Clownfish Can ‘Count’ Stripes on Other Fish to Spot Intruders, Study Suggests | Smart News

Clownfish that live among sea anemones are not as kind as Pixar Finding Nemo can suggest. Reef-dwellers are actually very territorial, and they can be aggressive. “Just keep swimming” is not a mantra for life; it’s their signal to other fish—or even people—that are too close for comfort.

“It’s literally among the most aggressive animals on our planet,” said Justin Rhodes, a marine neuroscientist at the University of Illinois Urbana-Champaign who has been bitten by clownfish until they bleed. Scienceby Christa Lesté-Lasserre.

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

However, 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 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?

New research suggests that clownfish don’t just 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 Journal of Experimental Biology.

To find out, the research team raised a school of common clownfish—with three vertical white stripes—in isolation, making sure they hadn’t seen any fish above their species.

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 white stripe running horizontally from head to dorsal fin; Clarke’s anemonefish, which is black and yellow with two vertical white stripes; and saddleback clownfish, a black and orange fish that usually has three vertical white patches.

The researchers observed the common clownfish directing the most aggression—such as chasing or biting—to other common clownfish and the least aggressive toward the orange skunk clownfish, which has a horizontal stripe. Clarke’s and saddleback clownfish were lightly pressured. These results suggest common clownfish recognize—and are most aggressive toward—their own species.

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

The team then introduced small groups of three clownfish to different 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 clownfish aggression.

Clownfish attacked three-striped decoys ten times more often than zero stripes, twice as often as one-striped decoys and 1.3 times more often than two-striped ones.

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

Because clownfish show two stripes when they are 11 days old and don’t get a third until a few days later, the team wondered if the school’s attack on the two-striped decoy was related to their development. , according to a statement.

“There are probably factors other than the white vertical lines that are important in discriminating both species,” lead author Kina Hayashi, an ecologist at the Okinawa Institute of Science and Technology in Japan, tells GuardianNicola Davis. “But this experiment at least suggested that the number of white vertical lines is important in discriminating both species and deciding whether to attack or not.”