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

By Martin Luehrmann 29 October 2022
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Research reveals clownfish use ultraviolet stripes to signal lower social rank and stay out of harm’s way with dominant anemone-mates.

Anemonefishes, better known as clownfishes, are perhaps the most famous among the inhabitants of one of Australia’s iconic landmarks, the Great Barrier Reef. Thanks to their distinct colouration and their debut on the big screen, which made them a household name, most people could easily tell an anemonefish from any other reef fish species. Whether their distinctive orange-and-white bars serve a purpose, however, largely remains a mystery – to both movie-goers and biologists.

A new study published in the journal Behavioural Ecology has identified how clownfish body colouration and their visual system may work together to help these charismatic little fish navigate the complex social landscape that governs their anemone-bound lives.

“Everybody knows Nemo,” says lead author Dr Laurie Mitchell. “But if you look more closely, it’s quite surprising how little we really know about them.”

Anemonefishes live in groups, sheltered from predators among the stinger-clad tentacles of sea anemones. While dangerous to other marine life, anemonefishes are protected against the anemone’s stingers by a layer of slimy mucus that covers their skin.

Under the leadership of a lone dominant female and its dominant male breeding partner, the groups are organised in strict social hierarchies in which subordinate fish – all male – can only move up if those above them die. Clearly, the benefit of the anemone’s protection must be worth the wait for ‘your turn with the lady’. At a lifespan of 30 years, a rather frustrating outlook.

To keep the peace amid all this waiting, clear communication of dominance and subordination is imperative. Anemonefish that don’t effectively communicate their social rank to their anemone-mates or knowingly ignore the established order, risk mortal injuries from fighting or even expulsion from the anemone – on reefs teeming with predators with a healthy appetite for Nemo “this is effectively a death sentence,” Mitchell says.

Dominant male and female barrier reef anemonefish near Lizard Island, Great Barrier Reef. Image credit: Justin Marshall

Among animals that establish social hierarchies, one of the most common and obvious dominance signals is size. In gorillas, for example, the most dominant individual tends to also be the largest. The same applies to anemonefishes, where the dominant reproductive female and male rapidly grow to become the largest fish in the group upon assuming their roles. Their subordinates, however, remain smaller. But how do they communicate their social status amongst each other?

The researchers hypothesised that anemonefishes could use colour cues of their skin to help them determine social status when size alone may not suffice. “Earlier research from our lab had revealed that the Barrier Reef anemonefish, Amphiprion akindynos, has these UV-reflective colour patterns, as well as the visual system to see them,” Mitchell says. “And in fact, their visual system can not only detect the patterns, but it can see them so well, almost as if it was specifically tuned to perceive the colour contrast between the orange and the white/ultraviolet bars. What we didn’t know was why this is or what they may be using these colour patterns for.”

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

“We had found that smaller fish, such as juveniles and adolescents, are ‘brighter’ in the UV than larger and dominant fish,” explains senior author Dr Fabio Cortesi. “So this sparked the idea to test whether their UV colouration is related to dominance behaviours in similar ways as size is.”

To test this, the researchers travelled to Lizard Island on the Great Barrier Reef in far north Queensland and designed an experiment to reveal the role ultraviolet light plays in anemonefish dominance behaviour. After catching the fish on the reefs surrounding the island, they staged contests between individual fish from different anemones but of similar size in small aquarium ‘arenas’, separated only by a transparent perspex divider. They observed the fishes’ behaviour towards each other with the UV-component selectively removed from the light illuminating the aquaria via UV-blocking light filters. This way, the UV-reflectance of a fish in one part of the arena could be rendered similarly invisible to the observing fish as it is to us or any other animal blind to ultraviolet light. As the fish had never met, they should seek to determine who was dominant, a process characterised by a range of dominance and submissive behaviours, such as leaning sideways and pivoting towards or away from the opponent, or snapping to intimidate them.

A dominant barrier Reef anemonefish (left) snaps at its opponent during staged contests. Video: Laurie Mitchell. 

What they found was striking. Fish under UV-filters were more likely to win contests against size-matched opponents not under UV-filters. As overall aggression remained unaltered, this indicated that fish with low UV-reflectance were perceived as dominant and that fish with high UV-reflectance were perceived as submissive.

In a second set of contests, this time between larger and smaller fish, the effects were amplified by a dramatic increase of aggression of the larger fish towards smaller opponents when those were placed under UV-blocking filters.

The authors concluded that among anemonefishes, higher UV-reflective skin – a feature of juvenile fish – serves as a reinforcing signal of subordination akin to ‘waving the white flag’ in the face of larger, adolescent males that could perceive them as threats to their social standing and inflict serious injury.

“Living in this very strict social system, the smallest individuals are at the mercy of the larger ones,’ Mitchell says. “They have to communicate clearly that they’re subordinate. We have shown that, in addition to their smaller size and various well-documented submission behaviours, there also is a basis in their colour pattern. So by having a higher, bolder UV signal, they’re able to communicate more clearly that they’re subordinate to older fish.”

Just how important this added information truly is to fish that live and reproduce in the wild, though, remains less clear.

“The authors clearly show that the UV-component of anemonefishes’ skin has an impact on social interaction,” says Professor Justin Rhodes, a behavioural neuroscientist from the University of Illinois who is interested in the genetic basis of socially induced sex-change in anemonefishes but was not involved in the study. “However, I think that the UV is one [of several] components that the animals could use to determine social status, and I am not sure whether it’s needed. In our lab we rear anemonefish under light without a UV component, and it does not seem to affect the fish at all. They still establish dominance hierarchies and reproduce without UV reflectance. Once they figure out who’s dominant, and then stay in that same environment for a long time – years and years – it’s not like they constantly need to check up and see who’s dominant. They know each other. So these are probably very important cues for fish figuring out dominance – some of the same size – when they first come to the anemone. [But] once a hierarchy is established, the dominance assessment is over, there is no fighting anymore.”

Only further research can help us fully understand anemonefishes’ UV signals and how they tie in to these iconic fishes’ fascinating lifestyles.

The original research was published in the journal Behavioural Ecology.