Biogeographical diet variation within and between the rabbitfishes Siganus corallinus, Siganus doliatus, Siganus trispilos and Siganus virgatus

Feeding habits of herbivorous fishes play an important role in shaping form and function of coastal marine ecosystems. Rabbitfishes (Siganidae) are important consumers of macroalgae on Indo-West Pacific coral reefs. However, it is unclear how their diet varies among and within species at biogeographical scales, casting doubt on their precise functional roles across different regions. The present study assessed the inter- and intraspecific diet variation of four rabbitfishes (Siganus trispilos, Siganus coralinus, Siganus virgatus, and Siganus doliatus) factored by morphological relatedness among populations from Ningaloo Reef (western Australia), the Great Barrier Reef (GBR, eastern Australia) and the Yaeyama Islands (Okinawa Prefecture, Japan). Results showed that region had a strong effect on diet, effectively reducing the expected effect of morphologic similitude. While intraspecific differences were only significant when populations inhabited different regions; interspecific differences were not as predicted, with different morphotypes having similar diets when populations inhabited the same regions. Rabbitfishes consumed more corticated and filamentous macroalgae on the GBR, more foliose and membranous macroalgae at the Yaeyama Islands, and more leathery macroalgae at Ningaloo Reef. The findings indicate that rabbitfishes have high diet plasticity, and hence their functional role as mediators of competition between macroalgae and corals can change across biogeographic regions. Local context is therefore important when assessing the diet and functional role of herbivorous fishes. As climate change unfolds, shifts in the distribution, trophic behaviour and function of species are expected, making the study of trophic plasticity more important.

Methods
Species of study and diet information

The study focused on four closely-related species of rabbitfish (Siganus virgatus, S. doliatus, S. corallinus, and S. trispilos; Siqueira, Bellwood & Cowman 2019) that have populations in three regions set apart by thousands of kilometres: Okinawa (Yaeyama Islands), the Great Barrier Reef (GBR; Turtle Group and Lizard Island), and Ningaloo Reef (Coral Bay). Specimens of S. virgatus and S. trispilos were collected by spear [Murdoch University ethics permit number R3349/21 and Department of Fisheries (DPIRD) exemption 3699] between 9 am and 11 am in backreef and lagoon habitats of Bateman (23°02'28"S, 113°47'04"E), Five Fingers (23°10'54"S, 113°45'51"E) and Yalobia (23°12'22"S, 113°45'33"E) reefs, Ningaloo Reef, Western Australia, during June 2022 and March-April 2023. Captured specimens were euthanised, stored in ice, and dissected fresh at the Coral Bay Research Station (Murdoch University). The alimentary tract was removed, dissected open, and the gut content preserved in a solution of 80% ethanol. Diet data from these samples were collected in the laboratory using a stereo microscope, similarly to previous studies (Nanami 2018). Food items were spread evenly over a petri dish divided into a grid of 100 cells, identified to the lowest taxonomic level possible and assigned a proportion of the total gut content based on the proportional number of grid squares covered using the program ImageJ (rsb.info.nih.gov/ij/). Macroalgae in the diet were grouped into morpho-functional groups: Filamentous/corticated, foliose/membranous, leathery, and calcareous. Categories other than macroalgae comprised seagrass, cyanobacteria, phytoplankton, mobile zoobenthos, sessile invertebrates, detritus and sediment. Diet information for the populations of rabbitfishes from the GBR, was obtained from a diet database created by Hoey et al. (2013). Diet data from Okinawa were obtained from Nanami (2018) using the software Datathief III (Flower, McKenna & Upreti 2016).

References

Hoey, A. S., Brandl, S. J., & Bellwood, D. R. (2013). Diet and cross-shelf distribution of rabbitfishes (f. Siganidae) on the northern great barrier reef: Implications for ecosystem function. Coral Reefs, 32(4), 973–984. https://doi.org/10.1007/s00338-013-1043-z 

Nanami, A. (2018). Spatial distributions, feeding ecologies, and behavioral interactions of four rabbitfish species (Siganus unimaculatus, S. virgatus, S. corallinus, and S. puellus). PeerJ, 6, e6145. https://doi.org/10.7717/peerj.6145