The mathematical abilities and intelligence of anemonefish may have been greatly underestimated
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When the Pixar cartoon film, Finding Nemo, came out in 2003, it was a sensation, and suggested to the audience that its colorful stars were kind and peaceful creatures. Except they aren’t: they actually are feisty little fish that vigorously compete for and enthusiastically defend their anemone homes from others of their own kind. Which raises the question: how do anemonefish distinguish members of their own species from other striped fishes living in the neighborhood?
Kina Hayashi, a Research Fellow at the Okinawa Institute of Science and Technology, noticed that anemonefish species that live in the same locations tend to have a variety of stripe patterns — ranging from three vertical white bars to none. Could these fish actually be capable of counting the number of white bands on other fish to identify and distinguish friend from foe?
To find out, Dr Hayashi and collaborators raised a school of 120 common clownfish, Amphiprion ocellaris, from eggs and in isolation from all other clownfish species. When the youngsters reached approximately six months of age, Dr Hayashi and collaborators exposed them to other clownfish species sporting distinct stripe patterns that differed from their own, including Clarke’s anemonefish, A. clarkii, orange skunk clownfish, A. sandaracinos, and saddleback clownfish, A. polymnus (all pictured in this piece) — as well as individuals of their own species, and filmed their reactions.
Dr Hayashi and collaborators quickly discovered that their cute little Nemos were actually aggressive little monsters: they faced off with 80% of the other three-striped common clownfishes for around 3 seconds, with one such showdown lasting 11 seconds.
“Common clownfish… attacked their own species most frequently,” Dr Hayashi observed.
In contrast, other anemonefishes faced less territorial aggression: orange skunk clownfish, which have a white stripe along the top of their body, were (mostly) left alone, whereas Clarke’s clownfish, which has three white bars, and saddleback clownfish, which has two white patches, were mildly bullied.
To learn if it was the number of stripes rather than their specific shapes or placement that triggered these little Nemos’ fierce territorial aggressions, Dr Hayashi and collaborators made orange fish-shaped models with one, two or three bars, presented these to groups consisting of three young anemonefish in individual tanks and filmed their reactions.
As Dr Hayashi and collaborators predicted, the young anemonefish showed little interest in the plain orange model and only occasionally nipped at the model with one white bar. The two-barred model, however, received more bullying. Further, the anemonefish repeatedly demonstrated that they absolutely did not like sharing their space with the three-barred model.
But because common anemonefish have three stripes, why did they bully the two-bar model? Bad mathematical abilities? Bad eyesight? childhood trauma? Perhaps not. Dr Hayashi suggested that the clownfish’s aggression towards the fish model with two bars may be related to their development: common clownfish initially have two white bars at around 11 days of age before gaining the third bar three days later. So they may perceive two-barred fish as younger interlopers to be chased away from their anemone home, whereas fish with fewer (or no) bars are obviously different species and therefore, are unlikely to steal their anemone host.
“We conclude that A. ocellaris use the number of white bars as a cue to identify and attack only competitors that might use the same host,” Dr Hayashi and collaborators concluded. “We considered this as an important behavior for efficient host defense.”
This study, which adds detail to previous work (more here), does have some limitations. For example, the young Nemos only ever saw members of their own species, so it’s difficult to know whether their aggressive behavior is innate or learned due to their childhoods. But it’s useful to point out that previous studies have found that anemonefishes use their markings to identify each other as individuals as well as other species.
“Amphiprion ocellaris may therefore recognize fish with bar patterns as competitors and frequently attack and chase them out to defend their host anemone,” Dr Hayashi and collaborators explained in their study. “These previous studies indicated that the white bars could be an important color pattern for distinguishing competitors for territory in anemonefish.”
Kina Hayashi, Noah J. M. Locke and Vincent Laudet (2024). Counting Nemo: anemonefish Amphiprion ocellaris identify species by number of white bars, Journal of Experimental Biology 227(2):jeb246357 | doi:10.1242/jeb.246357
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