Damselfish Eye Confuses Would-Be Predators

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A damselfish (Pomacentrus amboinensis) prey from one of the predator treatments released onto the coral reef. The false eyespot is very pronounced.
Oona Lonnstedt /JCU

Young damselfish grow large fake eyes and bulk up their body to confuse predators and avoid being eaten, Australian researchers have found.

Marine biologist Oona Lönnstedt from James Cook University and colleagues published their research in the Nature journal Scientific Reports.

"Juvenile damselfish have lightly colored bodies and a conspicuous eyespot on the rear dorsal fin that fades away as individuals approach maturation," said Lönnstedt.

Watch one use its wits - and many arms -- to get a snack.
Ken Lucas/Getty Images

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Their study shows that false eyespots grow and real eyes shrink in response to the presence of predator. It is also the first to demonstrate that the presence of predators can affect both growth and color patterns in their prey.

Like some species of insects and fish, damselfish are highly vulnerable to predation when they are juveniles, and eyespots are thought to have evolved to deter predators.

A Taste of the Enemy

To explore what impact the presence of predators have on the development juvenile Ambon damselfish, (Pomacentrus amboinensis), young damselfish were caught at the end of their larval phase, before they had been exposed to predators.

One group was placed in a tank back in the lab and conditioned to dusky dottybacks (Pseudochromis fuscus). These were placed in the tank inside a transparent plastic bag for 30 minutes at a time, while skin extracts and other odor cues were simultaneously injected into the tank.

Another group was similarly exposed to the herbivorous goby (Amblygobius phalanea), while a third group was not exposed to any other fish.

The researchers found significant differences in both the behavior and morphology between the groups of study fish.

"Prey exposed to predators for six weeks grew deeper bodies, developed larger eyespots and exhibited stunted eye growth compared to prey exposed to herbivores or those that were isolated from other fish," said Lönnstedt.

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Increased body depth is a common prey response to gape limited predators, she adds. Not only do deeper bodies deter attacks, they also improve speed, acceleration and maneuverability.

"But what is intriguing is the finding that the juvenile prey grow larger eyespots and display smaller eyes when continuously exposed to predators."

Lönnstedt believes the large eyespot tail area of the damselfish, along with smaller eyes, gives the impression that the fish is pointing in the other direction, "potentially confusing predators about the orientation of the prey."

She adds that it could also lure predator to attack the tail rather than the head.

"An attack on the head would damage vital parts allowing almost no chance of survival."

Experience Counts

The researchers also found that damselfish exposed to predators were more cautious in their behavior: they foraged significantly less, were less active, and spent more time sheltering than the control fish, especially in the early stages of the experiment.

"Reduced activity levels increase prey survival by making the prey less conspicuous to the predator," said Lönnstedt. "(It) also saves energy, allowing individuals to allocate more into growth and/or development."

When they were released back on to the reef, the "experienced" fish had drastically higher survival rates, suffering only 10 per cent mortality, while the naïve controls were fives time more likely to have been eaten in the first 72 hours on the reef.

While false eyespots represent a short-term adaptation to the presence of predators, Lönnstedt says the results demonstrate that getting to know the enemy from an early age has important consequences for survival down the track.

-- Rachel Sullivan