Small Damselfish Fools Predators by Developing "Eyespots" on Tail

A tiny fish fools its killers by sporting a false "eyespot" on its tail, a new study has found. Interestingly, the fish has even evolved to have smaller real eyes and bigger "false eyes" to distract its predators.

The study was conducted by researchers at Australia's ARC Centre of Excellence for Coral Reef Studies (CoECRS). They found that when threatened constantly, the tiny damselfish can grow a large eyespot at the tail and reduce the size of its actual eyes. A result of this tactic is that the killer has no idea about the prey's direction and often fails to launch a good attack.

Earlier research had shown that these fish have eyespots on their tails but, whether these spots were favored by evolution or were just accidents wasn't known.

"It's an amazing feat of cunning for a tiny fish. Young damsel fish are pale yellow in colour and have this distinctive black circular 'eye' marking towards their tail, which fades as they mature. We figured it must serve an important purpose when they are young," said Oona Lönnstedt, a graduate student at CoECRS and James Cook University.

"We found that when young damsel fish were placed in a specially built tank where they could see and smell predatory fish without being attacked, they automatically began to grow a bigger eye spot, and their real eye became relatively smaller, compared with damsels exposed only to herbivorous fish, or isolated ones," Lönnstedt said in a news release.

Researchers also found that young damselfish with larger eyespots had a five times higher survival rate than damselfish with a normal eyespot.

"This was dramatic proof that eyespots work - and give young fish a hugely increased chance of not being eaten," she said in a news release. "We think the eyespots not only cause the predator to attack the wrong end of the fish, enabling it to escape by accelerating in the opposite direction, but also reduce the risk of fatal injury to the head."

The fish also employs other strategies to survive such as reducing activity levels when near a predator and developing a chunkier body shape. Previous research had shown that this fish could even spot a killer in darkness.

The study paper "Predator-induced changes in the growth of eyes and false eyespots" by Oona M. Lonnstedt, Mark I. McCormick and Douglas P. Chivers is published in the journal Scientific Reports.

There are about 250 species of damselfish mostly found in the Atlantic and Indo-pacific oceans. These fish are bright-colored with shades of red, orange, yellow, or blue and rarely exceed 15 cm (6 inches) in length.