The partnership between clownfish and sea anemones is one of the most iconic in the animal world. Unlike in Pixar’s film Finding Nemo, clownfish seldom stray far from their anemone. During the day, they dart through the water overhead to catch morsels of food. At night, they snuggle deeply within the stinging tentacles. And the nocturnal half of this routine is still providing us with fresh surprises.
For ages, everyone thought that the two partners were a joint self-preservation society. The anemone’s tentacles provide the clownfish with protection from predators, while the clownfish chase away butterfly fish that would eat the anemone. More recently, Nanette Chadwick from Auburn University in Alabama showed that the fish also fertilise the anemone with their ammonia-rich waste.
Now, Chadwick’s team has found that the fish provide another secret service: They help the anemones breathe at night.
Sea anemones can do very little to control the flow of water across their bodies, and they rely on local currents to bring in oxygen and nutrients. For decades, scientists have suggested that the clownfish could help, but no one has actually tested that idea.
Chadwick and student Joseph Szczebak did so by studying the two-band clownfish and its host, the bubble-tip anemone, both collected from the Red Sea near Aqaba in Jordan. By separating the partners and measuring the oxygen concentrations in the surrounding water, Szczebak and Chadwick showed that, in the dark, they consume around 40 percent more oxygen together than apart. This only happened if they could actually made contact. If they were separated by a mesh barrier, and could see and smell but not touch one another, they used up less oxygen.
To understand why, Szczebak filmed the partners at night with infrared cameras, and saw that the clownfish were far more active than anyone suspected. While other marine biologists had claimed that clownfish stay still through the night, Szczebak saw the opposite—they spent most of their time moving. They would wriggle forcefully to wedge themselves between the anemone’s tentacles, and often made 180 degree turns in the process. And they never behaved like this when anemones were absent.
The fish’s frenetic dance aerates the anemone. Sway though they might, the tentacles aren’t great at moving water back and forth and stagnant zones can build up around them. That limits the movement of prey, nutrients, gases and more, and severely limits the anemone’s ability to grow. But the clownfish actively moves and opens up the tentacles, while encouraging water to flow between them. The fish uses up more oxygen because of the effort it makes and the anemone does so because of its partner’s antics, explaining the duo’s 40 percent hike in oxygen use.
The same dynamics are at work elsewhere on coral reefs. Many fish like gobies and damselfish live in corals at night, and face chokingly low levels of oxygen due to weakly flowing water. Both are known to beat their fins faster than normal to create a refreshing flow.
The fact that clownfishes do the same thing is a new slant to an old story. But there are still chapters waiting to be told. Consider this: Szczebak found that a clownfish is more likely to make its wedging, U-turning movements if it’s downstream of its anemone. Is the host releasing chemicals that spur the fish to start its dance? And what exactly is the clownfish doing? Anemones can expand and contract, and it may be that the fish are just trying to give themselves more room by inflating their hosts. Is aerating the anemone the point, or just a side effect of movements that accomplish some other purpose?
Reference: Szczebak, Henry, Al-Horani & Chadwick. 2013. Anemonefish oxygenate their anemone hosts at night. J Exp Biol. http://dx.doi.org/10.1242/jeb.075648