Feeding ecology of surgeonfishes (Acanthuridae) in the northern Red Sea, with particular reference to Acanthurus nigrofuscus (Forsskål)

Three surgeonfishes (Teleostei:Acanthuridae), inhabiting Red Sea reefs near Eilat, Israel, are similar in size and overlap to varying degrees in use of habitat. They differ in diel cycles of feeding behavior, specifics of habitat use and general strategies used to process food. Ctenochaetus striatus (Quoy et Gaimard) spends ≈30% of its time in active feeding on reefs and sandy substrata within its weakly defended home range, ingests both algae and large quantities of inorganic grit, triturates foods in a gizzard-like stomach, fails to retain a bolus of food in the posterior intestine overnight and generally engages in more complex social interactions than do the other species. In contrast, Acanthurus nigrofuscus (Forsskål) and Zebrasoma xanthurum (Blyth) feed 55-70% of the time, do not ingest large quantities of grit, lack the gizzard-like stomach, retain a bolus of food overnight in the posterior gut and exhibit social systems much different than C. striatus. A. nigrofuscus contrasts with the other species by feeding in the shallows next to shore, rather than on fringing reefs, and by forming much larger feeding groups. Groups of A. nigrofuscus feeding at two sites differed in group size, diel cycles of feeding activity and total feeding time·day^-1. These differences do not appear to correlate with differences in food availability or selectivity in feeding. They do correlate with distance from shelter to feeding sites and degree of exposure on the feeding grounds. Body condition (robustness) of A. nigrofuscus declines during summer and autumn despite active feeding, indicating that apparent maximization of feeding time may be insufficient to meet energy and nutrient demands during all seasons. Local differences in physical and biotic conditions on reefs probably serve as a powerful selective pressure for behavioral and ecological plasticity in reef fishes with vagile larvae.