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