Stripey Damselfish Use ‘Motion Dazzle’ As An Anti-Predator Strategy

How to hide from one’s enemies in plain view? Dazzle them with your brilliant stripes!

I’ve written about why some reef dwelling fishes, particularly clownfish or anemonefish, are striped and how the direction of their stripes — vertical or horizontal — predicts their aggressiveness (read more about that here). But stripes on different species of reef dwelling fishes can serve other purposes.

For example, take the case of the whitetail dascyllus, Dascyllus aruanus. These small marine reef fish are often known by the peculiar common name, the humbug damselfish (Figure 1). These handsome shoaling fish have brilliant black and white vertical stripes. These stripes seem to act as camouflage that allow the fish to blend in with the corals in the background as long as they remain motionless. But humbug damselfish are very active during the day as they move about searching for a meal. What is the purpose of these highly contrasting stripes if the fish are zooming around almost constantly?

A research team based at Macquarie University investigated. Their newly published study suggests that highly contrasting stripes on an animal’s body not only may serve as camouflage, but may also create a “motion dazzle” effect when the animal is actively moving around, thereby confusing potential predators into misjudging their precise location within the group so individual stripey animals have a better chance of avoiding becoming lunch.

“Our findings show that humbug damselfish adapt their behavior based on their environment,” said the study’s lead author, Louise Tosetto, a postdoctoral research fellow at Macquarie University, where she specializes in animal behavior, their sensory systems and especially their visual ecology.

“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,” Dr Tosetto added. “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,” Dr Tosetto explained.

To better understand how these fish use their stripes to stay safe, Dr Tosetto and collaborators studied how different backgrounds might affect the fish’s ability to confuse predators using motion dazzle. First, they filmed ten wild-caught humbug damselfish as they swam around in aquariums with different striped backgrounds, then they used computer models to simulate how predators, such as coral trout (coral grouper), see this movement.

Dr Tosetto and collaborators also studied the anatomical features of the humbug damselfish retinas to better understand how clearly the fish may be seeing the different striped patterns in their environment.

First, Dr Tosetto and collaborators found that when the fish were exposed to backgrounds where predators would find it hard to see them clearly, the damselfish tended to move closer to the background and increase their overall movement, so their own stripes would create confusing motion cues — motion dazzle — an anti-predator behavior.

However, when the background had clearly visible, broad stripes that predators could easily see, the fish then benefitted in two ways. First, the edges of their bodies became less visually distinct, so they were harder to see. Second, the combination of their own body stripes and the background stripes created a visual commotion of motion signals, so predators were unable to accurately judge the speed or predict the direction that the humbug damselfish were moving in.

“This suggests that humbugs can see different backgrounds and might then change their behavior to improve their protection from predators,” Dr Tosetto explained.

But there’s another wrinkle in this story: Humbug damselfish also live and move in tightly interacting social groups amongst the branching corals. Their collective movement combined with their bold stripes creates even more visual confusion for predators that are seeking to target one individual in the shoal, particularly as these small stripey fish are feeding on the reef. Their alternating black and white patterns create even more confusion for hungry predators trying to single out and target just one fish from the group.

This social camouflage adds an extra layer to their survival strategy, highlighting the complexity of how these small fish evade threats, according to the senior author of the study, shark sensory biologist Laura Ryan.

“This is an essential baseline study that provides new insights into the motion dazzle phenomenon,” Dr. Ryan pointed out.

“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.”

Source:

Louise Tosetto, Nathan S. Hart, & Laura A. Ryan (2024). Dazzling damselfish: investigating motion dazzle as a defence strategy in humbug damselfish (Dascyllus aruanus), PeerJ 12:e18152 | doi:10.7717/peerj.18152