Fidgeting clownfish benefit anemones
Clownfish boost oxygen flow around their anemone hosts at night, scientists have found.
The relationship between the reef animals is well known, with the fish hiding in the anemone's stinging tentacles to avoid predators.
But US researchers have discovered the anemones also benefit from the night-time presence of the fidgety fish.
As clownfish move around, they boost water flow over the anemone and increase its oxygen consumption.
The findings are published in the Journal of Experimental Biology.
At night there is less oxygen available on the reef because photosynthesis ceases once the sun goes down. But night is also when the main predators of clownfish hunt.
"While many reef organisms can pick up and move to other areas with more oxygen, clownfish stick by their anemones; retreat is not an option," explained Dr Joseph Szczebak from Auburn University, Alabama, US who led the study.
To understand more about the nocturnal relationship of the clownfish and anemone, Dr Szczebak and colleagues travelled to the Marine Science Station in Aqaba, Jordan.
Diving in the nearby Red Sea, the scientists were able to record how oxygen levels changed when the fish were in the anemones and when they were separated.
They found that both the fish and anemones consumed 1.4 times more oxygen when they were together than when apart.
The fish also moved around more when within their anemone hosts than when on their own.
"When clownfish rest in the tentacles of their anemone host, they engage in certain behaviours more often than when they are alone," said Dr Szczebak. "These behaviours appear to enhance water motion through anemone tentacles."
"Anemone oxygen consumption increases with water flow, suggesting that any flow-related side effects of clownfish behaviour will indeed increase anemone breathing rates."
Based on observations, they defined three particular behaviours: fanning, wedging and switching, which Dr Szczebak compared to tossing and turning in bed.
"During fanning, clownfish were motionless among the tentacles, aside from rhythmically flapping their pectoral fins," he said.
"During wedging, the clownfish forcefully wiggle deeper into the anemone's bed of tentacles, causing a flutter of tentacular activity.
"Lastly, during switching, clownfish rapidly changed their orientation within the anemone."
These observations contrast with previous theories that clownfish are passive at night in order to evade predators. Instead, the fish studied were active for more than 80% of the night.
"By constantly moving about their anemone and waving its tentacles like flags, it would seem these clowns are drawing more attention to themselves as a potential snack," Dr Szczebak said.
"However, regardless of this apparent risk, they wave on."
He suggested that the health of their hosts could be the motivation for this energy-consuming behaviour.
"Yes, the level of activity that these fish engage in is sure to increase their oxygen demand; however, consider the alternative: if their anemone was to deflate or die due to a lack of oxygen, where would that leave the clown?"
The researchers are continuing to investigate the relationship to understand if the "aeration" is the only intention of the fish.
"This research joins a growing body of knowledge that is using the ecological and physiological connections of coral reef relationships to show just how highly evolved and interdependent coral reef systems are," Dr Szczebak told BBC Nature.