Microhabitat segregation and cannibalism in an endangered freshwater isopod, 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, however, is poorly understood. In this paper, we examine microhabitat use, cannibalism, and individual responses to the presence of conspecifics in Thermosphaeroma thermophilum, an endangered isopod crustacean species that is endemic to a single, thermal spring in Socorro, N.M. USA. In samples from the natural habitat, juveniles (mancas) were found mainly 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, males 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, therefore, that cannibalism in T. thermophilum generates age-, size-, and sex-specific predation risks which are responsible for microhabitat segregation between mancas and adults, and between males and females. Since interspecific predation in the spring is negligible, cannibalism appears to play a significant role in population regulation and behavioral evolution in this species. We recommend, given the current "endangered" status of this species, that microhabitat heterogeneity be maintained in its native spring because it provides refuges from cannibalism and may support a larger and more viable natural population.