Habitat use and diet of two stream gobies of the genus Rhinogobius in south-western Shikoku, Japan

The importance of interspecific competition to habitat use by two congeneri c stream gobies, Rhinogobius sp. LD (large-dark type) and CB (cross-band ty pe), was studied by: (i) examining differences in habitat use by each speci es along the course of the stream; and (ii) comparing microhabitat use and the diet of each species between in allopatry and in sympatry in tributarie s of the Shimanto River, south-western Shikoku, Japan. Rhinogobius sp. LD m ainly used riffles through the course of the stream. Although CB also mainl y used riffles in the lower reaches where CB was numerically dominant, the greater use of riffles by CB was not observed in upper reaches where LD was dominant. Microhabitat analysis revealed that both LD and CB preferred low er bottom velocity in faster current in both allopatry and sympatry. Tn sym patry, however, LD used coarser substrate and faster current than CB, and b oth LD and CB used narrower ranges of microhabitat conditions than in allop atry. Dietary analysis indicated that both allopatric and sympatric LD pref erred baetids, which are apt to drift, suggesting they adopted ambush forag ing. Allopatric CB preferred leptophlebiids and chironomids, which are also apt to drift, whereas sympatric CB did not prefer such invertebrates. Over all results of this study suggest that LD and CB compete for better feeding habitats, CB shift their habitat use and foraging mode under the influence of LD, and current velocity and substrate coarseness are key factors in th eir habitat segregation.