Food habits of the farmer damselfishStegastes nigricansinferred by stomach content, stable isotope, and fatty acid composition analyses

Abstract

AbstractThe territorial damselfish,Stegastes nigricans, maintains algal farms by excluding invading herbivores and weeding unpalatable algae from its territories. In Okinawa, Japan,S.nigricansfarms are exclusively dominated byPolysiphoniasp., a highly digestible filamentous rhodophyte. This study was aimed at determining the diet ofS. nigricansin Okinawa and its dependency on these almost‐monoculture algal farms based on stomach content and chemical analyses. Stomach content analyses revealed that all available food items in the algal farms (i.e., algae, benthic animal inhabitants, trapped detritus) were contained in fish stomachs, but amorphous organic matter accounted for 68% of the contents. Therefore, carbon and nitrogen stable isotope ratios and fatty acid (FA) compositions were analyzed to trace items actually assimilated in their bodies. Stable isotope analyses showed that benthic animals were an important food source even for this farmer fish. Two essential fatty acids (EFAs), 20:4n6 and 20:5n3, which are produced only by rhodophytes among available food items, were rich in the muscle tissue ofS. nigricansas well as in algal mats and detritus, suggesting that algal mats contribute EFAs toS. nigricansdirectly and indirectly through the food web. In conclusion,S. nigricansingested algal mats, detritus, and benthic animals maintained within its farm. Algae and detritus were original sources of EFAs, and benthic animals, which were much more abundant in the farms than in outside territories, provided a nitrogen‐rich dietary source for the fish.