It turns out that more than one is an herbivore.
Living among the world’s coral reefs, there are fishes that eat algae. We should call them herbivorous fishes, though usually that indicates plants in most people’s minds. Among the algae grazers are damselfishes, parrotfishes, blennies, and surgeonfishes. Herbivorous fishes are necessary for a healthy reef. In consuming the algae, they help maintain corals’ dominance. Corals without these herbivores can be overwrought with algae. However, these fishes don’t all swim about cleaning off overgrown algae on the reef; it’s more detailed and amazing.
Some damselfishes tend to an algae garden. They plant their garden, eating their preferred food and pooping the spores onto their future garden plot. The fish will spend time weeding, picking out the algae it doesn’t like, so it’s preferred algae has more room to grow. These fishes also valiantly and aggressively defend their garden plots. They will nip and attempt to chase away those that stray too close, including larger fishes and the huge, in their eyes, human snorkelers that float above and peskily dive down to get a closer look at the coral head that has an unnoticed plot of algae garden off to the side.
The aggressive territoriality of Damselfishes leads to higher algal species diversity. The fish tend to keep away algal grazers that would grind the growers to nothing while themselves damping down the dominant algae species, allowing other species to grow.
Interestingly, parrotfishes should be considered omnivores since in every bite of the coral animal, they consume the symbiotic algae housed within and the coral flesh. Parrotfishes also graze on dead coral, scraping the growing algae with their parrot-like teeth.
The reef houses a large number of small blennies and gobbies. Many are predators whose contribution to the ecology of the reef is underrepresented. Several of these fish groups chow down on algae, among them, the Highfin Blenny, Lawnmower Blenny, and Rainford's Goby. The array of algae eaters is well-known to aquaria enthusiasts as these fishes can be used to control algae growth in their tanks.
The surgeonfishes are a common and delightful sight on the reef. Many have a sharp set of spines (scalpel-sharp, thus the name surgeonfish) on their caudal peduncle, used for defense. The family of these fishes is Acanthuridae, which makes reference to this sharpness; acanthus from the root ak (sharp in Latin) or ake (point/thorn in Greek). Some of the algae-eating species feed on algae growing on other fishes and thus act as cleaner fish.
There are other fishes that eat algae; one new addition is Batfish. This common name is problematic as it is used for other species of fish (see images below). Platax pinnatus is the batfish found to feed on algae. It’s in the spadefish family, Platax pinnatus. Previously known as a feeder on small invertebrates, this species fed on algae in macroalgal domination sections, enabling a transition to more coral being present. Unfortunately, these fish are rare and are not going to save the coral reefs from algae overgrowth.
In the study (Bellwood 2006) that discovered the batfish feeding on algae, 43 other herbivorous fishes on the reef were shown to not “control” the algae growth…Hmmm…let me adjust and change directions to an earlier statement that herbivorous fishes help maintain a healthy reef. The overgrowth of algae on the reef is not damped down primarily by fishes (or secondarily) but by the invertebrates, who are less discerning grazers, especially the urchins. The long-spine urchin, Diadema antillarum had yet another bad year in 2022; they had a really bad year in 1983 with a mass mortality event first recognized off Panama that quickly spread via currents and following boat traffic lanes to the whole of the Caribbean. The urchin populations have never fully recovered. There lies the possible algae overgrowth problem: fewer urchins. But take heart: the warming oceans will kill everything much sooner, so overgrowing algae will be the least of our problems.
Interestingly, when Diadema antillarum populations dropped, the blue tang (Surgeonfish family, an algae eater) population increased; this may have been due to the increase in the algal resource.
If you want to see algal eaters, head to the tropics, specifically the coral reefs. The relative number of fishes that graze on algae decreases at higher latitudes. I’ve stuck with marine fish today, but if you truly want to see herbivorous fish, go to the tropical freshwaters. As aquarium enthusiasts know, freshwater systems have many choices for algae-eating fish.
I'm ending with a reversion back to the batfish. A number of fishes have the common name batfish. Here are images of some of them:
My favorite batfish crawls along the bottom. very un-fish like
Bellwood DR, Hughes TP, and Hoey AS. Sleeping Functional Group Drives Coral-Reef Recovery. Current Biology 16(24): 2434 – 2439.
Roberts C. 2019. Reef Life: An underwater memoir. Profile Books.
Robertson DR. 1991. Increases in surgeonfish populations after mass mortality of the sea urchin Diadema antillarum in Panamá indicate food limitation. Marine Biology 111(3): 437–444
Tyson, Nd. 2022. Starry Messenger: Cosmic Perspectives On Civilization. New York: Henry Holt and Company. (Audiobook).