2003 Volume 58 Issue 1 Pages 51-73
Abstract: In the Ariake-Sound estuarine system, western Kyushu, Japan, two congeneric callianassid shrimps, Nihonotrypaea harmandi and N. japonica, which inhabit deep-reaching burrows on intertidal sandflats, generally occur separately along salinity gradient. The former and latter species appear mainly in(mixo-)euhaline and mixo-polyhaline waters, respectively. The two species co-occur in a restricted water range of intermediate salinity. The present study examined the spatial variation in density of the two species along the intertidal gradient on four sandflats selected from that water range, in relation to various environmental parameters including elevation, duration of exposure, sediment grain-size composition, sediment thickness and firmness, and subsurface-water salinity, temperature, oxygen concentration, and redox potential. On the respective sandflats in the exclusive occurrence ranges, each species inhabited the entire sandflat. In their co-occurrence range, N. japonica and N. harmandi separately inhabited the upper and lower shores, as a rule. Neither subsurface-water salinity nor temperature variations along the intertidal gradient could explain this zonation pattern. In contrast, the other environmental parameters varied in a shore-level-specific way, and corresponded closely to the zonation. In general, N. japonica occupied a broader fundamental niche, which would be associated with the harsher conditions of its primary habitat, i. e., extensive tidal flats in the middle estuary. In particular, a greater tolerance of this species for hypoxic conditions during a longer exposure period was strongly suggested. The abundance pattern of the two species also suggested the competitive dominance of N. harmandi, which might result in the exclusion of N. japonica from the lower shore.
