FORAGING BY THE STOPLIGHT-PARROTFISH SPARISOMA-VIRIDE .1. FOOD SELECTION IN DIFFERENT SOCIALLY DETERMINED HABITATS

Food selection by the Caribbean stoplight parrotfish Sparisoma viride was investigated on a fringing coral reef of Bonaire, Netherlands Anti lles. For different reef zones, the diet composition for each life pha se was determined by description of randomly selected bites, and compa red to the availability of food resources, as determined with the aid of chain-link transects. S. viride employs an excavating grazing mode, and feeds almost exclusively on algae associated with dead coral subs trates. Preferred food types are large and sparse turfs growing on car bonate substrates inhabited by endolithic algae. Crustose corallines, with or without algal turfs, are not preferred. Feeding forays were lo nger on the preferred food types. Foraging preferences are related to nutritional quality of the food types and their yield, i.e. the amount s of biomass, protein and energy that can be ingested per bite, as cal culated from the size of grazing scars and the biochemical composition of the algae. In spite of selective foraging, a large proportion of b ites is taken on inferior food types. Endolithic algae constitute an i mportant food resource for scraping herbivores, such as S. viride, The se algae have relatively high energetic value, and allow a high yield as a result of weakening the carbonate matrix by their boring filament s. The yield of algal resources also depends on the skeletal density o f the limestone substrates. On deeper reef parts (> 3.5 m depth), low- density substrates predominate, resulting in higher yields of algae pe r bite than are attained from high-density substrates that predominate on shallower reef parts. The increased availability of high-yield foo d and substrate types coincides with the occurrence of haremic territo rial behaviour in S. viride males on the deeper reef parts. Territorie s are defended against conspecifics and have an important function as spawning sites. It is argued that the access to superior food resource s on the deeper reef makes territorial defence feasible for S. viride.