Bustling around their reef homes, Red Sea dascyllus damselfish (Dascyllus marginatus) do quite well out of their co-habiting arrangement with hood coral (Stylophora pistillata). ‘The coral provide the fish with shelter from predators and a place for social interactions and egg laying’, says Nur Garcia-Herrera, from the Alfred-Wegener-Institut, Germany. Yet the agreement isn't entirely one sided. ‘In return, the damselfish remove sediments from the coral surface, protect it from predators such as butterflyfish or crown-of-thorns starfish, and excrete nutrients such as nitrogen and phosphorus that the coral takes up’, Garcia-Herrera explains. But she and her colleagues, Amatzia Genin, from the Hebrew University of Jerusalem, Israel, and Sebastian Ferse and Andreas Kunzmann, from the Leibniz Center for Tropical Marine Ecology, Germany, also wondered whether the gentle water currents generated by the fish's ceaselessly fanning fins may also nurture the coral and enhance coral photosynthesis.
Joining Genin at the Interuniversity Institute for Marine Sciences of Eilat on the Red Sea coast, Garcia-Herrera collected corals and their fish lodgers from the sea floor and returned with them to the lab. There, she and Genin embarked on a complex series of experiments recording oxygen levels in the water as it flowed over the isolated fish, isolated corals, fish nestled inside the coral, fish that were excluded from the coral, and fish that had taken refuge inside dead coral. By repeating some of the measurements in light and dark conditions, Garcia-Herrera, Genin and Ferse were able to tease out how much oxygen was produced by coral photosynthesis during the day in contrast to the oxygen consumed by both organisms when there was no light.
Comparing the oxygen produced by coral photosynthesis when a fish was allowed to nuzzle inside with the oxygen produced by coral when the fish was near but excluded from the branches, the team was impressed to see the oxygen production rocket by 22% when a fish was secluded amongst the coral branches. In contrast, the team discovered that the oxygen consumption of the damselfish and corals did not vary – regardless of whether they were isolated or together – although the metabolic rates of the fish rose when their only refuge was a piece of dead coral, suggesting that the fish may have been stressed.
Garcia-Herrera and Genin then went swimming with the damselfish in the Red Sea to get a better appreciation of the fish's impact on the coral. ‘Diving in the Red Sea is a wonderful experience’, says Garcia-Herrera, who observed the fish continually beating their fins as they took shelter inside the coral at night, before venturing out during the day to feed on plankton. Garcia-Herrera also noticed that the fish never ventured far from their coral homes – darting back inside the haven whenever danger loomed – to the extent that the coral was occupied by at least one fish for almost a third of the day.
Combining their observations and physiological recordings, the duo suspects that the fish's continually beating fins may generate sufficient water flow to boost coral photosynthesis when in residence. Garcia-Herrera says, ‘This is the first evidence of positive effects be a coral-associated fish on coral photosynthesis’; although she suspects that the fish's presence may only boost the coral's daily photosynthetic rate by up to 6% in practice. And she warns that the plucky little fish could come under increasing pressure if their coral refuges dwindle as a result of climate change, while the corals could become even more dependent on their roaming fish-fans for the oxygen that is fundamental for recovery from increasingly stressful conditions.