In Disney’s Finding Nemo, the clownfish Marlin worries that his son Nemo might be hurt and asks him to count how many stripes he has. Nemo gets the answer right: three.
But in another species, the tomato clownfish, all the stripes except one disappear as the young fish mature.
Now, in an article published in PLOS Biology, researchers present a series of experiments that appear to explain what triggers the change in appearance—both environmentally and genetically. They claim that the tomato clownfish, in response to an unpredictable world, seems capable of flexibly adjusting when it loses its stripes, based on signals from other fish and its habitat.
In particular, the presence of a pair of adult tomato clownfish in a real anemone accelerates the fading of the stripes, suggesting that the young fish can alter their physical appearance to help them find a foothold (or a position) in the local social hierarchy.
A small fish with a big personality
The adult tomato clownfish lives among the tentacles of bubble-tip anemones in the Western Pacific Ocean. It’s an impressive little creature, “especially the females, which have a darker red color like a tomato,” says Laurie Mitchell, marine biologist at the Okinawa Institute of Science and Technology. “The male is quite smaller and tends to have a lighter coloration.”
What all adults have in common, however, is “a single white stripe on the head,” says Mitchell.
But tomato clownfish juveniles, which are only a few weeks old, have two or three white stripes—one on the head, one on the body, and sometimes one on the tail. At least, that’s how they start when they settle into an anemone for the first time and join a hierarchy strictly defined by the older tomato clownfish already there.
“That’s the beginning of their socialization,” says Mitchell. “It’s when they need to interact for the first time with others of the same species to form a functional social hierarchy.” If integration is not successful, the adults bite the young fish hard or expel them from the anemone, which is “certain death,” says Mitchell.
The young fish that manage to integrate into an anemone end up losing all their stripes except one—leaving only the white stripe on the head.
“The timing of this loss is very variable,” he says. “It’s highly unpredictable,” occurring at some point between one and twelve months of age, approximately.
Mitchell knew, from other studies, that a different species of clownfish (the classic Nemo variety) uses the amount of white coloration to identify members of its own species—and increase its aggression accordingly. He wondered if the tomato clownfish’s color change could also be some kind of social signal. So, he and his colleagues decided to investigate what was causing the disappearance of those white stripes—and what it might mean for the fish.
Four tanks, one unmistakable conclusion
The first step was to raise tomato clownfish fry in the lab.
“They’re quite fragile as larvae,” says Mitchell. “They’re basically like human babies, very demanding.”
They feed only on live zooplankton. They’re also quite sensitive to light and water quality. It took time, but Mitchell succeeded.
Then, he carefully transferred the nearly three-week-old fish to one of several experimental tanks. The first had nothing but water. The second contained water and a plastic anemone. In both tanks, 20 days later, the juvenile fish were practically the same—”the solid white stripes were still quite visible,” says Mitchell.
The third tank contained a live anemone. There, the white stripes faded only a little after 20 days.
It was in the fourth tank—the one with a live anemone inhabited by a pair of adult clownfish—that things changed. The juveniles quickly began to lose all their stripes except the one on the head.
After those same 20 days, “they were almost completely invisible,” says Mitchell. “They had basically diffused completely into the reddish-orange skin around them.”
At 62 days old (that is, just under a month later), all the fish in the aquariums with the live anemone alone had also lost all their stripes except the one on the head.
Mitchell discovered a series 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, shriveling, 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-coded hierarchy
Here’s the dynamic that Mitchell believes initiates the color change: when young fish arrive at an anemone in the wild, their small size and multiple stripes signal that they pose no threat to the hierarchy.
“They’re almost reclusive—they keep moving among the tentacles,” he says. “But after that, there’s no need to maintain that multi-striped form, because when it disappears, they’ve already integrated into the hierarchy and the function has been fulfilled. Even as a low-level member, you’re still a member.”
And with live anemones, the juveniles may become “more territorial in what is perceived as a more suitable habitat,” says Mitchell. “So, essentially, you have the same social pressure, but weaker.”
In summary, tomato clownfish display their colors when they lose the stripes to adapt to the new social group.
“This is an incredibly interesting paper,” says Theresa Rueger, coral reef ecologist at the University of Newcastle, who was not 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 that allow us to understand how animals change these colors as they grow.”
She says it offers insights into biodiversity more broadly—and how coloration is influenced by the social environment and used as a signal within it.
“What they really nailed here was the mechanism,” says Peter Buston, marine evolutionary ecologist at Boston University, who also did not participate in the study. He reflects on the diversity of color changes among different clownfish species, including those that add stripes as they age, rather than losing them. “It’s interesting to me that different social systems may have exploited this potential signal in different ways,” he says.
In other words, clownfish of all kinds offer researchers plenty to learn—within a very colorful school.
Source: npr.org