Clownfish Can ‘Count’ Stripes on Other Fish to Identify Intruders, Study Suggests

Notoriously aggressive, common clownfish may be using basic mathematics to determine if another fish is a friend or foe

Two common clownfish, one facing left and one facing right, swim in a golden anemone
Common clownfish have three white stripes, which they "count" to identify other members of their species as potential threats, a new study suggests. Nick Hobgood under CC BY-SA 3.0 DEED

Clownfish living in sea anemones aren’t as amicable as Pixar’s Finding Nemo might suggest. The reef-dwellers are actually highly territorial, and they can be quite aggressive. “Just keep swimming” isn’t a mantra for life; it’s what they signal to other fish—or even humans—that get too close for comfort.

“These are literally among the most aggressive animals on our planet,” Justin Rhodes, a marine neuroscientist at the University of Illinois Urbana-Champaign who has been bitten by clownfish to the point of bleeding, tells Science’s Christa Lesté-Lasserre.

Previous research has shown that ocellaris clownfish (the common clownfish, as seen in Finding Nemo) are especially unfond of intruders of their own kind. To determine if an unknown fish is a friend or foe, they check its markings. If they don’t see any vertical white stripes—the common clownfish’s signature look—the fish is usually free to enter. But if they see striping similar to their own, they will defend their home.

Still, a big question remained for scientists. Not only do twenty-eight total species of clownfish swim the seas—some with as many as three stripes, others with none—but countless other patterned reef fish share the habitat. With all this diversity and color, how exactly do clownfish recognize members of their own species?

A graphic indicating the common clownfish's observed aggressive behavior towards other clownfish species and decoys
Common clownfish were more aggressive toward their own kind and the decoy that most resembled their species. Kina Hayashi

New research suggests clownfish do not simply check to see if stripes are present on another fish, but they actually count the stripes, too, according to a paper published last week in the Journal of Experimental Biology.

To learn this, the research team raised a school of common clownfish—which have three vertical white stripes—in isolation, ensuring they had never seen any fish beyond their own kind.

When the fish were six months old, the team set up several different tanks with cameras to film the interactions taking place inside. First, they placed 50 common clownfish in individual tanks and introduced more “newcomer” common clownfish. They also added fish from three other species: orange skunk clownfish, which are light orange and have a single white stripe running horizontally from head to dorsal fin; Clarke’s anemonefish, which are black and yellow with two vertical white stripes; and saddleback clownfish, black and orange fish that typically have three vertical white patches.

The researchers observed the common clownfish directing the most aggression—such as chasing or biting—toward the other common clownfish and being least aggressive toward the orange skunk clownfish, with its one horizontal stripe. The Clarke’s and saddleback clownfish were only mildly pressured. These results suggested common clownfish recognized—and were most aggressive toward—their own species.

Video from Amphiprion ocellaris experiments from Marine Eco-Evo-Devo Unit at OIST

Then, the team introduced small groups of three clownfish to various decoys, made from resin and painted with either zero, one, two or three stripes. The number of stripes on the decoy, they found, was a significant factor in the clownfish’s aggression.

Clownfish attacked the three-striped decoys ten times more often than those with zero stripes, two times more often than those with one stripe and 1.3 times more often than those with two stripes. 

Alonso Delgado, a marine evolutionary biologist at Ohio State University who was not involved in the study, tells Science that he finds the research convincing.

Four "decoy" clownfish, with stripes ranging from zero to three, and painted orange, attached to strings
The team tested how common clownfish reacted to four decoys and found they most frequently acted aggressive toward the one with three stripes, which mirrors their own species. Kina Hayashi

Because clownfish display two stripes when they are around 11 days old and don’t gain the third until a few days later, the team wonders if the school’s aggression toward the two-striped decoy is related to their development, according to a statement.

“Perhaps there are other factors besides the white vertical lines that are important in discriminating the same species,” lead author Kina Hayashi, an ecologist at the Okinawa Institute of Science and Technology in Japan, tells the Guardian’s Nicola Davis. “But this experiment at least suggested that the number of white vertical lines is important in discriminating the same species and deciding whether to attack or not.”

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