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Damsel fish relies on myterious illusory ‘eyes’ to throw off predators

Camouflage is a common defense trick among some types of crabs, lobsters, and other critters that forage along the ocean floor, but at least one breed of fish has evolved a visual trick that works to trick predators out in the open ocean. The damsel fish, according to Australian researchers, throws them off by deploying a pair of artificial eyes.

Researchers at the ARC Centre of Excellence for Coral Reef Studies (CoECRS), An Australian research institute, report observing small damsel fish displaying eye-like markings on their tails. The markings are black, circular spots with white outer rims. From a distance, they look exactly like their real eyes—albeit without the finer details of their irises and lenses.

Oona Lönnstedt, a graduate student at CoECRS and James Cook University, explained that while these eye spots are always present on the tail, the fish can instantly make them grow in size whenever they perceive danger. And they simultaneously shrink the size of their real eyes. The combined effect momentarily confuses an oncoming predator into thinking that the would-be prey is moving in one direction, when the would-be prey is actually darting off the opposite way just in the nick of time.

Lönnstedt and her colleagues saw it for themselves while watching damsel fish in a custom-built tank in which carnivorous fish that might eat them were swimming in cordoned-off chambers, close enough that the damsel fish could see and smell them but physically blocked from being able to attack the damsel fish. The latter, seeing and smelling danger, quickly performed their false-eye trick.

Scientists noticed these “eye spots” decades ago, and many scientists suspected that the eye spots helped the fish evade prey. But no one had any conclusive proof of the fish putting these eye spots to active use, much less growing or shrinking them as needed. The CoECRS observation was a definitive first.

The eye spot is most prominent in the younger damsel fish, Lönnstedt added. As the fish mature, their eye spots fade. The eye spots thus serve to give the fish an extra defense when they need it most—i.e., when they are at their smallest.

Damsel fish have long had a reputation for adaptability. The fish has evolved into more than 50 distinct species, with a domain that extends to both saltwater and freshwater areas throughout the Pacific and Indian oceans.

Their robustness makes them an immensely popular choice of fish for household aquariums, as even novice aquarium keepers find taking care of them relatively easy. They are, for instance, capable of digesting far more types of food than many other species.