All prey fish exposed to predator cues over a 6 week period had significantly larger eyespots (F) and smaller eyes than fish from the control treatments (C)

Juvenile damselfish have been found to grow larger fake "eyes" towards the rear of their bodies (relative to their real eyes) to direct any potential attacks from predators away from their heads.

This adaptation has been described by marine biologists from James Cook University in Australia -- Oona Lonnstedt and Mark McCormick -- along with Douglas Chivers from the University of Saskatchewan in Canada. The team has been studying the fish and their predators in their lab.

Many insects and fish have developed false eyespots -- large, dark circles surrounded by a lightly coloured ring that seems to represent an iris around a pupil. However the adaptive significance of these eyespots has been long debated by ecologists. Four broad hypotheses have been proposed regarding their function: they are there for deterring predators; they deflect attacks to non-vital regions of the body; they are used for status signalling or they are an evolutionary remnant that serve no purpose. 

Previous studies have shown that other animals' false eyes have helped them to deflect fatal attacks. A conspicuous black tail tip on long-tailed white weasels appears to reduce predation by predatory birds; hawks attacking the rodents in snowy environments were more likely to attack the conspicuous tail, rather than the head or body. Another study found that predators tended to attack insect prey towards conspicuous eyespots that had been painted on.

Young damselfish have lightly coloured bodies and conspicuous eyespots on their rear dorsal fin, which fade away as individuals grow older. The researchers aimed to study how threat cues from a common predator, Pseudochromis fuscus or dusky dottyback, affected the development of the young fish over six weeks. The team hypothesised that if the damselfish were kept in constant contact with their predators, their eyespots might not fade as they grew up.

The team gathered juvenile damselfish from the Great Barrier Reef using lighttraps as well as some dusky dottybacks (a common predator) and some herbivorous gobies. The fish were first of all trained to recognise the smell and sight of their predators. This was done by placing the dusky dottybacks into transparent plastic bags and then into the damselfish's tanks while simultaneously injecting the predator's odour cues into the water.

They then monitored the appearance and shape of the fish before dividing them into different groups. Some were placed into predator-prey tanks with a main section (containing either the predator or the herbivore goby) and six prey compartments. These sections were separated by Perspex barriers with holes in them. Water would flow through the predator compartment and then on through the prey compartments. A control group was placed into tanks with neither a predator nor a herbivore.

The predators were fed two damselfish morning and night to make sure that the cue stimulus (the chemical excretions used to communicate) provided to the prey was realistic. After six weeks the damselfish were measured again to check for changes in their bodies.

The team found that those fish exposed to predators for six weeks tended to grow deeper bodies (which rules out predation by those fish with smaller mouths and improves speed and acceleration), larger eyespots and stunted eye growth compared to prey exposed to herbivores or those that were isolated from other fish.

"The large eyespot in the caudal area of prey taken together with the smaller eye in the head region give an impression of the true eye being present in the posterior end of the body, potentially confusing predators about the orientation of prey," say the authors of the study.

This means that predators anticipate that the prey will move in the opposite direction when they initiate an attack, which might allow the prey to escape. Even if a strike is successful, the predators are less likely to damage vital parts of the body. A fish is much more likely to survive an attack on its tail and rear end than an attack to the head region.

The study appears to show that the mere presence of predators is enough to give prey a survival advantage in their natural environment.

This was put to the test by releasing the prey fish back onto individual patch reefs (25x15x20cm) in the field. Individual fish were given their own patch reefs and left to settle in with a cage over them for an hour, before having the cage removed. Scuba divers then checked whether the fish were still there twice a day for four days. Fish were assumed to have been caught by a predator if they had gone missing from their patch reef.

Those fish exposed to predators had dramatically higher survival rates, suffering only 10 percent mortality 72 hours after release, compared to 60 percent mortality in control groups.

"The results emphasise the importance of experience with predators to prey survival early on in life," the authors say.

The full study is published in Scientific Reports.


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