The secret life of Nemo: new study finds clownfish keep peace on reefs using ultraviolet flags

By Martin Luehrmann

October 29, 2022

Studies show that clownfish use the ultraviolet band to signal lower social rank and to stay out of harm’s way with their dominant anemone mates.

Anemonefish, more commonly known as clownfish, are perhaps the most popular among the inhabitants of one of Australia’s iconic Great Barrier Reef. Thanks to their distinctive colors and their debut on the big screen, making them a household name, most people could easily distinguish the anemone fish from any other species of reef fish. Whether their distinctive orange-and-white bars serve a purpose remains a mystery to moviegoers and biologists alike.

A new study published in the journal Behavioral Ecology determined how clownfish body color and vision can work together to help these charismatic little fish navigate the complex social landscape that governs their lives in association with anemones.

“Everyone knows Nemo,” says lead author Dr Laurie Mitchell. “But if you take a closer look, it’s quite surprising how little we know about them.”

Anemone fish live in groups, protected from predators between the hooks of sea anemones. Although anemone fish are dangerous to other marine life, they are protected from anemone stings by a slimy mucus layer that covers their skin.

Led by a single dominant female and her dominant male breeding partner, groups are organized into strict social hierarchies in which subordinate fish—all males—can ascend only if the superior dies. Obviously, the benefit of protecting anemones should be worth waiting for your “turn with the lady”. At 30-50 years of life, it’s a pretty annoying look.

A clear communication of dominance and subordination is necessary to maintain peace among all these expectations. An anemonefish that doesn’t effectively communicate its social rank to its fellow anemones, or that deliberately ignores established order, risks fatal injuries from fights or even expulsion from the anemone — “effectively a death sentence” for Nemo on reefs full of predators with a healthy appetite, Mitchell says.

A dominant male and female barrier reef anemonefish near Lizard Island, Great Barrier Reef.

Among animals that build social hierarchies, one of the most common and obvious signals of dominance is size. For example, in gorillas, the most dominant individual also tends to be the largest. The same is true of anemone fish, where once the dominant reproductive female and male take over their roles, they grow rapidly to become the largest fish in the group. Their subordinates remain smaller. But how do they communicate their social status to each other?

The researchers hypothesized that anemonefish may use their skin’s color markings to determine their social status when size alone is not enough. “Previous research from our lab found that the Barrier Reef anemonefish, amphiprion akindynos, it has UV-reflective color patterns and a visual system to see them,” Mitchell said. specifically tuned to perceive We didn’t know why that was or what it could use these color samples for.

Related: Saving Nemon: how climate change threatens anemonefish and their homes

“We found that small fish, such as juveniles and juveniles, are ‘brighter’ in UV light than larger and dominant fish,” explains lead author Dr Fabio Cortesi. “So this led them to the idea of ​​testing whether UV color is related to dominance behaviors in a similar way to size.”

To test this, the researchers went to Lizard Island on the Great Barrier Reef in far north Queensland and set up an experiment to discover the role that ultraviolet light plays in anemonefish dominance behaviour. After fishing on the reefs surrounding the island, they set up competitions between individual fish in small aquarium “arenas” of different anemones, but of the same size, separated only by a clear perspex divider. They observed the behavior of the fish towards each other with the UV component selected from the light illuminating the aquariums through UV-blocking light filters. In this way, the UV reflection of a fish in a part of the arena can be made invisible to observing fish, just as it is to us or any other UV-blind animal. Fish must try to determine who is dominant because they have never met, a process characterized by a range of dominance and submissive behaviors, such as leaning to the side and moving toward or away from an opponent or flashing to scare them.

What they found was surprising. Fish under UV filters were more likely to win races against size-matched competitors not under UV filters. Since overall aggression remained unchanged, this indicates that fish with low UV reflectivity are perceived as dominant and fish with high UV reflectivity are perceived as submissive.

In a second set of races, this time between larger and smaller fish, the effects were amplified by a dramatic increase in the aggressiveness of the larger fish toward their smaller rivals when placed under UV-blocking filters.

The authors concluded that among anemones, the higher UV-reflective skin—a characteristic of juvenile fish—serves as a reinforcing signal of submission, similar to “waving the white flag,” to larger, juvenile males who may perceive them as a threat. taking a public position and causing serious injury.

“The smallest individuals living in this very strict social system are at the mercy of the larger ones,” Mitchell said. “They should make it clear that they are submissive. We have shown that, in addition to their small size and various well-documented presentation behaviors, there is also a basis in their color pattern. Thus, by having a higher, bolder UV signal, they can more clearly signal their dominance to older fish.

It is not yet clear how important this additional information is to fish living and reproducing in the wild.

Justin Rhodes, a behavioral neuroscientist at the University of Illinois, says the ultraviolet component of anemone fish’s skin affects social interactions. but did not participate in the study. “But I think UV is one [of several] components that animals can use to determine social status, and I’m not sure if it’s needed. In our lab, we grow anemone fish under light without the UV component and it seems to have no effect on the fish. They still establish dominance hierarchies and reproduce without UV reflection. Once they’ve established who is dominant, they stay in the same environment for a long time – years and years – it’s not like they have to constantly check and see who is dominant. They know each other. So these are probably very important cues for fish to determine dominance when they first come to the anemones – the same size. [But] Once a hierarchy is established, the dominance assessment is over, no more fighting.

Only further research can help us fully understand anemone fish’s UV signals and how they relate to the fascinating lifestyles of these iconic fish.

The original study was published in the journal Behavioral Ecology.

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