Researchers May Have Solved Darwin's Paradox of How Reefs Are So Productive

Photo: Jim E Maragos, U.S. Fish and Wildlife Service (Wikimedia Commons)

Cryptobenthic fish may sound weird, but they’re not so different from you and me. They’re vertebrates just like us, though their hearts, bones, and brains are quite tiny. But unlike us, they do two things extremely well.

“They’re the masters of premature death,” Simon Brandl, a postdoctoral researcher at Simon Fraser University, told Earther. “And that is their big contribution to coral reefs.”

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Indeed, in research published on Thursday in Science, Brandl and his colleagues show that without these tiny, death-loving fish, reefs would be in big trouble. And in doing so, they’ve helped answer a 200-years-old puzzle called Darwin’s Paradox, which asks how reefs exist in otherwise desert-like parts of the ocean.

Dive or snorkel on a reef, and you’d be forgiven for not noticing cryptobenthic fish. The group of 17 small swimmers only grow about 2 inches in length and spend time tucked into the nooks and crannies coral’s rough surfaces provide. Sure, they come in shimmery varieties like other large reef fish, but they’re decidedly not the angelfish, damselfish, and multicolored tangs that are the stars of the show.

A Bluebell Benny cryptobenthic fish pokes its head out of a coral hiding place.
Photo: Tane Sinclair-Taylor

But it turns out that these rinky-dink fishes are basically the reason life flourishes on reefs. On the surface it doesn’t look like they do much, but it turns out their penchant for dying is what keeps all the other fish fed. That tiny fish feed the big fish isn’t exactly groundbreaking, but what the new results show is that cryptobenthic fish produce an astounding number of larvae to keep the food chain balanced. And those larvae forsake a long trip to the open ocean that other baby reef fish take—where they would likely die even more prematurely—allowing more to come back and replenish the on-reef stock.

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Data collected from various reefs around the world, including in Belize, Australia, and French Polynesia, show that nearly two-thirds of all larvae come from just the 17 types of cryptobenthic fish. Those larvae rapidly replace adults that get gulped by larger fish, allowing cryptobenthic fish to account for 60 percent of all fish eaten on a reef. In essence, they act as fertilizer that allows reef communities to live in parts of the ocean that may otherwise be barren. The findings provide one answer to Darwin’s Paradox, though other research has laid out other explanations as well. But regardless, the new relationship opens up critical grounds for discussing reefs and conservation.

Nobody is fishing for these wee fish, but they are threatened by other issues like climate change. Warming waters are killing coral. As coral die and erode away, the fish lose their habitat, meaning the formerly vibrant reefs could actually become deserts as the food chain dissolves completely. On a biodiversity level, it makes sense to preserve the reefs and ensure the tiny fish continue to have a home. But Brandl pointed out that 500 million people also rely on reefs for fishing and livelihoods, so in a way, the cryptobenthic fish are actually the basis for the reef economy as well.

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Brandl’s research isn’t done, either. He said the next step is to figure out where the larvae hang out before returning to reefs, including why they have a higher survival rate than the larvae of larger reef fish.

“That will help us maintain this cryptobenthic fish conveyor belt that feeds coral reefs,” he said.

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