Coral reefs are teeming with life, despite being surrounded by nutrient-poor waters. An international research team believes they have unraveled Darwin's paradox.

Charles Darwin once puzzled over how coral reefs in the tropics could be so vibrant while existing in oceans with little available food. How can the richness of these ecosystems be explained? Researchers say they are closer to solving this mystery, known in scientific circles as Darwin’s paradox.

In the clear, nutrient-scarce waters of tropical seas, coral reefs act like oases in the desert. In an otherwise barren environment, they host a vast diversity of life – from the tiniest organisms to the most colorful fish. Recently, an international research team published a new theory in Science about these biological hotspots.

The Key Role of Tiny Reef Fish

The researchers suggest that the answer to Darwin’s paradox lies in the countless small fish living near the seafloor of coral reefs. These fish serve as a crucial food source for larger predators, fueling the reef’s life. “These fish are like candy,” explains lead researcher Simon Brandl from Simon Fraser University in Canada. “Small packets of energy that are quickly consumed by any creature that can bite, grab, or slurp.”

Homebound Fish

These fish species are typically only a few centimeters long. Unlike most fish larvae, which drift into the ocean, the larvae of these species stay close to home. The fish reproduce rapidly, their larvae grow fast, and they are quickly eaten – usually within weeks.

The researchers found that these tiny fish account for nearly 60% of the diet of larger reef-dwelling fish. With their “hidden productivity,” they form a cornerstone of coral reef ecosystems. Their role had previously been overlooked because they are eaten so quickly, Brandl explains. “They’re devoured before they can even be counted.”

Sponge Waste as Food

Jasper de Goeij, a tropical marine biology lecturer at the University of Amsterdam, views this as a valuable addition to existing theories but does not consider the Science study the sole explanation for coral reef biodiversity. Small fish are not the entire story.

De Goeij’s research at Curacao revealed that sponges play a critical role in reef food supply. Sponges, which line the inner crevices of coral reefs, filter bacteria, single-celled algae, and dissolved substances (like sugars) from the water and convert them into waste. Sponge waste becomes food for crabs, worms, and snails – which are then eaten by larger animals.

“Sponges are underestimated organisms,” says De Goeij. “They are essential to the energy cycle. The largest source of potential food is dissolved in the water, and sponges utilize it efficiently. They renew their cells quickly, excreting old cells as organic waste, essentially fertilizing the reef.”

Nutrient Capture and Recycling

Fleur van Duyl, a researcher at the Royal Netherlands Institute for Sea Research (NIOZ), finds it remarkable that small fish and their offspring provide such a vital food source. However, she is not convinced they are solely responsible for reef biodiversity. According to Van Duyl, the key lies in the coral and its symbiotic organisms, which efficiently capture and recycle scarce nutrients.

“Corals extract dissolved substances and plankton from passing water, providing nitrogen and phosphorus to the algae they coexist with. In return, the corals receive energy and nutrients from the algae through photosynthesis. Some reefs thrive even when the surrounding waters are almost barren. Coral reefs start small, but the ecosystem gradually enriches itself,” Van Duyl explains.

The Food Chain Pyramid

Erik Meesters, a coral ecologist at Wageningen Marine Research, downplays the role of small fish larvae, suggesting that overall reef richness is not primarily due to them. “Larvae make up a tiny fraction of all plankton. The biodiversity of reefs is more closely linked to nutrients that well up from deeper, colder waters, leading to an abundance of plankton that supports the base of the food chain.”

Is Darwin’s Paradox Finally Solved?

For Meesters, the mystery is no longer a paradox. “We are gradually understanding reef processes better. The study by Brandl and colleagues adds another piece to the puzzle, highlighting how efficiently energy and carbon – the building blocks of life – are recycled in reefs.”

Van Duyl agrees that the paradox has largely been demystified. “We know that various factors contribute to the richness of ecosystems in nutrient-poor environments. We just don’t yet know the exact contribution of each element.”

According to De Goeij, the puzzle is mostly solved by recognizing how reef organisms make the most of dissolved substances in the water and efficiently recycle nutrients. “It’s like reusing a peanut butter sandwich a hundred times over.”