The truth about clownfish

I am a zoological pedant, obsessively and compulsively sensitive to the misrepresentation of animals in the media. This makes me the last person you’d want to take to the cinema, particularly when an animated movie is in town.

In Kung Fu Panda, for instance, I could not see why Po’s diet should have consisted of noodle soup when it could have been bamboo. In the Ice Age movies, I was puzzled that a squirrel with carnivorous, sabre-toothed dentition should have had such an eye for acorns. Finding Nemo, by contrast, contains a lot of accurate science, yet the truth about these bright little reef fish is still more incredible than most people appreciate.

In the opening sequence, a female clownfish (Coral) admires the anemone in which she and her partner (Marlin) have set up home. “A lot of other clownfish have their eyes on this place,” says Coral. This is an appropriate observation because real-world clownfish are fiercely competitive over anemones, entirely dependent upon them for their survival and rarely straying from the safety of their tentacles. It’s true that in an aquarium clownfish can live quite happily in the absence of anemones, but that’s only because there are no predators to hoover them up.

Like model tenants, clownfish offer their anemone landlords something in return. They fearlessly ward off butterflyfish, which are known to attack and feed on an anemone’s tentacles. With clownfish in residence, an anemone can relax, its tentacles typically swollen to around 80% of the max. Remove the clownfish and it shrinks to around 50%, a shrewd precaution because it is usually just a matter of hours before a butterflyfish attacks.

This is not all. Like all reef fish, clownfish excrete a large amount of ammonia. “It seemed logical that they would be fertilizing their host through their excretions,” says Nanette Chadwick, a marine biologist at Auburn University in Alabama who has studied the relationship between clownfish and anemones in detail. In a neat experiment published in 2005, she and her colleagues brought several giant anemones into the lab and sliced them in two, adorning one half with a pair of clownfish and leaving the other with no fishy residents. The halves with clownfish and hence ammonia were much quicker to regenerate than the fishless controls, probably because ammonia helps to fuel an increase in photosynthetic microscopic algae that provide the anemones with energy in exchange for somewhere to live.

Most recently, Chadwick and colleagues have demonstrated that clownfish also help to aerate the anemone at night. During the daytime, when the sun is shining, the photosynthetic algae can provide the anemone with all the oxygen it needs. In the dark when the algae are effectively asleep, the wiggling motions of the resident clownfish help aerate the anemones.

With the resident clownfish providing so many benefits, it is inevitable that the anemones should struggle in their absence. “When you collect the fish, the anemone dies,” says Chadwick. In the wake of Finding Nemo, this is an uncomfortable truth because the movie triggered a sudden and insatiable demand for Nemo fish. “It’s cheaper to get them off the reef rather than culture them,” says Chadwick. Indeed, surveys conducted last year on the Great Barrier Reef in Australia reveal that several species of clownfish are now absent from sites at which they once thrived and collecting is the likely culprit.

Finally, there is another reason I am critical of Finding Nemo. It failed to mention perhaps the most remarkable truth about clownfish: they go in for protandry or sequential hermaphroditism.

Just seconds after Coral and Marlin admire their anemone and the fertilized eggs that are developing inside it, the domestic bliss is shattered by a barracuda. Marlin and one egg (that will hatch out as Nemo) are the only survivors. When they get separated, the doting father spends the rest of the movie attempting to find his son. In the real world, things would have turned out rather differently. “If the female dies or migrates to a different anemone, the male changes sex and becomes the dominant female,” says Chadwick. Cool.

What’s more, larval clownfish are naturally buoyant, so when an egg hatches the little fish immediately float away from their home in search of a new anemone. “If they don’t their parents eat them,” Chadwick tells me. So Coral’s untimely death would most likely have triggered Marlin to undergo gender reassignment then dine out on his/her only son. It’s a clear instance of fact one-upping fiction. But I can see why Disney-Pixar didn’t use it.

This article was first published on BBC Earth in 2014.