>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.
>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.
>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.
>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.
>Learn more about cryptobenthic fish in this video.