Synopsis
We studied territorial behaviour of the acanthurid surgeonfish Acanthurus sohal and pomacentrid damselfish Plectroglyphidodon leucozona on fringing reefs in Ras Mohammed National Park, Red Sea, Egypt. We investigated trespass times, the number of agonistic acts, their intensity and the agonism elicited (number agonistic acts/total trespass time) for all intruders. Five families (Pomacentridae, Chaetodontidae, Acanthuridae, Labridae and Scaridae) were dominant among the 14 we recorded in our study sites. Seven species belonging to four families were resident inside A. sohal territories. Of the other 66 species we observed, 45 are intruders. The territory-holder did not react equally agonistically toward all these intruders. A. sohal showed a high number of agonistic acts against conspecifics and the parrotfish Cetoscarus bicolor. The intensity of the agonistic encounters, however, showed a different picture, with a low level for conspecifics and C. bicolor, respectively. A. sohal showed a high degree of agonistic intensity toward only one parrotfish, Chlorurus sordidus. The highest values of "˜Agonism elicited' were recorded against Scarus ghobban, followed by Siganus rivulatus and Zebrasoma desjardinii . Despite these high values, the intensity was relatively weak. The territories of A. sohal were large (average 12.5 m²); those of P. leucozona were ca. 20 times smaller (average 0.6 m²). A. sohal spent most of their time swimming (patrolling) along the territory border, stopping briefly to feed on algae. On the other hand, P. leucozona spent most of their time swimming and sheltering
