Microhabitat use by bisexual and unisexual fishes (Poeciliopsis: Poeciliidae) in an artificial stream.

Two all-female species of viviparous fish depend on males of bisexual species for sperm in order to reproduce. The unisexual P. monacha-latidens mates with males of P. latidens (the sexual host) while the unisexual P. monacha-lucida mates with males of P. lucida (its sexual host). Ecological divergence of each unisexual from its sexual host is promoted by any competitive interactions while ecological convergence of the unisexual toward its sexual host is promoted by the unisexual's sperm requirements. This study measured the habitat use in an artificial stream of five Poeciliopsis species (P. monacha-latidens, P. monacha-lucida, P. latidens, P. lucida and P. monacha) when alone and in several combinations. The habitat use of both unisexuals was examined for evidence of 1) competition with their sexual hosts, 2) use of monacha-like behavior, and 3) competition with each other. The two unisexuals interacted differently with their sexual hosts. P. monachalatidens competed with P. latidens. These two species differed more in their habitat use when they were sympatric than when they were allopatric. They frequented different habitats when they were sympatric, P. latidens preferring fast-water areas and P. monacha-latidens preferring slowwater areas. The distribution of P. monacha-latidens was more similar to P. monacha when the unisexual was with its sexual host than when it was alone, indicating that this unisexual may use monacha-like behavior to escape from competition with its sexual host. In contrast, P. monachalucida did not compete with P. lucida. These two species were more similar in their habitat use when they were sympatric than when they were allopatric. The habitat use of P. monacha-lucida when alone and when with P. lucida showed no consistent pattern when compared with the habitat use of P. monacha. Apparently, P. monacha-lucida did not use monacha-like behavior when with its sexual host. The two unisexuals did not compete. Thus, forces favoring ecological divergence are more important for P. monachalatidens and forces favoring ecological convergence are more important for P. monacha-lucida.