Patterns of habitat and food utilization in two coral-reef sandperches (Mugiloididae): competitive or noncompetitive coexistence?

Abstract

Use of food and habitat of the two sympatric, territorial mugiloidid fishes Parapercis polyophthalma (Cuvier) and P. clathrata Ogilby, which have similar body size and morphology, were examined in the fringing coral reef area at Iriomote Island, one of the Ryukyu Islands of Japan. Both species preyed mainly on decapod crustaceans, and the dietary overlap value was intermediate (0.528). Differences in resource utilization between the two species were conspicuous in habitat rather than in food. ≈60% of the P. polyophthalma population was found on sandy flats of the reef flat zone, while > 90 % of the P. clathrata population occurred at channel areas. In addition to this macrohabitat segregation, both species showed a substantial microhabitat separation at channels where they coexisted with nearly the same densities: P. polyophthalma was numerically dominant on a sandy rubble substratum, whereas P. clathrata was abundant on a rubble substratum. A reciprocal removal experiment demonstrated that this microhabitat separation was an outcome of current competitive interactions between them. P. polyophthalma is an aggressively dominant species and so can predominantly occupy its optimal microhabitat i.e., sandy rubble substratum, at channels, while the subordinate P. clathrata is constrained to shift its microhabitat to a rubble substratum unsuitable for the dominant. Although a density-manipulation experiment for the macrohabitat segregation was not carried out in this study, circumstantial evidence suggested that the segregation is probably attributable to differences in habitat adaptation between the species which need not be the consequence of competition. These results indicate that the structure of the two-species guild may be determined by both competitive and noncompetitive mechanisms.