In 2024, scientists found that clownfish use white stripes to spot rivals; the fish attacked most when intruders matched their three-stripe pattern

Forget that friendly, cute clownfish you remember from Finding Nemo. In real life, these little orange fish are some of the most territorial animals on a coral reef. According to a study published in the Journal of Experimental Biology, common clownfish can count the white stripes on other fish to identify family members and potential threats, and the fish they attacked the most were the fish that looked just like them.

Nemo was never that chill

Clownfish make their home inside sea anemones and fight fiercely for their tiny patch of home. They bite, chase, and don’t tolerate other fish in their territory easily, especially other clownfish. Justin Rhodes, a marine neuroscientist who studies clownfish at the University of Illinois Urbana-Champaign, has suffered serious bites from these fish that resulted in bleeding. Speaking to Science magazine, he said that clownfish are some of the most aggressive creatures on the planet. This aggressive behavior appears to be highly calculated. Scientists had already found that clownfish use white stripes as identification marks. Common clownfish have three vertical white stripes on them. Other fish that come without any stripes are generally left alone. However, if the other fish look alike, then the fights can become ugly very soon.


So how do they actually tell fish apart?

There are 28 known species of clownfish, per the study's own count of described anemonefish species, and reefs are full of other striped fish as well. So researchers from the Okinawa Institute of Science and Technology, led by ecologist Kina Hayashi, set out to discover just how these fish find “their own kind” in such a crowded, colorful neighborhood.

For the experiment, scientists raised several common clownfish in isolation from other species. As a result, these fish had never encountered other species besides their own. When the fish were six months old, 50 of these common clownfish were introduced, one at a time, in enclosed cases inside their tank, to three other species: the single-striped orange skunk clownfish, the two-striped Clarke's anemonefish, and the three-striped saddleback clownfish. The study reports the fish reserved their most aggressive chasing and biting for other common clownfish, their own species, while the single-striped orange skunk clownfish drew little response, and the two others provoked only mild aggression.

The decoy experiment that sealed the deal

To ensure that stripe number was the causal factor, the researchers built resin decoys resembling clownfish and painted them with either no stripes, one stripe, two stripes, or three stripes. Each experiment consisted of introducing groups of three clownfish to the decoys and observing their behavior on video. The study found that the clownfish were ten times more likely to attack the three-striped decoy, which had a pattern that exactly replicated theirs, as compared to the plain, non-striped decoy. The three-striped decoy drew about twice as much aggression as the one-striped decoy and 1.3 times as much as the two-striped decoy.

Hayashi told The Guardian that the number of white vertical stripes seems to be significant in judging whether a clownfish is of the same species as the one it is facing, and whether or not the clownfish is willing to attack. She also noted that young clownfish develop their first two stripes around 11 days old and a third stripe a few days later, which might explain why the two-striped decoy still drew considerable aggression.

Why should you care about fish math?

The 2024 findings were a small but striking reminder that counting is not just a human skill, or even one limited to mammals. The little reef fish seems to be counting visual features, tallying stripes, essentially, to determine who lives with it in its den and who should be chased away. This offers a neat correction to the childhood mythology many viewers absorbed from Finding Nemo. These fishes are not docile creatures who only wait for their babies to return home. As per the researchers, they are rather sharp, territorial, and astute enough to identify their rival.

The bigger picture

Alonso Delgado, a marine evolutionary biologist at Ohio State University who was not involved in the study, called the research convincing when he spoke to Science magazine. For now, there is no clear explanation of what causes aggression towards two-striped decoys. Even the scientists acknowledge that some other features may affect their behavior, apart from the number of stripes. One thing, however, can be stated definitely: matching stripes are pretty much a warning sign for a clownfish, and in a reef of lookalikes, counting to three can be a matter of survival.