Aquarium

Clown anemonefish ‘recognizes’ the stripes of its potential rivals

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By Milthon Lujan

Figure showing the aggressive behavior of Amphiprion ocellaris, or clown anemonefish, in response to different species of anemonefish, both live and models. Credit: Kina Hayashi​​​​​
Figure showing the aggressive behavior of Amphiprion ocellaris, or clown anemonefish, in response to different species of anemonefish, both live and models. Credit: Kina Hayashi​​​​​

The vibrant colors of coral reef fish have long fascinated scientists, but new research delves deeper and uncovers a unique way in which clownfish use their distinctive white stripes to identify rivals and preserve their territory.

Clownfish are vibrant inhabitants of coral reefs, known for their symbiotic relationship with stinging sea anemones, displaying distinctive white bar patterns on their bodies. And it turns out, these barcodes are more than just fashion statements: they are crucial for efficient territorial defense.

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The study, conducted by researchers from the Okinawa Institute of Science and Technology (OIST) and the Institute of Cellular and Organismic Biology (ICOB), focused on the clownfish (Amphiprion ocellaris), a popular aquarium inhabitant. They observed that aquarium-raised clownfish were more likely to aggressively attack intruders of their species than other types of anemone fish. But the question persisted: how do they recognize their kind?

The Experiment

Did you know there are 28 species of anemone fish, each with a specific pattern of white bars? The study suggests that these stripes are not just pretty; they are crucial for identifying rivals.

By observing how a colony of clownfish (Amphiprion ocellaris), the species featured in the main character of Finding Nemo, reacts to intruders in their sea anemone home, OIST researchers have discovered that fish recognize different species of anemone fish based on the number of white bars on their bodies.

“The frequency and duration of aggressive behaviors in the clownfish were higher towards fish with three bars like them,” explains Dr. Kina Hayashi from OIST’s Marine Eco-Evo-Devo Unit, the first author of the article published in the Journal of Experimental Biology, “while they were lower with fish having one or two bars, and lower with those having no vertical bars, suggesting they can count the number of bars to recognize the intruder’s species.”

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Clownfish are usually gracious hosts, allowing many different species to visit their sea anemone. However, if a member of their species, not part of the colony, enters their home, the larger fish in the colony, known as the alpha fish, will aggressively bite and drive away the intruder.

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In this regard, researchers raised clownfish (Amphiprion ocellaris, known for their 3 white bars) in aquariums and introduced “intruders”: real fish and models with different bar patterns. Surprisingly, clownfish attacked their kind (3-bar models) more frequently, suggesting they recognize their own “uniform.”

Model behavior

To confirm their theory, the research team presented clownfish with models displaying 0, 1, 2, and 3 bars. The fish approached the 3-bar model aggressively as true rivals, further solidifying the link between stripes and species recognition.

The plastic models used to measure the clown anemonefish’s aggressive behavior. Photo: Kina Hayashi.
The plastic models used to measure the clown anemonefish’s aggressive behavior. Photo: Kina Hayashi.

The two-bar fish models were attacked less frequently, while those with one or zero bars received the least aggressive response.

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Previous studies have shown that clownfish react much more strongly to models with vertical bars than horizontal ones, suggesting that the quantity of white color or the overall presence of white bars is not the decisive factor. Combined with the observation that plastic discs, lacking species-defining features other than vertical bars, elicited the same response as live fish, researchers suggested that fish seem to be counting the number of vertical white bars to gauge their level of aggression towards intruders.

Researchers also discovered a strict hierarchy in clownfish anemone colonies that determines which fish attack the intruder. In nature, a colony is typically composed of an alpha female, a beta male, and several gamma juveniles. Social position within the colony is determined by very slight size differences. The anemone fish obtains its third and final bar as it grows, which is why the current alpha employs harsh methods to maintain the status quo, including expelling colony members if they grow too large.

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Defense Mechanism

Although the researchers used immature fish that have not yet metamorphosed into males or females, they still observed the same size-based hierarchy, with the largest juvenile assuming the alpha role and leading the charge against the intruder.

This behavior makes a lot of sense in the competitive reef world. By easily identifying and attacking competitors with the same bar pattern (who might covet their host anemone), clownfish maximize their chances of defending their valuable home. This efficient system ensures they have the best chances of survival and reproduction.

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By focusing their aggression on competitors with the same barcode, clownfish can maximize their chances of securing and defending their valuable territory. This saves energy and reduces unnecessary fights, ultimately promoting their overall physical fitness.

Conclusion

“It’s interesting to study anemone fish due to their unique symbiotic relationship with sea anemones. But what this study shows is that there is much we don’t know about life in marine ecosystems in general,” says Dr. Hayashi. The study serves as a sobering reminder of the need to preserve fragile coral reefs where fish like clownfish inhabit. If the clownfish, popular both as a pet and in the media, can surprise us with their ability to count bars and maintain strict social hierarchies, it raises the question of how many remarkable animals and animal behaviors are still undiscovered in these threatened ecosystems.

This research sheds light on the complex social life of clownfish and how they use visual signals to survive. It also opens the door to further studies on how other reef fish species use their various colors and patterns to communicate and compete.

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Reference
Kina Hayashi, Noah J. M. Locke, Vincent Laudet. 2024. Counting Nemo: Anemonefish Amphiprion ocellaris identify species by number of white bars. Journal of Experimental Biology, DOI: 10.1242/jeb.246357