Critter of the Month - Clown Anemone Fish

A gender-bending critter reveals what life is like as a male and then female. Made famous as Nemo... it's the anemonefish.

Peter - My name is Peter Buston, I'm assistant professor in marine biology and evolutionary ecology at Boston University. And the critter that I've worked on for a very long time is the clown anemone fish, made famous by the movie Nemo.

They're always found in close association with giant tropical sea anemones, which areabout, on average about half a metre across. The anemones provide the fish with everything they need. They provide them with protection from predators, a place for the fish to lay their eggs, and they never stray from the anemone's tentacles.

So within each anemone there's a single group of clownfish, anywhere from 3 to 6 individuals. The largest is always the female, and she's dominant to everybody else. The 2nd largest is the male. And the 3rd largest is a non-breeder, it's neither reproducing as a male nor as a female. The same goes for the 4th, 5th, 6th individual.

If we start from the bottom there's a queue for breeding positions so these non-breeders are waiting in line to inherit breeding positions. Lower ranked individual remains small, it remains 80% of the size of the immediate dominant. And it remains small because if it gets any bigger, the dominant will evict it.

If, for instance, the female of the group dies, then the male will change sex and take the position of the female. And the largest non-breeder, that's the one at the front of the queue will start breeding as a male and everybody else moves one up in the queue. It sometimes takes them 10, 15, 20 years to inherit a breeding position, but it's the best way of going about it because there are no breeding positions vacant elsewhere, because all the sea anemones are occupied and all the sea anemones have a little queue like this going on.

The species is really easy to work with. You can go onto the reef and you can map out all of the anemones and you can go back and find the fish exactly where you left them the day before. You can identify individuals based on natural variation in their markings the same way that you tell your friends apart. I was able to tell apart 350 individuals in the population.

They're super-photogenic so if you go down onto the reef, even with all the other colours around these clown anemone fish really stand out. There's loads of interesting questions but also you can do very cool, long term observations of a known population of individuals. And that's what attracted me to working on them.