Abstract
Unisexual sperm-dependent species depend on a sexual host for reproduction, and must live in sympatry with their sperm donor. If niche overlap between the species is substantial, the intrinsic faster population growth of the unisexual over sexual species can cause competitive exclusion of the host from resources, causing the demise of the unisexual species. However, theoretical models predict that coexistence is possible, even without niche differentiation, if the unisexual species is a poorer competitor than the sexual host and if the effect of the unisexual species on the exploitative abilities of the sexual species is smaller than the effect that the sexually reproducing individuals have among themselves. We tested these predictions in the unisexual-bisexual mating complex of Poecilia formosa, and one of its sexual hosts, P. latipinna. Fishes were housed from parturition for 76 days with both conspecific and heterospecific individuals under both limited and ad libitum food regimes. Only one of the predictions of the models was met: the effects of intraspecific competition for P. latipinna were stronger than the effects of interspecific competition. Poecilia latipinna raised with conspecifics with limited food stored fewer fats than both heterospecific P. formosa, and conspecifics raised in any other treatment.
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