As clownfish move around at night, they boost water flow over the anemone and increase its oxygen consumption.
At night there is less oxygen available on the reef because photosynthesis ceases once the sun goes down.
US researchers measured and compared the net dark oxygen uptake of fish–anemone pairs when partners were separate from each other, together as a unit, and together as a unit but separated by a mesh screen that prevented physical contact.
They found that both the fish and anemones consumed 1.4 times more oxygen when they were together than when apart.
"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.
When clownfish rest in the tentacles of their anemone host, they engage in certain behaviours more often than when they are alone. These behaviours appear to enhance water motion through anemone tentacles.
—Dr Joseph Szczebak from Auburn University, Alabama, US who led the study