This awkward fish works harder than you

The ocean is full of strange creatures. The parrotfish is no exception.

Its teeth are fused into a sharp beak, giving it a birdlike appearance. It’s hermaphroditic, changing sex partway through its life. And to sleep, some parrotfish engulf themselves in a mucus cocoon.

Odd and awkward-looking as it may be, this creature is a true hero of the ocean.

Rising global temperatures, various diseases, and coastal development have been killing off the world’s coral reefs, iconic ecosystems that support as much as a quarter of all marine life. By some estimates, the live area of coral globally has declined by half since the 1950s.

But the situation would almost certainly be worse if it weren’t for parrotfish.

Parrotfish are essentially janitors who are very good at their jobs. While cruising around the reef, these animals — which live in oceans all over the world — scrape colonies of bacteria and algae off rocks using their beaks. If left unchecked, that algae can grow out of control, smothering reefs and preventing new corals from growing. And that makes it hard for reefs to recover after a bout of, say, extreme ocean warming kills off a bunch of coral. So where you find hungry parrotfish, coral has more room to grow.

The problem is that, on many reefs, the number of parrotfish — and especially large ones in the Caribbean — has plummeted. Other algae grazers like sea urchins, meanwhile, have vanished, too. Some scientists say that’s why Caribbean reefs have failed to recover following climate-related impacts like bleaching and superstorms; there’s simply too much algae for coral to regrow.

On the flip side, these dynamics offer a bit of hope for an ecosystem that seems all but doomed: By protecting parrotfish, alongside efforts to rein in climate-warming emissions, countries might have a better shot at saving reefs.

Reefs are turning green

If there’s one thing people know about coral reefs it’s that they’re colorful — an intricate mosaic of blues, reds, pinks, and oranges. 

But more and more, just one color is starting to dominate: green. 

In step with the decline of coral is the rise of algae, or seaweed. When corals die, this green, plant-like organism grows quickly on top of their skeletons. And as it spreads, that seaweed can prevent corals from regrowing. 

Baby corals, which start their lives swimming in the ocean, need a bit of bare rock to grow on and harden into adults. When the seafloor is covered in algae, larval coral has nowhere to develop. Seaweed can also release chemicals that harm coral and, when it grows abundantly, shade out reefs.      

“The biggest enemy of corals is really seaweed,” said Nancy Knowlton, a marine scientist and author, formerly with the Smithsonian National Museum of Natural History. “It goes without saying that reefs will recover better if they don’t have to deal with lots of seaweed.”

Research shows that in the last 50 years or so, algae has proliferated in coral reefs worldwide, and especially in the Caribbean.  

Algae thrives on human waste, such as sewage, and runoff from farmland. This water pollution is full of nutrients like nitrogen and phosphorus that algae need to grow. So as it runs into the ocean, algae booms.

Plus, one of the most voracious algae-eaters, the long-spined black sea urchin, began dying in the Caribbean in the 1980s, likely from a waterborne pathogen. Caribbean reefs lost, on average, more than 90 percent of their urchins in a matter of weeks, and those populations have yet to recover.

Now, the important job of constricting algae — of giving corals a better shot at growing and recovering from die-offs — has fallen to certain vegetarian fish, including the parrotfish. In some parts of the Caribbean, parrotfish may be the only thing standing between a relatively healthy reef and one shrouded in green noxious gunk.

Parrotfish to the rescue

The life of a parrotfish mostly consists of munching on rocks and dead corals, grinding it into sand, and releasing it through their rear ends. Some of the world’s beaches are largely made of parrotfish poop.

It’s not totally clear what parrotfish are actually eating. Research suggests that their main source of food is colonies of bacteria including cyanobacteria and other microbes that live on rock surfaces, often alongside more visible clumps of seaweed. Parrotfish likely don’t seek out the seaweed itself — the stuff known to be harmful to coral growth and recovery. But when they’re grazing on microbes, they still end up removing it from rock surfaces, according to Andrew Shantz, who studies parrotfish at the University of Hawai’i at Mānoa. 

“Irrespective of what they’re targeting, they end up removing algae from the reef,” Shantz told Vox. “That gives room for corals to come in and settle or grow and occupy that space.”

