Do clownfish count stripes?

The different number of stripes in the different anemonefish species apparently has a special meaning. Pictured: Amphiprion ocellaris, © Kina Hayashi

They get particularly angry at three: The number of white stripes that clownfish recognize on a visitor to their home anemone apparently determines how aggressively or calmly they react. They use the number of stripes to distinguish competing conspecifics from “harmless” anemonefish species. This emerges from experiments with fish dummies with different stripes. Versions with two, one or no stripes bothered the test animals significantly less than those that were drawn like themselves. It is another example of the sometimes complex abilities of fish, say the scientists.

The animated film “Finding Nemo” made the clownfish (Amphiprion), also known as clownfish, famous. It is about the adventures of a three-striped representative of this genus of damselfish, which includes a total of 28 species with different markings. They are found in the coral reefs of the tropical Indo-Pacific including the Red Sea. What they have in common is their way of life in a community with sea anemones: clownfish live in small groups between the tentacles of their respective home anemones. The cnidarians offer the inhabitants, who are adapted to their poison, protection from attackers. In return, it is primarily the alpha animal of the group that defends the anemone and the group provides it with additional food through its leftover food and excretions.

It is known that the clownfish not only attack predators of the anemone, but also chase away members of their own species. However, visitors from other anemonefish species usually leave them alone. This is because the different clownfish species are specialized on certain anemone species, which means that they usually do not compete for living space between species. However, a member of the same species can dispute the anemone and is therefore attacked. In this context, the research team led by Kina Hayashi from the Japanese Okinawa Institute of Science and Technology has now investigated the extent to which the different stripe markings of the various anemonefish species play a role in the perception of the threat. There are species that have three white vertical stripes like “Nemo”, but also species with two or one stripes and monochrome.

The researchers chose the three-striped clownfish species Amphiprion ocellaris for their experiments. The test animals were raised from eggs in aquariums and then used for the experiments when they were around six months old. First, the team examined the fish's reactions to the presence of other anemonefish species with different markings as well as to visitors of their own species. Fundamentally, it was once again shown that the A. ocellaris clownfish attack their own species most intensively. However, the orange anemonefish (Amphiprion sandaracinos), which has no vertical stripes, received little attention. The researchers then moved on to a test that was intended to show more precisely to what extent the number of strips plays a role in the reactions.

Three stripes have the strongest effect

To do this, they confronted established groups of three clownfish with various plastic dummies.

Summary presentation of the study results. © Kina Hayashi

These were either uniformly orange or had one, two or three white stripes. Apart from the vertical bars, the shapes had no other species-defining features. As the team reports, their observation results showed that the fish actually respond to the number of stiffs. The test animals showed the most aggressive behavior towards the artificial intruder, which, like them, had three white stripes. The strength of the defense reaction corresponded to that of real conspecifics. The plastic models with two bars, however, were attacked less often, while models with one or no bars caused the least aggressive reactions.

As the researchers explain, previous research has already shown that clownfish respond significantly more strongly to models with vertical rather than horizontal bars, suggesting that the amount of white color or the general presence of white stripes is not the deciding factor. “Our results therefore indicate that the clownfish are able to count the number of bars in order to recognize the type of intruder,” summarizes Hayashi. “Ultimately, the study also highlights how much we don’t yet know about the creatures in marine ecosystems,” says the researcher.

Source: Okinawa Institute of Science and Technology (OIST) Graduate University, The Company of Biologists, specialist article: Journal of Experimental Biology, doi: 10.1242/jeb.246357

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