Eye as a key element of conspecific image eliciting lateralized response in fish

Visual lateralization in different aspects of social behaviour has been found for numerous species of vertebrates ranging from fish to mammals. For inspection of a shoal mate, many fishes show a left eye-right hemisphere preference. Here, we tested the hypothesis that in fish, there is a key cue in the conspecific appearance, which elicits lateralized response to the whole image of the conspecific. In a series of eight experiments, we explored eye preferences in cryptic-coloured Amur sleeper, Perccottus glenii, fry. Fish displayed left-eye preferences at the population level for inspection of a group of conspecifics, their own mirror image, and a motionless flat model of a conspecific. In contrast, no population bias was found for scrutinizing an empty environment or a moving cylinder. When fry were showed a model of a conspecific in a lateral view with the eye displaced from the head to the tail, they again showed a significant preference for left-eye use. On the other hand, 'eyeless' conspecific model elicited no lateralized viewing in fry. Finally, the left-eye preference was revealed for scrutiny of the image of a conspecific eye alone. We argue that in Amur sleeper fry, eye is the element of the conspecific image, which can serve as a 'key' for the initiation of lateralized social response. This key element may serve as a trigger for the rapid recognition of conspecifics in the left eye-right hemisphere system. Possible causes and advantages of lateralized perception of social stimuli and their key elements are discussed in the context of current theories of brain lateralization.