Coral reefs buzz with life even while being surrounded by underwater deserts, a phenomenon that has puzzled biologists for so long that it is known as "Darwin's paradox". The richness is so great, it is easy to overlook key components. That's why the importance of one of these – tiny, short-lived fishes – has only now been recognized.
It’s not news that medium-sized fish eat little fish, and in turn are preyed on by bigger ones. Yet the small creatures known as cryptobenthic fishes that form the basis of the food chain have attracted surprisingly little research.
“Tiny fish larvae absolutely dominate the larval communities near reefs," said Dr Simon Brandl of Simon Fraser University in a statement. Brandl discovered this isn’t just because small fish are so much more numerous, it’s also because their larvae behave differently from those of larger counterparts.
The larvae of larger reef fish disperse between reefs, crossing what can be wide expanses of open ocean to get to new homes. Many of them don’t make it, keeping the numbers of the larger species limited. By avoiding these journeys of exploration, Brandl said that cryptobenthic fish get “more bang for their buck.” Of course, if never leaving the home reef didn't have a price, all reef fish would do it. Cryptobenthic fish are more vulnerable to local disturbances, and obstacles to interbreeding produce new species that can eventually become inbred.
Cryptobenthic fish take living fast and dying young to an extreme. "These fish are like candy," said Brandl. "They are tiny, colorful bundles of energy that get eaten almost immediately by any coral reef organism that can bite, grab or slurp them up.” As many as 70 percent of the small fish on the reef at the start of a week will have been eaten by the end, yet there are always more to replace them.
By making up almost 60 percent of the fish mass consumed on reefs, Brandl reports in Science, cryptobenthic species around 5 centimeters (2 inches) long enable all the richness we love. “We never see it because the fish get eaten much faster than we could ever count them," said Brandl. "It's essentially a bag of candy that magically replenishes every morsel eaten."
Keeping that magic bag full is an aspect of reef conservation that has been overlooked until now, and Brandl hopes his work will start to bring attention to what needs to be done.
Rising carbon dioxide levels are affecting the capacity of fish larvae to find their way to new reefs using sound, and this is expected to get much worse. Whether there are similar threats to fish that don’t undertake such voyages is a question we haven't begun to answer.