It’s kind of like how you might weed a garden before planting seeds to give your seedings room to develop.

A number of studies have shown that when you exclude large fish including parrotfish from a reef, it gets covered in more algae, and that appears to limit the growth of some corals. One study in Belize, for example, documented less algae and more baby corals when large parrotfish were around. 

Similarly, a 2017 study in Nature Communications linked parrotfish to reef growth in Panama by examining historical records of fish teeth and coral fragments. The study relied on reef sediment cores: tubes of material extracted from the seafloor that contain layers of coral, sea shells, and animal remains. Those cores allowed researchers to see how fast the reef was growing and — by looking at the number and shape of teeth — how many parrotfish were on the reef.  

Studies like this support the simple idea that parrotfish help coral reefs, yet the relationship between fish and coral is complex and somewhat controversial in marine biology. Smaller parrotfish, for example, don’t seem to limit the amount of seaweed, even if there are a lot of them. Some studies have also failed to find links between fishing restrictions — which typically lead to more parrotfish — and the amount of algae and live coral. Parrotfish also snack on live coral to an extent, though scientists don’t suspect this causes much damage to reefs.

“The effect of parrotfish on reef dynamics is not always clear,” said Joshua Manning, a postdoctoral researcher at the University of Colorado Boulder who studies parrotfish. “It’s still safe to say that parrotfish are good for the reef.”

What a reef full of parrotfish looks like

People have been eating parrotfish for centuries in the tropics, and it’s still common today in many coastal communities throughout the world. (They taste like sweet shellfish, according to a quick Google search). While global population data is sparse, it’s clear that overfishing has caused parrotfish — and especially large parrotfish, which are favored by fishermen — to decline in some of these regions, like Jamaica and Micronesia. 

These declines have almost certainly contributed to the rise of algae. 

But there are also places that have protected parrotfish for decades, where these animals are still abundant and apparently doing their job well. The Dutch island of Bonaire, for example, has banned spearfishing — a common method for catching parrotfish — since the early 1970s. The island, which is just east of Curacao in the south Caribbean, also outlawed the harvest of parrotfish altogether in 2010. While some of Bonaire’s large parrotfish have still declined, it has at least double the number of parrotfish compared to most other Caribbean reefs, according to a 2018 report by the Dutch Caribbean Nature Alliance, a nonprofit. 

All those parrotfish help limit the growth of algae on Bonaire’s reef, according to Robert Steneck, professor emeritus at the University of Maine, who’s been studying Bonaire’s reef for more than 20 years. That in turn has helped the coral here survive, he said. Indeed, while much of the Caribbean’s coral has died off in recent decades from bleaching and disease, the reef in Bonaire is still intact; parts of it are still thriving.

What’s more, Bonaire’s reef has been able to bounce back from large-scale die-offs in the past, according to Steneck’s research. Parrotfish essentially make this ecosystem more resilient, he said.

The reality is more complicated. There are a number of reasons, beyond the abundance of parrotfish, why Bonaire’s reef is healthier than other parts of the Caribbean. The island lies below the path of most Atlantic hurricanes, for example. Bonaire’s coral is also not nearly as healthy as it once was. Bleaching has been harming the reef for years. And in the spring of 2023, a wildlife disease started sweeping through and killing off hundreds of corals, some of which were centuries old.

Against these mounting threats, parrotfish can do very little. When coral die-offs are unrelenting and pollution continues to flow into the ocean, reefs get overcome by seaweed. Once that happens, parrotfish can’t do much to bring them back to life, Manning said. “At some point, with the intensity and frequency of these disturbances, the parrotfish grazing is not going to be able to keep pace,” he said.

Nonetheless, reefs are still better off with more of them. Saving coral reefs depends, above all, on policies and corporate efforts to slash carbon emissions, but that doesn’t mean effective fishing regulations don’t also help.

What parrotfish reveal is that individual components of an ecosystem matter. Take one piece out and the system starts to fail.

“We need to protect them, even if only to give reefs a chance,” Manning said. “As long as we have parrotfish, we might have a chance at least prolonging the potential for reefs to come back.”