Ilyoplax pusillus (De Haan) (Decapoda: Ocypodidae) were investigated at four stations established on the intertidal flat of Waka River Estuary, middle Japan. In S. globosa, large males tended to be restricted to the upper level, while small males and juveniles were distributed more evenly. This feature is explained in connection with repulsive interactions between individuals. Juveniles of l. pusillus showed higher densities at upper levels inhabited by many conspecific sexable crabs, and the spatial associations between these juveniles and sexable crabs within the upper levels were highly positive. These spatial features of juvenile I. pusillus are attributable to their special habit that they can dwell in the adult burrows together with the adults. At the stations inhabited by both S. globosa and l. pusillus, juvenile S. globosa appeared to be restricted within rather upper levels, although no such tendency was detected at the stations devoid of I. pusillus. Thus, it is suggested that the distribution patterns of these two species are influenced by intraspecific or interspecific relations.