In the Disney film Finding Nemo, the clownfish Marlin worries that his son Nemo may have suffered an injury, and asks him to count how many stripes he has. Nemo gets the answer right — three.
But in another species, the tomato clownfish, all but one of those stripes disappear as the young fish mature.
Now, in a paper published in PLOS Biology, researchers present a set of experiments that appear to explain what triggers the costume change — both environmentally and genetically. They say tomato clownfish, in response to an unpredictable world, appear capable of flexibly adjusting when they lose their stripes based on cues from other fish and their habitat.
In particular, the presence of a pair of adult tomato clownfish on a real anemone accelerates the fading of the stripes, suggesting that young fish may alter their physical appearance to help them find a foothold (or finhold) in the local social hierarchy.
A small fish with a big personality
The adult tomato clownfish lives amongst the tentacles of bubbletip anemones in the western Pacific Ocean. It's a striking little thing, "especially the females, they're kind of a darker, tomato-like red color," says Laurie Mitchell, a marine biologist at the Okinawa Institute of Science and Technology. "The male is a lot smaller and tends to have a lighter complexion."
What all adults do share, however, is "a single white stripe down the head," says Mitchell.
But juvenile tomato clownfish, which are just a couple weeks old, have two to three white stripes — a head bar, a body bar and, sometimes, a tail bar. At least, that's how they start out when they're settling onto an anemone for the first time and joining a strictly enforced pecking order of the older tomato clownfish already there.
"That's the onset of their socialization," Mitchell says. "That's when they first have to interact with others of their own kind to form a functioning social hierarchy."
If they don't integrate properly, the adults severely bite the young fish or evict them from the anemone, which is "certain death," says Mitchell.
Young fish that do join an anemone successfully go on to lose all but one of their stripes — leaving them, in the end, with just the white head bar.
"The timing of this loss is really plastic," he says. "It's highly variable," happening sometime between roughly one and 12 months of age.
Mitchell knew from other work that a different species of clownfish (the classic Nemo variety) uses the amount of white coloration to identify members of its own species — and dial up its aggression accordingly. He wondered whether the tomato clownfish's color change might be some form of social cue as well. So he and his colleagues decided to investigate what was leading to the disappearance of those white bars — and what it might mean for the fish.
Four tanks, one unmistakable conclusion
The first step was to rear baby tomato clownfish in the lab.
"They're quite fragile as larvae," says Mitchell. "They're basically like human babies, very demanding."
They only eat live zooplankton. They're also quite sensitive to light and water quality. It took time, but Mitchell managed.
He then gingerly transferred the not-quite-three-week-old fish to one of several experimental tanks. The first had nothing in it aside from water. The second contained water and a plastic anemone. In both these tanks, 20 days later, the juvenile fish more or less looked the same — "pretty much quite visible solid white bars remain," says Mitchell.
The third tank contained a live anemone. There, the white stripes faded only slightly after 20 days.
It was in the fourth tank — the one with a live anemone inhabited by a pair of adult clownfish — where things were different. The juveniles rapidly began losing all their stripes except for the head bar.
After those same 20 days, "they were almost completely not visible at all," says Mitchell. "They had pretty much diffused completely into the surrounding reddish-orange skin."
By 62 days of age (so not quite another month later), all the fish in the tanks with the live anemone alone had also lost all striping aside from the head bar.
Mitchell found a host of changes in gene expression likely responsible for the color change, including those associated with cell death. The cells that produce the white coloration were "basically fragmenting and shriveling up and dying," he says. And Mitchell found that hormones produced by the fish's thyroid may have been responsible for triggering the change in gene expression.
Color-coding rank
Here's the dynamic that Mitchell believes sets the color change in motion: When the young fish first arrive at an anemone in the wild, their small size and multiple stripes signal that they're not a threat to the pecking order.
"They're almost recluse — they're going between the tentacles," he says. "But after that, there's no need to keep that multi-bar form because by the time it's gone, they've integrated into the hierarchy, and the function is fulfilled. Even though you're a bottom rank member, you're still a member."
And with live anemones alone, perhaps "juveniles become more territorial in what is perceived as more optimal habitat," says Mitchell. "So you essentially get the same social pressure but weaker."
In sum, tomato clownfish flex when they lose their stripes to fit into their new social group.
"This is an incredibly interesting paper," says Theresa Rueger, a coral reef ecologist at Newcastle University who wasn't involved in the research. "You get the ecological side of the story, understanding how the fish live their lives. But you also get the mechanisms for us to understand how animals change these colors as they grow up."
She says this offers insights into biodiversity more broadly — and how coloration is both influenced by the social environment and used as a signal within it.
"What they really nail here is the mechanism," says Peter Buston, a marine evolutionary ecologist at Boston University who didn't participate in the study either. He reflects on the diversity of coloration changes among different species of clownfish, including those that add stripes as they age instead of lose them. "It's interesting to me that different social systems might have exploited this potential signal in different ways," he says.
In other words, clownfish of all forms offer researchers plenty to learn — inside a very colorful school.
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