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Clownfish (symbolic image)
Photo: Pond5 Images / IMAGO
The animated film “Finding Nemo” showed that clownfish feel comfortable and safe in sea anemones. They defend their territory. And for this they may benefit from a property that researchers from the Okinawa Institute of Science and Technology (OIST) now claim to have discovered: namely, that clownfish can count. They published their results last week in the journal “Journal of Experimental Biology”.
According to a statement about the study, clownfish would actually be considered friendly hosts; many other species could easily visit the native sea anemone. But as we all know, hospitality has limits - with clownfish, it was reached when clownfish from another colony wanted to swim into the anemone. The so-called alpha fish would aggressively drive away the intruders.
"The frequency and duration of aggressive behavior in clown anemonefish was highest in fish with three bars, like themselves," said lead author Kina Hayashi from the Marine Eco-Evo-Devo Unit at OIST, according to the release. On the other hand, it is lower in fish with one or two bars and lowest in those without vertical bars. This suggests “that they are able to count the number of bars in order to identify the type of intruder.”
For the study, the researchers observed in two series of experiments how a colony of clownfish (Amphiprion ocellaris) reacts to intruders in their sea anemone home. The fish they used had grown up in the laboratory:
In the first series of experiments, Hayashi and her team placed different species of anemonefish with different numbers of white stripes in small boxes in a tank containing a clown anemonefish colony. They said they observed how often and for how long the fish aggressively stared at and circled the box.
In the second series of experiments, the researchers presented a colony of clown anemonefish with various plastic discs that were painted with lifelike anemonefish colors. They also measured how aggressive the fish would behave towards them.
The result: The clownfish showed the most aggressive behavior towards the intruders with three bars - as many as they have. According to the researchers, fish and plastic models with two bars were attacked slightly less often. They reacted least aggressively to one or no bar. The fish also showed the same reaction to the plastic discs and the live fish.
A strict hierarchy regulates which fish takes on the attack on the intruder, which is based on minimal differences in size - whereby their own stripes also play a role. It is said that anemonefish get their third and final stripe when they are big enough. The respective alpha fish therefore uses harsh methods to maintain their status. For example, by driving away other colony members.
The study is a sobering reminder to protect the fragile coral reefs where fish like the anemonefish live, Hayashi said. “If the clownfish can surprise us with its ability to count stripes and maintain strict social hierarchies, then the question arises as to how many remarkable animals and animal behaviors remain to be discovered in these threatened ecosystems.”
ani