IT IS one of the best-known relationships in nature: the anemone provides a tentacle-guarded home and the clownfish drives off predators that would chew its protector. Recent research has, however, shown there is much more going on. A study in 2009 revealed that waste excreted by clownfish provides vital nutrients to anemones. Now researchers have found clownfish can boost their hosts’ oxygen supplies at night too.

Clownfish and anemones live on coral reefs where oxygen levels in the water often plunge at night as photosynthesis shuts down. Some fish species are known to wave their fins over their coral homes to help keep the coral oxygenated. Joseph Szczebak of Auburn University, in Alabama, and his colleagues wondered if clownfish might do something similar for anemones.

It seems that they do. In a forthcoming edition of the Journal of Experimental Biology, Mr Szczebak and his colleagues report that they studied the oxygen consumption of clownfish and anemones in the laboratory, using special tanks which can measure oxygen levels in the water as it is pumped in and out of the tanks. Readings were taken for 20-minute periods during the night and day with just a fish in a tank, just an anemone or both in a tank together.

Infra-red cameras quickly hinted that something was going on. The fish became active at night when they had an anemone to hang out with. They would twirl around it, push its tentacles up and down, and whip their tails about. It was like a sort of dance. The combined level of oxygen consumption when the fish and anemone were together was significantly higher than when they were kept apart. Mr Szczebak speculates that the antics of clownfish allow more water to flow over the anemone, thus increasing the amount of oxygen the anemone can collect. How lucky anemones are to have such good friends.