Clown anemonefish seem to be counting bars and laying down the law

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 which fish attack the intruder. In the wild, a colony typically consists of one alpha female, one beta male, and several gamma juveniles. The social position within the colony is determined by very slight differences in size. Anemonefish get their third and final stripe when they grow large enough, which is why the current alpha uses harsh methods to uphold the status quo, including chasing out colony members if they grow too large.

Though the researchers used immature fish that have yet to metamorphize into males or females, they still observed the same size-based hierarchy, with the largest juvenile taking on the role of alpha and leading the charge against the intruder. 

“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”, says Dr. Hayashi. The study is a sobering reminder to preserve the fragile coral reefs that fish like the anemonefish inhabit. If the clown anemonefish, which is popular both as a pet and in the media, can surprise us with their abilities to count bars and maintain strict social hierarchies, then it begs the question of how many remarkable animals and animal behaviors have yet to be discovered in these ecosystems under threat.