Australasian Science Magazine

By Stephen Luntz

Juvenile damselfish respond to the presence of predators by changing their body shape and expanding the size of their eye spots, but the mechanism for this development is still unknown.

Many fish, like butterflies and moths, produce large eye spots on their less vulnerable parts, apparently so that predators will attack there and give them a better chance to escape in an unexpected direction. At worst they may suffer less damage.

However, there is substantial variation in the size of these spots, even within a species. When James Cook University graduate student Oona Lönnstedt placed damselfish in a range of different conditions she found the spots changed with the circumstances.

“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 says. Fish that could see or smell predators had a similar response, while fish exposed to herbivorous fish or no fish at all kept their false eyes small and their real eyes large.

The frightened damselfish also changed their body shape, growing wider and shorter, apparently to be harder to swallow, as well as taking refuge more often. Once released into the wild, Lönnstedt announced in Scientific Reports that the predator-exposed fish tended to survive longer.

Lönnstedt’s supervisor, Dr Mark McCormick, admits that some aspects of the damselfish behaviour remain a puzzle. For example, since predators are always a threat in the wild, why don’t all the damselfish maximise the size of their eye spots?

“Making the real eye smaller reduces visual acuity,” McCormick explains. “Some people have suggested that there is an energetic cost to producing the pigments for a large spot, but it seems like a lame argument.” However, he admits that he hasn’t come up with anything better.

Damselfish lose the false eyes as they become sexually mature. McCormick thinks that for juveniles the eyespot provides a secondary benefit, reducing aggression within the hierarchy among mature damselfish for priority to food and mates. However, he says: “If you keep the spot when mature you may miss out on mating opportunities”.