Fighting off the intruder: context-dependent territory defence in the damselfish Stegastes fuscus

Abstract

Dusky damselfish (Stegastes fuscus) behaviour was investigated under natural and controlled conditions to evaluate how territory and familiarity affect aggressive behaviour. In the natural environment, fish occupies territories of 2.74 ± 1.3 m2 and the most frequent behaviours exhibited were monitoring swimming, feeding, shelter occupation and agonistic interactions. Larger-territory individuals spend more time in monitoring swimming than smaller-territory ones, which results in a reduction of food intake. Agonistic interactions were more frequent with heterospecifics than conspecifics. Whenever agonistic interaction occurred, confrontations were not frequent. Territorial behaviour in the lab was evaluated considering two variables: prior residence and familiarity between conspecific opponents. Resident fish invested more against intruders irrespective of the intruder’s identity (familiar or unfamiliar). When no previous territory was established, S. fuscus showed lower aggressive behaviour, and it was even lower when a familiar fish was the opponent. Data from the field and the lab suggest that S. fuscus territoriality and aggression are related to the size of the defended area and the competitor’s identity. Thus, reduced reef areas may result in increased interindividual aggression, while the intruders’ recognition may be a key trait controlling agonistic behaviour in the reef community where S. fuscus is inserted.