A bothersome fish might be the cause of bleached tropical coral off the coast of the Florida Keys, as indicated by research conducted by the University of Georgia.

The rainbow parrotfish is a stunning teal green and yellow fish commonly encountered in the Atlantic from Florida to Argentina.

The fish’s beak-like mouth could be responsible for the annual coral bleaching in Buttonwood Sound during April and May, a period not typically associated with the elevated temperatures usually blamed for bleaching.

The investigation revealed that over one in three corals in regions habitually visited by parrotfish were bleached, with around a quarter of the bleached corals exhibiting bite marks. In locations devoid of the fish, only 3.2% of the corals were bleached, and none of those showed any bite marks.

“What implications does this have for the coral? Well, if I were a coral, I would fear encountering these parrotfish,” remarked William Fitt, the principal author of the study and a professor at UGA’s Odum School of Ecology. “I don’t believe there are any benefits to having parrotfish present for this coral.”

Low algae, numerous bite marks leave corals exposed

The research utilized NOAA data that recorded water temperatures hourly from 2010 to 2017 each April and May. Only three days in May 2017 showed water temperatures high enough to potentially cause coral bleaching.

The bleached and healthy corals revealed no differences in development, growth, or maturation.

However, much of the bleached coral displayed numerous bite marks, resulting in reduced tissue and diminishing the algae that resides within the coral. This stress may be the catalyst for coral bleaching, according to the researchers.

Further examination of the corals also indicated that levels of beneficial algae had diminished during the spring, a season when it would generally increase, hinting that something was causing stress to the coral.

That factor, Fitt suggested, is probably the parrotfish. The fish migrates into the area each year in early spring and departs in the summer, coinciding with the increased coral bleaching.

Coral bleaching may result in disease, starvation, mortality

Coral bleaching occurs when coral is stressed by high temperatures or other conditions.

The normally golden-brown coral expels beneficial algae living in its tissues, transforming into a pale pinkish hue. In healthy coral, this algae nourishes the coral and provides its unique coloration.

Bleaching can be fatal for coral. Even when it’s not lethal, it renders coral susceptible to disease and starvation. Repeated episodes of bleaching, like the annual event in the sound, can ultimately kill the coral.

Fitt compared the predation by parrotfish to that of mosquitoes.

“It might be a micropredator that’s merely irritating,” he stated. “But it’s more than just an annoyance if there’s a million mosquitoes on you. It depends on how many bites you endure.”

Most corals possess stinging cells that discourage potential predators from feasting on them. This specific type of coral likely has fewer of those cells or weaker ones, Fitt noted, clarifying why the parrotfish can easily consume it.

It remains uncertain how widespread this phenomenon is beyond the Keys. However, it is extremely prevalent where Fitt conducts his studies.

Although it is unclear exactly what long-term impact this predation will have on the coral in the area, it likely isn’t beneficial.

“The most critical point is that parrotfish are causing such significant stress to this coral that it’s producing a lot of this pink coloration, which could signify major distress for the coral,” Fitt explained. “If you observe a pink coral, that coral is facing problems.”

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