Look out! Clownfish can count

Clownfish, the orange and white fish made famous by Disney’s Finding Nemo, can probably count up to three, new research suggests.

A study by Japan’s Okinawa Institute of Science and Technology shows clownfish single out intruders based on their number of white bars, showing more aggression towards those with 3 stripes (like them).

That’s because the species – officially called clown anemonefish (Amphiprion ocellaris) – will happily share their home with other kinds of anemonefish but ward off other adults of their own species, using aggressive behaviour like chasing and biting.

In the first stage, individual clownfish were placed in separate tanks with a flowerpot standing in for a home base. Researchers then recorded how the clownfish reacted to the arrival of an ‘intruder’ fish – Clark’s anemonefish (A. clarkii), Orange skunk clownfish (A. Sandaracinos), Saddleback clownfish (A. polymnus) and clown anemonefish  – placed inside the tank within a transparent water filled box.

The more bars an intruder had, the more frequently the clownfish responded aggressively. The most frequent aggression was shown towards its own species. 

Prevented from biting the intruder by the transparent box, clownfish used other aggressive tactics like rushing at the intruder, or following the intruder fish while facing it.

The least aggression was shown to Orange skunk clownfish, a species with no vertical white bars, and one white stripe along its back.

“The frequency and duration of aggressive behaviors in clown anemonefish was highest toward fish with three bars like themselves,” explains author Dr Kina Hayashi.

In the second stage, 120 immature clownfish were placed in tanks in groups of 3 and given 8 days to establish social relationships. 

The colourful creatures usually live on a host anemone in small groups made up of an alpha female, beta male and several juveniles.

Into each tank, researchers dangled a model – a sort of clownfish puppet – made from resin clay with a set of white bars (ranging from 0 to 3) drawn on using marker.

The clownfish responded to the model intruder following a similar pattern, more aggression towards models with 3 white bars, and the least towards fish without vertical stripes.

The 28 species of anemonefish are coloured black, red, orange or yellow patterned with a series (between 0 and 3) of white stripes or bars.

Researchers say the findings, published in the Journal of Experimental Biology suggest clown anemonefish can count the number of white bars as a way to distinguish between different types of fish.