Great Barrier Reef Blenny (Ecsenius stictus) Tane Sinclair-Taylor

We have long overlooked many of the important inhabitants of coral reefs. Tiny fish that hide in the nooks and crannies may provide much of the food that supports larger animals on and around healthy coral reefs.

“There are these whole worlds happening that people are just not aware of on coral reefs that involve these incredibly beautiful little fish,” says Isabelle Cote of Simon Fraser University in Canada.

Efforts to conserve and restore coral reefs focus on the corals themselves and on larger fish, she says. For instance, no one thinks of providing the small-scale shelter the tiny fish need.


Cote’s colleague Simon Brandl has been studying “cryptobenthic” reef fish that are less than 50 millimetres long as adults – basically, the ones you don’t see when snorkelling or diving on a reef. When he looked at surveys of plankton near reefs around the world, he was surprised to discover that 70 per cent of the fish larvae were of these cryptobenthic species.

Larger fish produce more eggs overall than the tiny fish, says Cote. But these eggs have large yolks and float in the plankton for weeks. This allows the larvae to disperse over vast distances but relatively few make it back to reefs. By contrast, the eggs of tiny fish develop fast and the larvae stay near reefs, so they end up outnumbering the larvae of bigger fish.

That means they are an important food source for other animals. In fact, when the team modelled what happens based on the available data, they concluded that that these tiny fish provide 60 per cent of all the fish biomass eaten on reefs.

“There’s this massive turnover,” says Cote. In other words, the total biomass of these tiny fish at any one time is relatively small. But because these fish live fast and die young, they provide most of the eaten biomass over an entire year. The mortality rate can be as high as 70 per cent a week.

What biologists have been doing, says Cote, is the equivalent of going to a conveyor-belt sushi restaurant and trying to work out how much food it sells by counting how many plates are on the conveyor at any one time. What actually matters is how fast the plates taken by diners.

The team think their findings could help explain “Darwin’s paradox” – how coral reefs so rich in life thrive in nutrient-poor waters. Or as Darwin put it, how these oases occur in oceans that are otherwise deserts.

But despite their importance we still know little about crytobenthic species. In fact, many remain unknown. “There a whole lot of species we have not discovered,” Cote thinks.

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Journal reference: Science, DOI: 10.1126/science.aav3384