Salvador Zarco-Perello , Andrew Hoey, Storm Martin

Feeding habits of herbivorous fish play an important role in the form and function of coastal marine ecosystems. Rabbitfishes (Siganidae) have been recognized as important consumers of macroalgae in the coral reefs of the Indo-Pacific region. However, it is unclear how their diet varies among and within species at biogeographical scales. The present study assessed the inter- and intra-specific diet variation of rabbitfishes (Siganus trispilos, Siganus coralinus, Siganus virgatus, and Siganus doliatus) factored by phylogenetic and morphological relatedness among populations from Ningaloo (western Australia), the Great Barrier Reef (GBR, eastern Australia) and Okinawa (Japan). Results showed a strong effect in diet by geographic distribution, effectively reducing the expected effect of phylogenetic and morphologic relatedness. While intra-specific differences were only significant when populations inhabited different regions as expected; inter-specific differences were not as predicted, with different morphotypes (non-sister species) having similar diets when populations inhabited the same regions. Differences were driven by higher consumption of corticated and filamentous macroalgae by populations from the GBR, higher consumption of foliose and membranous macroalgae by rabbitfishes in Okinawan reefs and higher diet proportions of leathery macroalgae in populations from Ningaloo reefs. The findings indicate that rabbitfishes possess a relatively high diet plasticity, potentially driven by regional differences in algal resources, and hence their functional role as mediators of competition between macroalgae and corals can change across biogeographic regions. This highlights the importance of considering local context when assessing the diet of herbivorous fishes to know their functional role in the ecosystem accurately. As climate change unfolds, shifts in the distribution, trophic behaviour and function of species are expected, making the study of trophic plasticity more crucial.