A role for encrusting, endolithic sponges in the feeding of the parrotfish Scarus rubroviolaceus? Evidence of further trophic diversification in Indo-Pacific Scarini

The speciose scarinine clade of coral reef parrotfishes display significant variation in trophic cranial morphology, yet are often described as generalist herbivores. The hypothesis that many parrotfishes target micro-photoautotrophs is a new framework within which to clarify parrotfish diets. Here, we investigate the dietary targets of Scarus rubroviolaceus using the feeding substrata extraction method and then compare the results to fourteen other syntopic parrotfish species. Scarus rubroviolaceus were followed on snorkel until repeated biting was observed. A 22 mm × 20 mm core was extracted around the bite. We identified and quantified the bite core biota by scraping the top 1 mm from bite cores for microscopy and 16S/18S small subunit rRNA metabarcoding. Filamentous cyanobacteria density on S. rubroviolaceus bite cores did not differ from the other fourteen parrotfish species, Calothrix (Nostocales) being the most frequently observed filamentous cyanobacteria for all fifteen parrotfish species. The 18S metabarcoding analysis detected the encrusting, endolithic sponge taxon Clionaida in the S. rubroviolaceus bite cores. We investigated the possibility of spongivory across all fifteen parrotfish species including an analysis of sponge-associated microbiota detected on the bite cores. This revealed a new axis of trophic partitioning with varying levels of spongivory amongst the fifteen Indo-Pacific parrotfish species. The bite cores of Cetoscarus ocellatus, Chlorurus spilurus, Chlorurus microrhinos, Scarus frenatus and S. rubroviolaceus particularly indicated spongivory. Our findings develop our understanding of parrotfish diet and provide further evidence that parrotfishes are specialized feeders and partition benthic trophic resources.