These tiny, mysterious fish may be key to solving coral reef ‘paradox’

If a snorkeler or scuba diver is lucky enough to spy a cryptobenthic fish—named for its elusive nature—all they may glimpse is a brief flash of color. But these tiny swimmers may be a cornerstone of coral reefs, making it possible for bigger, more charismatic fish and many invertebrates to thrive, according to a new study. And they could help solve a mystery that stumped even the father of evolution, Charles Darwin.

“By not dealing with these little guys, we were missing a big part of what’s happening on reefs and the food web,” says Nancy Knowlton, a coral reef biologist at the Smithsonian National Museum of Natural History in Washington, D.C., who was not involved in the work.

Coral reefs are the marine equivalent of tropical rainforests, teeming with fish, crabs, sponges, and other creatures. Yet, as Darwin noted in 1842, the beautiful blue of surrounding waters means there’s very little of the microscopic plankton that makes up the base of most marine ecosystems. Over the past century, researchers have tried to solve “Darwin’s Paradox” by proposing that reefs have hyperefficient systems for recycling the nutrients already there—or are sustained by nutrients washed in from the open ocean.

Simon Brandl, a coral reef ecologist at Simon Fraser University in Burnaby, Canada, wondered whether cryptobenthic fish could be the key instead. These secretive, sometimes brightly colored fish are often less than a centimeter long and dash quickly back into crevices in the coral when startled. They can weigh as little as 0.1 grams (about as much as a single pea), making them the smallest marine vertebrates—and potential food for reef inhabitants.

When Brandl first started studying these fish in Australia, “I was blown away by how many there are and how diverse they are,” he recalls. About 3000 species are known, and there’s an estimated 1000 more.

To figure out just how abundant the tiny fish are, Brandl and his colleagues put 58 bell-shaped nets over individual corals in Belize, French Polynesia, and Australia. Setting up the nets scared away anything other than the cryptobenthic fish. When the researchers added an anesthetic that caused the fish inside to float, they counted up to 100 cryptobenthic fish per square meter, they report today in Science. A literature search by Brandl’s team also revealed that, unlike most fish larvae, the young of cryptobenthic fish don't stray too far from their natal reefs. Almost 70% of the larvae found within 10 kilometers of a reef belong to these tiny swimmers, the team found.

Brandl and colleagues then looked at previous studies that estimated lifespans and death rates for multiple reef species. Because every creature that dies on a reef becomes “fish food,” the scientists were able to infer how many were eaten each day—including the cryptobenthic fish. Their final calculations suggest that cryptobenthic fish account for almost 60% of the fish biomass consumed on the reefs. Because these fish can cycle through seven generations a year, their returning young provide a constant food supply. “Cryptobenthics do one thing particularly well: getting eaten,” Brandl says.

The discovery is “a major contribution to our understanding of coral reef productivity,” says Nick Graham, a coral reef ecologist at the Lancaster University Environment Centre in the United Kingdom who was not involved with the work. Douglas McCauley, an ecologist at the University of California, Santa Barbara, agrees. “We didn’t have a good handle on these numbers previously,” he says. The numbers, he says, reveal how important these fish are to reefs.

Knowlton thinks small fish may fuel other marine ecosystems such as coastal mangroves. And “there are huge numbers of other really small things,” such as crabs, shrimp, and snails that may also play outsized roles, she says.