Its day job helps prevent an iconic ocean ecosystem from collapsing.
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 M?noa.
“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.”