Small damsels at risk of being eaten by larger fish not only increase the size of the false 'eye spot' near their tail, but also reduce the size of their real eyes, distracting predators and dramatically boosting their chances of survival.
For decades scientists have debated whether false eyespots, or dark circular marks on less vulnerable regions of the bodies of prey animals, played an important role in protecting them from predators – or were simply a fortuitous evolutionary accident.
Now researchers from Australia’s ARC Centre of Excellence for Coral Reef Studies (CoECRS) have found the first clear evidence that Ambon damselfish (Pomacentrus amboinensis) can change the size of both the misleading spot and their real eye to maximise their chances of survival when under threat.
"It’s an amazing feat of cunning for a tiny fish," says says Oona Lönnstedt, a graduate student at CoECRS and James Cook University. "Young damselfish 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.
"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.
"We believe this is the first study to document predator-induced changes in the size of eyes and eye-spots in prey animals."
When the researchers investigated what happens in nature on a coral reef with lots of predators, they found that juvenile damsels with enlarged eye spots had an amazing five times the survival rate of fish with a normal-sized spot.
"This was dramatic proof that eyespots work – and give young fish a hugely increased chance of not being eaten.
"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," she explains.
The team also found that when placed in proximity to a predator the young damsels also adopted other protective behaviours and features, including reducing activity levels, taking refuge more often and developing a chunkier body shape less easy for a predator to swallow.
The paper "Predator-induced changes in the growth of eyes and false eyespots" by Oona M. Lonnstedt, Mark I. McCormick and Douglas P. Chivers appears in the latest issue of the journal Scientific Reports.
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