When you think of Pixar’s clownfish Nemo, you probably imagine that these striped pranksters are peaceful creatures. This image could use some nuance; intruders are usually heavily attacked. How Nemo recognizes these? By counting their stripes, new research suggests.

Maybe you can already see it in your mind: a peaceful group of clownfish moving happily through the water. But rest assured: Nemo also protects his house against intruders. For example, scientists have discovered that the clownfish species Amphiprion ocellaris, also officially called the three-banded clownfish, can probably identify intruders by counting their stripes. Not all clownfish species are a threat to the clownfish, but outsiders of the same species are often perceived as dangerous. This is because clownfish have to share the limited sea anemones among themselves. Intruders often come along with the intention of kicking out the existing group. The research has been published in the journal Journal of Experimental Biology.

Clownfish are endlessly fascinating creatures that are very complex. The fish can live independently in the wild, but what you often see is that clownfish choose to live in small groups. Previous research has already shown that clownfish have a true hierarchy among themselves that is very special. The largest fish is always a female. This female lays the eggs in front of the nest and attacks intruders by hitting them with her tail. These eggs are not fertilized randomly by the males in the group; only the largest male can do that. The rest of the clownfish are completely sterile. When the largest fish dies, something very special happens: the largest male begins a process of metamorphosis, during which it grows a set of ovaries and takes the place of the largest female. His old place is then taken again by the male that is largest at that moment. Previous research has shown that the other group members often ‘talk’ among themselves to determine the rest of the ranking. Would you like to know more about three-banded anemonefish? Then take a look here.

Fighting fish
The scientists have completed a step-by-step plan to make the discovery. So they started the research by first breeding a new nest of clownfish to ensure that the group had never seen other species before. Once the young clownfish were six months old, they were first introduced to other clownfish species: black clownfish with two vertical stripes (A. clarkii), yellow clownfish with one long horizontal stripe on top (A. Sandaracinos) and saddle clownfish with three vertical stripes (A. polymnus). This showed that the fish with more stripes were also harassed more by the ‘alpha fish’ of the young group. However, it only really hit when invaders of the same species were introduced: these fish were attacked in 80% of all cases. Scientist Kina Hayashi contributed to the research. “The frequency and duration of attacks were highest against fish with three stripes like the clownfish themselves,” Hayashi explains. “At the same time, we saw that fish with only one or two vertical stripes were attacked much less. This seems to suggest that they can count.”

Plastic modellen
To prove this new hypothesis, Hayashi’s team came up with a plan. The idea here was to make small groups of three clown fish, and introduce them to fish-shaped plastic models with different numbers of stripes. What turned out? Once again, the researchers saw that the clownfish were most aggressive towards the plastic models that had three stripes, while the models with one and no stripes were left alone. The plastic two-stripe model was sometimes attacked; Scientists think they can explain this because very young clownfish are born with two stripes and only later develop their third stripe. So it could be that the fish think that the plastic model with two stripes is a young clownfish.

Smarter than you think
According to Hayashi, the results of the research are especially significant because they show that life in the ocean can be a lot more intelligent than we currently think. “Clownfish are interesting to study because they have such a unique and symbiotic relationship with sea anemones,” Hayashi concludes. “What this research shows above all is that there is so much we don’t yet know about the maritime ecosystem in general.”

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