Coral reefs depend on a large number of fish the size of a jellybeans

There is an old paradox about coral reefs: they occupy tiny parts of the ocean, and nutrient-poor parts, yet they are home to one-third of the world's fish species. Millions of people depend on it for food. What explains their meteoric productivity?

Since Charles Darwin recognized the paradox for the first time, scientists have struggled to solve it. One theory suggests that the sloping topography of the reefs captures and concentrates nutrients and microscopic plankton from the surrounding waters. Another is the role of sponges and other invertebrates in recycling dead organic matter into something that can be eaten.

Navy scientists from Canada, France, Australia and the United States believe they have found another solution to Darwin's paradox. In an article published this week in Science, they argue that a multitude of small reef fish – the smallest vertebrates in the ocean – feed the nutritional engine of reef ecosystems, providing food for larger and more attractive creatures.

These puny fish – gobies, blenny, cardinal fish and others – are usually less than two inches long. Some are so tiny that it would take 40 to have the weight of a quarter. Yet they can account for nearly 60% of the fish biomass and half of the fish species on a reef – "the half hidden" says Simon Brandl, coral reef ecologist at Simon Fraser University in Vancouver, which has directed the new study.

The secret to the success of small fish – and their importance on the reef – is a way of life reminiscent of insects: they reproduce prolifically, grow rapidly and lead very short lives.

"They made a change that is not like any known vertebrate organism," says Brandl. "They entered the territory of the white flies, the territory of the cicadas, with a long larval stage and only a few weeks into adulthood. They reproduce, then they are gone, engulfed by a predator. "

The official name of these neglected animals is "cryptobenthic reef fish": "Crypto" because they melt into their environment, either visually or by their behavior, and "benthic" because they live near or even at the inside the seabed.

Melting into a coral reef often requires a bright coloring. Cryptobenthics can be crimson with turquoise, yellow striped scarlet, lime green, tin soaked in orange, mango dipped in purple. They have spots, flecks, racing streaks, vertical bars and hallucinogenic swash. There are innumerable variations.

Because of their small size, many of these fish occupy areas a few hands wide, and they have very specific habitat preferences. Some live only on one species of coral. Others live only on sand, or on rubble or in worm tubes.

Scientists have so far discovered more than 2,800 species of these fish and are discovering new ones at a rate of about 30 per year. Sometimes they have to use chemical anesthetics to chase tiny fish from their dens.

With inhabited submersibles, biologists have even discovered new cryptobenthics on reefs hundreds of meters deep – a process that Brandl likens to "looking for a needle in a haystack in a dark barn using a forklift." ".

The great meaning of these fish, says Brandl, is their way of life. Their rapid growth, rapid turnover and extreme mortality serve as a pump that constantly maintains biological energy in the reef food web. Cryptobenthics eat microscopic substances – filamentous algae, coral mucus, tiny crustaceans – and convert them into food for the hundreds of large species of fish that define coral reefs in the spirit of those who cherish them.

The rolling of the little fish is immense. The maximum age of the dwarf goby in the wild is 59 days – the shortest lifespan of all known vertebrates. Another goby spends longer in the larval stage than in adulthood. No other fish is known to have this way of life.

According to the Brandl team, up to 70% of cryptobenthics die every week, mostly in the jaws of larger animals. They are consumed by virtually all predators of reefs – fish, shrimp, crabs and even some predatory molluscs. Juvenile grouper diets have been found to be nearly 90% cryptobenthic.

To keep up, small fish breed very quickly and all year round; Little gobies, for example, can produce more than seven generations a year. More importantly, cryptobenthics are very successful at ensuring the survival of their offspring.

Most large fish throw their eggs into the sea and let the currents spread them off – a cover strategy that ensures there will always be a few hoppers in flight to replenish the population in the event of a disaster. "The disadvantage is that you have to produce a considerable number of products so that some can survive the deadly trap of the ocean," says Brandl.

Cryptobenthic reef fish have the opposite strategy: they produce fewer offspring, keep them close to home and invest heavily in these resources by providing an unusual degree of parental protection.

Some incubate their eggs in one pocket, others in the mouth, others put them behind their pectoral fins – this is called brood in the armpits. Two groups give birth to live young – a rarity among coral reef fish.

Even species using more conventional spawning methods, in which the female attaches a sticky egg mass to a coral branch, sponge or rock, provide these eggs with postnatal care. It is mostly the men who take care of the surveillance of the eggs, in particular to ventilate them to keep them well oxygenated and to clean them to avoid the accumulation of detritus. Some species may even offer the eggs antibiotic protection against infection.

All this parental investment would be useless if the larvae were swept away from the reef by strong currents. While large fish larvae actively search for these currents, they are avoided by larval fishes of cryptobenthic reefs, which settle on their domestic reef.

They manage to linger on the periphery of the reefs, while standing out of the reef-based predators and deep seabed.

"We do not know how they do that," says Brandl. "I suppose they go down close to the bottom, perhaps a few hundred meters from the reef, where the currents can be weaker and predators are less likely to notice them." This is one of our next research questions: where are the larvae going?