Clownfish can count, but how did they learn?

The common clownfish, famously depicted as Nemo in the beloved Pixar film, possesses the ability to count, according to a new study from the Okinawa Institute of Science and Technology.

This astonishing discovery challenges the serene and simplistic depiction of clownfish. 

The study, led by Dr. Kina Hayashi and colleagues Noah Locke and Vincent Laudet, delves into the complex social behaviors of anemonefish.

Defending their territory 

Contrary to the peaceful existence suggested by their on-screen portrayal, clownfish are fiercely territorial creatures.

They zealously guard their anemone homes against same-species intruders with a level of discernment that hints at unexpected cognitive capabilities.

The study stemmed from a curiosity about how clownfish distinguish between friend and foe in their densely populated reef environments, where encounters with other striped fish species are frequent. 

Social dynamics

Anemonefish, including the common clownfish (Amphiprion ocellaris), exhibit a variety of stripe patterns, ranging from three vertical white bars to none, suggesting that these patterns could play a role in species recognition and social dynamics within the reef.

To explore this theory, Dr. Hayashi and her team embarked on a meticulously designed experiment. They raised a cohort of common clownfish from eggs, ensuring these fish had no prior exposure to other anemonefish species. 

At approximately six months old, the team observed the young clownfish’s interactions with both their own species and other anemonefish species, documenting the responses to perceived intruders. 

Remarkably, the common clownfish demonstrated a pronounced aggression towards their own kind, especially those bearing the characteristic three white stripes, while showing varying degrees of tolerance towards other species, based on the number and arrangement of their stripes.

Visual cues help clownfish count

The researchers further investigated this behavior by introducing the clownfish to models painted with different numbers of white bars. 

The reactions were telling; the clownfish exhibited a clear preference for challenging models that mirrored their own three-striped pattern, suggesting a sophisticated level of species and self-recognition based on visual cues.

“The frequency and duration of aggressive behaviors in clown anemonefish was highest toward fish with three bars like themselves, while they were lower with fish with one or two bars, and lowest toward those without vertical bars, which suggests that they are able to count the number of bars in order to recognize the species of the intruder,” explained Dr. Hayashi.

Dr. Hayashi proposes that this behavioral trait is not just a simple act of aggression but a nuanced form of social interaction rooted in the necessity to protect one’s habitat from competitors. 

This discernment, based on counting the number of stripes, allows clownfish to effectively identify and deter potential rivals, ensuring their survival and the integrity of their homes within the complex ecosystem of the reef.

Clownfish social hierarchy 

The researchers also discovered a strict hierarchy in the clown anemonefish colonies that determines which fish attack the intruder. 

Anemonefish get their third stripe when they get grow large enough. Social positions within a colony are determined by slight size differences, and typically consist of one alpha female, one beta male, and several gamma juveniles. The team found that the alpha will even chase out members of its own colony if they grow too large. 

When the researchers studied immature fish that did not yet have their third stripe, the same size-based hierarchy was observed. The largest juvenile took on the role of alpha and led the charge against the intruder. 

Implications of clownfish counting

“Anemonefish are interesting to study because of their unique, symbiotic relationship with sea anemones. But what this study shows is that there is much we don’t know about life in the marine ecosystems in general,” said Dr. Hayashi.

The study sheds new light on the cognitive abilities of fish, particularly in terms of visual recognition and social discrimination. It challenges long-standing perceptions of fish intelligence and opens new avenues for research into the cognitive processes underlying animal behavior in marine environments. 

The research is published in the Journal of Experimental Biology.

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