MICROHABITAT SEGREGATION AND CANNIBALISM IN AN ENDANGERED FRESH-WATERISOPOD, THERMOSPHAEROMA-THERMOPHILUM

Intraspecific microhabitat segregation is expected to arise when there are age- or sex-specific differences in predation risk. The degree to which conspecific predation (cannibalism) can generate this risk, how ever, is poorly understood. In this paper, we examine microhabitat use , cannibalism, and individual responses to the presence of conspecific s in Thermosphaeroma thermophilum, an endangered isopod crustacean spe cies that is endemic to a single, thermal spring in Socorro, N.M. USA. In samples from the natural habitat, juveniles (mancas) were found ma inly on vegetation, whereas adults were found predominantly on bottom sediments. Females were found on vegetation more often than males. In laboratory containers without refuges, males cannibalized females, mal es and females cannibalized mancas, and mancas cannibalized each other , even in the presence of alternative food. When placed in containers provided with refuges, mancas actively avoided adults. We suggest, the refore, that cannibalism in T. thermophilum generates age-, size-, and sex-specific predation risks which are responsible for microhabitat s egregation between mancas and adults, and between males and females. S ince interspecific predation in the spring is negligible, cannibalism appears to play a significant role in population regulation;Ind behavi oral evolution in this species. We recommend, given the current ''enda ngered'' status of this species, that microhabitat heterogeneity be ma intained in its native spring because it provides refuges from canniba lism and may support a larger and more viable natural population.