Acanthuridae is a family of reef fishes that includes many of the herbivorous fishes commonly found on coral reefs. It contains members of the genera Acanthurus, Ctenochaetus, Naso, and Zebrasoma. In general, Acanthurus spp. and Zebrasoma spp. are considered grazing fishes that consume filamentous turf algae. Ctenochaetus spp. generally consume a lot of detritus, and Naso spp. are more likely to eat large fleshy macroalgae or even be omnivorous. Collectively referred to as surgeonfishes (a name that alludes to the scalpel like blade on the base of their tail) or tangs, these fishes are incredibly important members of coral reef communities. They exert top down control on the growth of algae that has the potential to overgrow corals. They also remove sediment from shallow reef environments where it can build up and negatively impact benthic organisms, and in some cases, they transport it to deeper water, which improves the quality of the shallow habitat (Goatley & Bellwood 2010).
Yesterday I was fortunate enough to dissect my first Zebrasoma flavescens, or yellow tang. While there isn’t much information about its diet specifically, this fish is often considered an herbivore with the assumption that it consumes turf algae like the other Acanthurid grazers. Brandl et al. (2015) found that other members of its genera such as Z. scopas and Z. veliferum consumed a majority of filamentous and thallate algae. Within the Kahekili Fisheries Management Area here in Hawaiʻi, Kelly et al. (2016) observed that Z. flavescens primarily consumes turf algae as well. Imagine my surprise then when I opened up their intestines yesterday and found them packed with detritus and sediment!
Though this is preliminary speculation, it appears that the diet of Z. flavescens may be far more similar to that of C. strigosus than A. triostegus. Interestingly, Kelly et al. (2016) looked at the composition of C. strigosus’s diet in addition to bite observations and found that more than half of the gut contents were detritus with a large portion of sand as well. This suggests that though C. strigosus are seen feeding on turf algae, they are likely combing through it to ingest detritus and sediment – a finding that has been supported by other studies focusing on its sister species C. striatus. In Kelly et al. (2016) the bite observations for C. strigosus and Z. flavescens are quite similar, and like most other surgeonfishes, Z. flavescens primarily took bites from turf algae. It seems, however, that Z. flavescens may also be targeting the detritus within the turf rather than the turf algae itself. This would suggest a significant difference in diet between Z. flavescens and its congeners Z. veliferum and Z. scopas. For those of us that get jazzed about fish guts, that’s pretty neat!
This all supports one of the main questions behind my dissertation research, which is what is an herbivore!?! To assist in management, classification, and analyses scientists like to neatly organize species into functional groups. This helps to look for patterns and understand the underlying processes in a context that is broader than may be possible with individual species. The problem with this approach is that we tend to perpetuate the preconceived identity of some organisms without having a closer look and really understanding the complexity of any species’ functional role. There is an ecologically significant difference between a fish that consumes detritus, a fish that consumes turf algae, and a fish that consumes macroalgae, yet we often fail to differentiate between these types of organisms. Through my work I hope to contribute to a better understanding of what those differences might be and help to disentangle the messy world of herbivores! For now, I only have speculation based upon limited observations of Z. flavescens guts, but I look forward to sorting through more digested detritus to get to the bottom of this story.
For more information on the studies that I referenced check out these awesome papers:
Brandl SJ, Robbins WD, Bellwood DR (2015) Exploring the nature of ecological specialization in a coral reef fish community: morphology, diet, and microhabitat use. Proc. R. Soc. B 282: 20151147.
Goatley CHR, Bellwood DR (2010) Biologically mediated sediment fluxes on coral reefs: sediment removal and off-reef transportation by the surgeonfish Ctenochaetus striatus. Marine Ecology Press Series 415: 237–245.
Kelly ELA, Eynaud Y, Clements SM, Gleason M, Sparks RT, Williams ID, Smith JE (2016) Investigating functional redundancy versus complementarity in Hawaiian herbivorous coral reef fishes. Oecologia 182:1151–1163.