Intra- and Interspecific Aggression By the Central American Midas Cichlid Fish, Cichlasoma Citrinellum

Intra- and interspecific aggression by the Central American cichlid fish, Cichlasoma citrinellum, was studied to examine the similarity in aggression toward different species and the degree to which interspecific responses depended on intraspecific cues. One hour encounters were staged between C. citrinellum focal individuals, and subordinate opponents of the same species, C. zaliosum, Pseudotropheus zebra and Haplochromis polystigma. C. zaliosum is a strikingly similar and sympatric species while P. zebra and H. polystigma are morphologically dissimilar allopatric species. Attention was focussed on the effects of species similarity and feeding cues on interspecific aggression. Analyses were based on the relationship between stimuli from opponents and responses by the focal individual. A "conditional chance" technique was developed for this purpose and its reliability was tested against a computerized agonistic exchange with known parameters. The conditional chance method proved to be more accurate than traditional chi-square or information theoretical measures. At least a portion of the causation of interspecific aggressive response depends on the same cues as intraspecific behavior. The intensity of the aggressive response decreased with increasingly dissimilar opponents, but the qualitative response characteristics remained largely the same. Notable exceptions were a failure to honor an apparent appeasement signal, Lateral Roll, when given by another species and a lack of aggressive response when a strongly dissimilar species withdrew or fled. Differences were found in the aggressiveness of response by different focal individuals. However, focal individuals disagreed most strongly in their response to the similar species, C. zaliosum. Two responsed very aggressively and, while support for the hypothesis is weak, they may have perceived their opponents as odd conspecific fish. The others showed less aggression and their response resembled that shown to dissimilar species. Observing the opponent feed acted as an experiential cue and served to increase the aggressiveness of response to all subsequent stimuli from that opponent. While the duration of this increase and the effects of different feeding types are not known, response to ecological cues such as feeding could help to focus "mistaken-identity" aggression on competitors. This could be a first evolutionary step toward minimizing the maladaptive effects of interspecific aggression without forsaking t