Defense of a multi functional territory against interspecific intruders by the damselfish Stegastes nigricans (Pisces, Pomacentridae)

DEFENSE OF A MULTI FUNCTIONAL TERRITORY AGAINST INTERSPECIFIC INTRUDERS BY THE DAMSELFISH STEGASTES NIGRICANS (PISCES, POMACENTRIDAE) A LEXANDRA M. H AMB Integrative Biology, University of California, Berkeley, California 94720 USA Abstract. Animals exhibit territorial behavior to defend essential resources such as food or offspring. Interspecific aggression is linked with territoriality and this behavior has been observed between species with overlapping ecology. This behavioral study complements existing research on interspecific aggression in vertebrates, specifically reef fish. It examines the possible functions of territoriality and motivations for aggression towards heterospecifics without ecological overlap. The territorial damselfish, Stegastes nigricans is an algae farmer that defends an individual territory used for a food supply, a breeding site, and shelter. Territorial interactions between S. nigricans and other fish species were examined through behavioral observations in Mo’orea, French Polynesia. Aggressive interactions between S. nigricans and intruding fish were analyzed based on four main factors: intruder species, diet, activity, and location in S. nigricans colony. The effect of S. nigricans colony size on the exclusion of intruders was also examined. Stegastes nigricans exhibited the most aggression towards two groups of intruders: non­algae eaters feeding in the interior of the colony and algae eaters swimming in the interior of the colony. It was found that large colonies of S. nigricans were not more effective at excluding intruders than small colonies. The three factors that were the most indicative of whether S. nigricans reacted aggressively towards an intruder were intruder species, diet, and location in the colony. Two common themes of interspecific aggression were found to be the defense of a food resource and the protection of reproductive assets. Key words: reef fish; Stegastes nigricans; Pomacentridae; interspecific aggression; Mo’orea, French Polynesia; territoriality I NTRODUCTION Territoriality can be defined as any behavior on the part of an animal, which tends to confine the movements of that animal to a particular locality (Etkin 1964). The emergence of territoriality can be related to essential life processes such as resource acquisition and the need to exclude other members from the area where these activities occur (Etkin 1964). Because of this, territorial behavior is intricately linked with agonism and animals maintaining territorial boundaries do so via aggressive actions (Etkin 1964). Interspecific aggression is an important aspect of territorial behavior in birds, fish and mammals (Thresher 1976). Interspecific interactions occur when two non­related species compete over a resource such as food or shelter (Thresher 1976). This type of behavior has been observed in species with overlapping ecology and the result of this interaction is the exclusion of one of the species by the other (Simmons 1951). It is thought that the primary function of interspecific territoriality is the protection of food resources from heterospecific competitors (Ebersole 1977). One ecological factor, such as the maintenance of a reliable food supply, can act as a selective force leading to the evolution of interspecific territoriality (Orians and Willson 1964, Low 1971). This theory can account for the positive correlation between increases in aggression and increases in diet overlap between species (Low 1971, Hixon and Brostoff 1983, Harrington and Losey 1990, Letourneur et al. 1997, Bay et al. 2001, Gochfeld 2010). Previous studies on reef fish have examined interspecific aggression by territorial species on non­territorial species (Losey 1982). The amount of time and energy spent engaged in interspecific defense demonstrates its importance in the ecology of fish (Low 1971, Myrberg and Thresher 1974). In contrast to birds, where interspecific aggression can be a case of mistaken identity, territorial fish are defending a residence against species with