Counting Fish: Clown Anemonefish Intelligence

Sunday - February 11 2024, 22:57 UTC - 9 months ago

New research shows that clown anemonefish are able to recognize different species based on the number of vertical white bars on their bodies. These fish also exhibit a strict hierarchy within their colonies, and are able to count the number of bars to inform their level of aggression towards intruders. This study reveals the intelligence and complex social interactions of these seemingly carefree swimmers.

We often think of fish as carefree swimmers in the ocean, reacting to the world around them without much forethought. However, new research from the Okinawa Institute of Science and Technology (OIST) suggests that our marine cousins may be more cognizant than we credit them for. By observing how a colony of clown anemonefish (Amphiprion ocellaris) – the species of the titular character in Finding Nemo – reacts to intruders in their sea anemone home, OIST researchers have found that the fish recognize different anemonefish species based on the number of white bars on their bodies.

"The frequency and duration of aggressive behaviors in clown anemonefish was highest toward fish with three bars like themselves," explains Dr. Kina Hayashi from the Marine Eco-Evo-Devo Unit at OIST, first author on the paper published in the Journal of Experimental Biology, "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." .

The clown anemonefish is normally a gracious host, allowing many different species to visit their sea anemone. However, should a member of their own species, and which is not part of the colony, enter their home, the largest fish of the colony, referred to as the alpha fish, will aggressively bite and chase out the intruder.

To figure out how these fish determine the species of their visitors, Dr. Hayashi and colleagues conducted two sets of experiments with immature clown anemonefish raised in the lab. In the first set, they placed different species of anemonefish, with different numbers of white bars, in small cases inside a tank with a clown anemonefish colony and observed how often and for how long the fish would aggressively stare at and circle the case. In the second set, the researchers presented a colony of clown anemonefish with different plastic discs painted with true-to-life anemonefish coloration and measured the level of aggression towards these models.

The clown anemonefish displayed the most aggressive behavior towards the intruders with three bars like themselves. Fish and plastic models with two bars were attacked slightly less frequently, while the ones with one or zero bars received the least aggressive response. Previous studies have shown that clown anemonefish react much stronger to models with vertical rather than horizontal bars, suggesting that the amount of white color or the general presence of white bars is not the deciding factor.

Combined with the observation that the plastic discs, which have no species defining traits other than the vertical bars, received the same response as the live fish, lead the researchers to suggest that the fish appear to be counting the number of vertical white bars to inform their level of aggression toward intruders. "The researchers also discovered a strict hierarchy in the clown anemonefish colonies that determines the size and order of the fish," says Dr. Hayashi. "The alpha clown anemonefish is usually the largest fish in the group, but if it dies, the next largest fish will change into a female and lead the group. This process continues until the dominant female dies, at which point the largest male will transform and maintain the colony's structure." .

It is truly fascinating to see these small anemonefish display such sophisticated behavior. The findings of this study shed new light on our understanding of fish intelligence, and highlight the complexity of their social interactions. Our marine friends are definitely more aware of their surroundings and each other than we may have thought.