The Tiny Fish That Break a Fundamental Rule of Vertebrate Life

By living fast and getting eaten in extreme numbers, these overlooked critters fuel the food webs that allow vast communities to flourish in coral reefs.

A Great Barrier Reef blenny
A Great Barrier Reef blenny (Tane Sinclair-Taylor)

Although coral reefs are home to bustling communities of gaudy marine life, half the fishes that live there are hardly ever seen. Aptly known as cryptobenthics—literally “hidden bottom-dwellers”—these species are mostly shorter than two inches and usually hidden in crevices. If you snorkel past, they’ll scurry away. But Simon Brandl of Simon Fraser University has made a career of studying them. And he and his team have now shown that cryptobenthics are a crucial component of healthy reefs because they, more so than other fish, are extremely good at dying.

Pretty much every predator on the reef eats cryptobenthics, and a full 70 percent of these tiny morsels are devoured every week. But since they also reproduce, hatch, and grow at equally phenomenal rates, new individuals are constantly replenishing the ones that are consumed. Entire generations can turn over in a matter of weeks. And this extreme life cycle, Brandl found, is the secret engine of the world’s coral reefs, fueling the food webs that allow their inhabitants to flourish in otherwise nutrient-poor waters.

Large, colorful fish such as groupers, wrasses, and parrotfish “are what people focus on because they’re the ones you see when you’re diving,” says Julia Baum, a coral researcher at the University of Victoria. “But I love it when a study like this comes out of the blue and says that maybe these tiny fish might be the ones that are fueling everything else on the reef. That’s very cool.”

Seventeen families of fish count as cryptobenthics, but aside from seahorses (and perhaps gobies), most are obscure. Blennies, dottybacks, clingfishes, and dragonets—they’d be household names if only they were a little bigger, Brandl thinks. “I suspect that if we were to blow them up to 20 times their size, we’d consider some of them to be the most beautiful fish on the reef,” he says.

Cryptobenthic fish include the Roughhead blenny, Belize sponge goby, Matador triplefin, Warteye stargazer, and Saddled blenny. (Credit: Jordan Casey)

The small size of these fish also limits how many eggs they can produce, “which means they have to be extremely religious about making sure those actually hatch,” Brandl says. Some incubate their eggs in their mouths, forgoing meals until the larvae break out. Seahorse males lug around their eggs in breeding pouches. Sandgazers carry around a ball of eggs under their pectoral fins—a behavior called “armpit breeding.” These tactics mean that cryptobenthics are far better than other fish at converting eggs into larvae. When researchers count the baby fish in the waters near a reef, about 70 percent will be cryptobenthics.

For most of these larvae, life will be brief—even if they escape the jaws of predators. The pygmy goby, for example, crams its entire existence into just two months—the shortest life span of any backboned animal. Much of that time is spent as a larva, which quickly quintuples in size into an inch-long adult that lasts for just a few weeks. Living fast and dying young: That’s the cryptobenthic way. “They’ve broken with the fundamental rule of being a vertebrate, which is that you spend most of your time as an adult,” says Brandl. “They’re almost getting into mayfly territory.”

All of this explains why standard surveys fail to capture how important these fish are. Ecologists typically study reef animals by measuring their “standing biomass”—that is, they grab all the adults in an area and weigh them. By this measure, cryptobenthics seem unimportant: Though there are plenty of them, they barely shift the scales, even together. But that metric is profoundly misleading, because it ignores just how many cryptobenthics are eaten, and how often they’re eaten.

By simulating the lives of these fishes, Brandl found that their generations tick by so quickly, and they’re so frequently preyed upon, that every individual is replaced seven times a year. That’s a huge mass of fish that we rarely ever notice, because it’s eaten almost as quickly as it is generated. Weigh the survivors at any one moment, and you’ll be unimpressed. But estimate the total mass of the dead, as Brandl and his team did, and you’ll find that cryptobenthics account for 60 percent of the fish flesh that is eaten on a reef. Abundant, calorific, and rapidly produced, “they’re the fast food of coral reefs,” says Gabby Ahmadia, who has studied cryptobenthics and is now a director of marine-conservation science at the WWF.

The hidden influence of cryptobenthics might explain a long-standing puzzle called Darwin’s paradox. Simply put, “corals need clear, tropical waters to survive, but those waters are usually poor in nutrients,” explains Luiz Rocha of the California Academy of Sciences. So how can such watery deserts support such heaving communities?

One possibility: Seabirds could import nutrients by feeding in faraway waters and then dropping guano on the reefs. (A recent study supported this idea by showing that when invasive rats kill island seabirds, the surrounding reefs also suffer.) Cryptobenthics could fulfill a similar role. As larvae, they feed in richer offshore waters before moving back to the reefs once they mature. All the energy locked in their bodies “gets transferred to the reef when they die as adults or are eaten,” says Rocha. They’re like a massive, invisible conveyor belt, channeling nutrients into the shallows.

For this conveyor belt to work, cryptobenthics have had to break yet another rule of marine life. The larvae of most reef fish spread over long distances by riding ocean currents, but those of cryptobenthics “don’t really go anywhere,” Brandl says. They stay within a few hundred meters of the reefs where they were born, and eventually swim home to fill the empty spots that the former generation has vacated.

This lack of wanderlust has shaped their evolution. Populations can become easily isolated from one another, so existing species are quick to split into new ones. But since those species are typically confined to a narrow geographical range, they’re also prone to extinction. What will happen to them in the future, as coral reefs are pummeled by heat waves, acidifying water, and increasing tropical storms? Ahmadia says that based on previous studies, they’re actually pretty resilient. “Cryptobenthic fish are more likely to withstand reef degradation and may play a role in the recovery of coral-reef systems,” she says.

Repeatedly, we see that the limits of our senses limit our understanding of nature. We neglect what we cannot see, whether cryptobenthic fish or the microbes that grow on corals. “I work on cryptic invertebrates, which have been ignored for the same reasons,” says Nancy Knowlton of the Smithsonian National Museum of Natural History. “If you go to a reef and break off a head of dead coral, it’ll be full of little hiding crabs, shrimps, and snails. But unless you really make an effort to find these things, they’re hard to study.”