Cross-species familiarity in shoaling fishes.

Preferential association with familiar shoal mates confers a number of potentially important benefits to individuals, including improved anti-predator effects and the reduction of aggression in competitive interactions. Until now, however, familiarity has been demonstrated purely between conspecifics. Here, we present evidence that familiarity preferences can override natural preferences for conspecifics. Individual focal fishes (chub, Leuciscus cephalus) were given a choice of two stimulus shoals of the same size composed of conspecifics or of heterospecifics (minnows, Phoxinus phoxinus) in a flow tank. A series of four treatments was carried out to investigate the effects of familiarity, induced by a 15 day association between the focal fish and the stimulus fishes, on the choices made by the focal fish. Focal fishes showed a significant preference for conspecifics over heterospecifics when both stimulus shoals were composed of non-familiar individuals. Focal fishes also showed a significant preference for stimulus shoals composed of familiar fishes over stimulus shoals composed of non-familiar fishes when both shoals were conspecific and when both shoals were heterospecific. Finally, the preference of focal fishes for conspecifics disappeared when the alternative, a shoal of heterospecifics, was composed of familiar individuals. The importance of this work is discussed in the context of species interactions in free-ranging shoals.