Coexistence: Symbiotic sharing of feeding territories and algal food by some coral reef fishes from the Western Indian Ocean

A symbiotic relationship between a damselfish (Stegastes fasciolatus) and 2 surgeonfishes (Acanthurus lineatus and A. leucosternon) is described. The damselfish, which is about 1/10 the size of the surgeonfishes, is concentrated in, and appears to prefer to be in, the feeding areas of the surgeonfishes. There is an average of one adult damselfish per adult surgeonfish feeding area. All 3 species defend their feeding areas against conspecifics and various other fishes that have similar diets, but there are few aggressive interactions between the cohabitants. Most of the interspecific defense of cohabited areas is provided by the surgeonfishes. The cohabitant species eat the same types of benthic microalgae, but only the damselfish eats microcrustaceans. It appears that the net cost to a surgeonfish of having a damselfish in its feeding area is very low because (1) biomass density of the damselfish is low, (2) it uses some food that the surgeonfishes do not use, and (3) it makes a small contribution to the defense of shared feeding areas. Even if there is a cost to the surgeonfish, the small size of the damselfish would allow it to take shelter from the surgeonfish, thus making it too costly for the surgeonfish to exclude it. Such cohabitation relationships, which may be quite common among reef fishes, represent a means by which the coexistence of species that use the same limiting resources is achieved.