Cryptobenthic Fish

Secretive, snack-sized fish fuel coral reefs.

Researchers are only recently beginning to understand how important tiny, reef-dwelling fish are to the ocean ecosystem.

With clear blue water and large schools of colorful fish, coral reefs are perhaps the most popular marine ecosystem for divers and underwater photographers. The combination of these two perks — beautiful turquoise water and lots of large fish — holds a mysterious contradiction: The water is so blue and clear because it contains very few nutrients, but the fish need to eat something. Charles Darwin first described this phenomenon, which has since been termed "Darwin's paradox."

Over the past two decades scientists have sought various explanations for the enigmatic productivity of reefs, presenting evidence for positive effects of resources from the open ocean that are trapped by reefs and a significant role for sponges that recycle organic waste. Researchers have discovered another piece of the puzzle, but unless you are a macrophotography aficionado you are unlikely to have noted the actors.

A recent study published in the journal Science shows the unexpected and remarkably large role of tiny, bottom-dwelling reef fishes for coral reef productivity. These fishes — termed cryptobenthic reef fishes (or simply "cryptos") — are rarely noticed, but they support the iconic schools of large, colorful fishes by providing almost 60 percent of consumed fish flesh on reefs. In fact, most divers will only notice them as tiny flashes of yellow, red, purple, pink, orange or green zipping around the coral reef bottom, frantically scurrying into little holes, nooks and crevices to avoid being eaten.

Yet these evasive maneuvers are often to no avail. Most of these fishes — including gobies, blennies, triplefins, cardinalfish and 13 other families — will live on coral reefs for only a few months, and many meet their untimely end within a few weeks of settling onto the reef. There are simply too many hungry mouths to feed; with an average size of about 1 inch, the cryptos are a perfect snack for many predators ranging from mantis shrimps to groupers. Living on the reefs is a true, albeit short, purgatory for these tiny fish.

With so many cryptos dying at such high rates, how are reefs not running out of these convenient snacks? The answer to this question lies in the larval stage. Almost all coral reef fishes develop through a pelagic larval stage in which the fish larvae cross the open ocean in search of a new home. This dangerous and often fatal journey eliminates vast numbers of reef fish offspring. Cryptos, however, have taken another route: The tiny fishes that live such short and hazardous lives on the reef appear to have given up the dispersal to reefs far and wide in favor of staying near the parents' reef. The larvae of the tiny fishes can develop in relative safety there.

From this tiny-fish kindergarten flows a steady stream of offspring back to the reef to provide replacements for every adult eaten, creating a nearly inexhaustible conveyor belt of snack-sized fish that predators can gobble up. The unusual ecology of cryptobenthic fishes resembles an ever-replenishing candy jar on coral reefs. Grab one, two or even a handful on the go, and the jar will refill almost immediately with the next generation of coral reef candy.

While cryptobenthic fish have an unusual propensity among fish to stay at their home reefs, leading to their robust populations that support larger fish, the changes taking place in their habitats threaten to upset the balance of reef environments.

These remarkable dynamics have clear consequences for coral reefs as we know them. While cryptos will never amount to a significant portion of fish biomass in our surveys — they are simply too small and disappear too quickly — they are a staple for many of the big fish that we associate so intimately with tropical coral reefs.

On the other hand, abandoning dispersal into the four corners of the tropical seas in favor of locally living fast and dying young also has severe implications for cryptobenthics themselves. Staying put means that populations of these tiny fishes that are separated by open ocean have limited exchange. In this isolation, populations diverge from each other until, over the scale of thousands of years, a new species arises. This makes the few families of cryptos extremely diverse, accounting for more than 2,500 species — almost half of all reef fish biodiversity.

As coral reefs are grappling with the effects of the climate crisis and other human-made stressors, this newly discovered role for the ocean's smallest vertebrates offers both hope and concerns. The peculiar homeward-bound strategy of cryptos means that many of them may be much more vulnerable to extinction than researchers previously assumed. Most species of these tiny fish depend on the fine-scale complexity that corals provide, and some of them have extremely specific habitat requirements: Their existence may be bound to a single species of coral, sponge or sea fan. Coral bleaching, tropical storms and land-based pollution are changing the composition of coral reefs at unprecedented rates, which may threaten the existence of many cryptobenthic species.

Knowing that coral reef fishes depend on these tiny, short-lived and seldomly noticed animals also offers scope for protecting the finely tuned engines that produce much of the iconic and important reef fish biomass. Across the globe, 500 million people depend directly on coral reefs and their services. For both reef fisheries and tourism, reefs without big fish would be catastrophic; after all, most divers do not strap on their tanks to catch a glimpse of a glorious frond of algae. Protecting the tiny fish that fuel the large schools of fish figures to be a conflict-free endeavor that could help safeguard this asset and ensure that our future image of coral reefs continues to feature large schools of big fish in clear, blue water.

While you are photographing the snappers, groupers or sharks on your next dive trip, take a moment to look for the snack-sized-candy fish that sacrifice their lives for the reef's greater good on a daily (or probably hourly) basis. If you have a macro lens, try taking a photo of them as well. If it weren't for their minuscule size, they might be considered some of the most beautiful fish on the reef. By being tiny and sporting a unique way of life, however, they are perhaps some of the most useful.
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Learn more about cryptobenthic fish in this video.