DIGESTION OF ALGAE BY STEGASTES NIGRICANS AND AMPHIPRION AKINDYNOS (PISCES, POMACENTRIDAE), WITH AN EVALUATION OF METHODS USED IN DIGESTIBILITY STUDIES

An investigation of the digestive physiology of two species of damselfish (family Pomacentridae) was undertaken, comparing a herbivore Stegastes nigricans with an omnivore/planktivore Amphiprion akindynos. Apparent digestibility of Enteromorpha flexuosa (Chlorophyta) was calculated using both total collection and a range of ash-marker methods. In addition, Van Soest's system of fibre analysis was used to determine the digestibility of algal cell walls. A comparison of total collection methods (comparing food intake v. faeces output) with ash-marker methods showed that the calculated digestibility estimate will depend on the method used. Total dry matter digestibilities varied from 75 to 20% for S. nigricans and from 79 to 15% for A. akindynos. Comparable ranges were obtained for organic matter, cell wall, nitrogen and soluble carbohydrate. Digestibility estimates calculated using total collection methods were larger and less variable than estimates calculated using ash-marker methods. Despite major methodological variation, there was no significant difference in the digestibility of algae between the two species.