Humbug damselfish use 'motion dazzle' to evade predators

New research suggests that high-contrast patterns on animals' bodies may serve a dual purpose: offering camouflage when stationary, then creating a 'motion dazzle' effect when moving, confusing potential predators into misjudging their location -- and helping them avoid being eaten.

The study, by Dr Louise Tosetto, Professor Nathan Hart and Dr Laura Ryan from Macquarie University's School of Natural Sciences and published in PeerJ on 26 September, is the first to show that humbug damselfish -- known for their striking black and white stripes -- use motion dazzle as a defence strategy.

"Our findings also show that humbug damselfish adapt their behaviour based on their environment," says Dr Tosetto.

"In their natural habitat, when they encounter backgrounds resembling their own striped patterns, like branching corals, they tend to move closer and reduce their movement. This likely helps them blend in and stay hidden from predators.

"However, when feeding outside the coral colony, where camouflage is less effective, they increase their movement and rely more on the confusing effects of their stripes, known as motion dazzle."

Methodical study

Humbug damselfish are small, striped coral reef fish that often live in social groups protected by branching corals. While their stripes help camouflage them when still, these fish frequently move to feed, risking predator attacks.

The research team studied how different backgrounds might affect the fish's ability to confuse predators using motion dazzle, by filming humbugs swimming in tanks against various striped backgrounds, then using computer models to simulate how predators like coral trout would see this movement.

They also used anatomical data from the retinas of humbug damselfish to help understand how clearly the fish might see different striped patterns in their environment.

The team found that when they were against backgrounds where predators would find it hard to see them clearly, humbugs tended to move closer to the background and increase their overall movement, so their own stripes would create confusing motion cues -- anti-predator behaviour.

But when the background had clear, wider stripes that predators could easily see, the fish gained two advantages. First, the edges of their bodies became less distinct, making them harder to spot. Second, the combination of their own stripes and the background stripes created confusing motion signals (motion dazzling), so predators couldn't accurately judge the speed or direction of the humbug fish.

"This suggests that humbugs can see different backgrounds and might then change their behaviour to improve their protection from predators," Dr Tosetto says.

Senior author Dr Ryan says the findings show that these fish have complex anti-predator strategies.

"This is an essential baseline study that provides new insights into the motion dazzle phenomenon," says Dr Ryan.

"Future research should investigate how factors like the water's light-filtering properties and interactions with other damselfish might affect motion dazzle. The humbug damselfish offers a perfect model to explore these important questions."