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The fin fanning of damselfish in the Red Sea helps boost the rate of photosynthesis of the algae that live inside corals.

We already knew that the damselfish (Dascyllus marginatus) lives in symbiosis with the coral Stylophora pistillata. The fish use coral branches as shelters and nests, and in return they remove sediment from the coral surface and excrete nutrients. But it turns out there is more to the relationship.

Nur Garcia-Herrera at the Alfred-Wegener Institute in Germany and colleagues measured oxygen levels inside the branches of coral kept in tanks either with or without fish.

They found that photosynthesis rates were higher during the day in tanks containing fish, probably helped by the fish’s fin strokes wafting away water containing high levels of oxygen. The presence of fish increased photosynthesis by 22 per cent.

“This is the first evidence of positive effects by a coral-associated fish on coral photosynthesis,” says Garcia-Herrera.

Coral ventilation

The effect is probably smaller in the wild, she says, as the fish spend only about a third of their time in coral reefs, according to her team’s field observations. Even so, it may boost the coral’s photosynthesis by up to 6 per cent.

“Many corals live in environments where currents are low and the concentration of pollutants and ocean acidification parameters are high,” says Garcia-Herrera. “Therefore, through the ventilation, the fish are helping the corals to cope with such hard conditions.”

The ventilation by the fish could provide some relief from the build-up of warm water and excess oxygen among the coral branches, says Sebastian Ferse, Garcia-Herrera’s colleague at the Leibniz Centre for Tropical Marine Research in Germany. This may help to counteract the bleaching risk associated with global warming.

“Another important point is that fish may spend less time among the branches if their predators are removed, for example if reefs are overfished,” says Ferse. “In that case, the damselfish may opt to spend more time foraging outside the coral. Overfishing may thus have previously overlooked negative side-effects on coral physiology.”

“All types of fish living alongside corals have some function,” says Petar Kruzic at the University of Zagreb, Croatia. “It makes sense that ventilation helps coral.”

But he says that rising sea temperatures and pollution are the main problems facing corals, and the new study doesn’t necessarily help with solving those.