Behavioural arousal and its habituation in the squirrel fish, Holocentrus rufus: the role of the telecephalon

Squirrel fish were captured, maintained for 2 days and then tested for their responses to stimuli in laboratory aquaria. A tap on the side of the aquarium or a shadow moving overhead elicited an arousal response involving a reduction in opercular movements, movement of the pectoral fins, and erection of the spiny dorsal fin. This latter response, when regularly provoked by a moving shadow at 15-s intervals, decreased in duration and habituated. When fish were restricted in a wire mesh trough and the stimulus was the onset of illumination to one eye at 15-s intervals, responses were quantitatively similar although shorter in duration. A series of stimuli delivered to one eye until habituation followed by a series to the other eye reduced the duration of initial responses and the number of trials for habituation of responses to stimuli delivered to the second eye. Responses were equivalent regardless of which eye (left or right) was first presented with stimuli. Section of the telencephalic anterior commissure considerably restricted the transfer of information about experience of stimuli presented to the first eye. Commissural section, combined with unilateral lesions in the posterior-medial telencephalon, severely impaired habituation of responses to the contralateral eye. Lateral lesions had no effect. The results are discussed in relation to the visual projection in Holocentrus and telecephalic involvement in arousal and its habituation in teleosts as a whole.