Food and microhabitat resource use by Rudarius ercodes and Ditrema temmincki coexisting in a Zostera bed at Aburatsubo, central Japan

Patterns of food and microhabitat utilization by two dominant species, the filefish, Rudarius ercodes, and the surfperch, Ditrema temmincki, in a Zostera bed were investigated from May through December in 1993 and 1994. In May, the filefish took mostly caprellid amphipods, detritus and polychaetes, while the surfperch consumed mainly cyclopoid copepods, detritus and gammaridean amphipods. In and after July, however, the contribution of detritus to the diets of both species increased greatly. Thus, the dietary overlap was lowest in May, but subsequently increased until November. Throughout the study period, both species were uniformly distributed horizontally from the margin to the center of the seagrass bed, but not vertically relative to the substrate and seagrass canopy. In June 1993 and 1994, vertical (i.e., microhabitat) distributions of both species were similar. After June, surfperch occurred more frequently in the upper parts of the water column, while filefish were abundant in the lower parts. In contrast to the dietary overlap, the microhabitat overlap was at a maximum in June, subsequently decreasing to a minimum in December. In 1994, when the surfperch density was lower than that in 1993, the filefish significantly increased in number and tended to occur more often in the upper parts of the water column, where it had been infrequently seen the previous year. These results suggested that resource partitioning resulting from interspecific competition plays a major role in the coexistence of R. ercodes and D. temmincki in the seagrass bed.