Is Nemo A Meanie? Clownfish Stripes Predict Aggressiveness

Most anemonefishes are striped and the direction of their stripes — vertical or horizontal — correlate with their levels of territorial aggression

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Dazzling colors and color patterns evolved as visual signals that operate over short distances. The most diverse pigment cell types of any vertebrate group are found in tropical fishes, and create an exhilarating variety of color patterns such as bars, stripes and spots (ref). This riotous visual variety is particularly effective in the excellent lighting and crystal-clear waters found in coral reefs, where fishes living there rely on them for camouflage, mimicry, individual and species identification, courtship and other social interactions. It’s interesting to note that whilst competition and cooperation within the coral reef community are established using visual signals, we still don’t know how particular colors and color patterns affect which species live within coral reef fish communities.

Anemonefishes, also known as clownfishes, are small, brightly colored and boldly patterned fishes that live on coral reefs in close association with sea anemones — hence, their common name. Only ten species of sea anemones are known to host clownfishes, and each anemone species is only compatible with limited subgroup of clownfish species.

There are 28 species of clownfishes and most of them have conspicuous white bars on a base color of orange, red or black, with the number of bars varying between species. The purpose of these bars is not well understood, but several hypotheses have been proposed to explain them. First, it is possible that the number of white bars serves to help species recognize each other because the differences in white bar numbers between species occupying the same area are significantly greater than what would be expected by chance (ref). Additionally, as young anenomefishes mature, their patterns change in some species — is this a dishonest signal meant to reduce adult aggression towards juveniles? The second hypothesis proposes that the contrast between the bold white bars and the bright base color is visually disruptive and thus, makes the fish’s silhouette difficult to detect (ref). The third hypothesis proposes that the conspicious colors and patterns advertize the toxicity of the clownfish’s host anenome. There is some data to support this proposal: a previous analysis did find that host venom strength and tentacle length are correlated with the color patterns of their anenomefish partners (ref).

To better understand the behavioral implications associated with clownfish stripes, an international team of marine biologists from Okinawa Institute of Science and Technology (OIST), The University of Ryukyus and the Institute of Cellular and Organismic Biology collaborated to investigate how anemonefish color patterns are associated with the frequency of aggressive behavior in coral reefs.

“Our findings are the first to highlight that banding patterns of fish in the vicinity of an occupied anemone influence the behavior of other resident fish”, said the lead author of the study, Kina Hayashi, a Research Fellow with the Marine Eco-Evo-Devo Unit, which operates under the guidance of senior study co-author, Vincent Laudet.

To do this research, the team conducted field studies with wild fish living at five different sites (two study sites on reefs around Miyakojima, one around Ishigakijima and two around Iriomotejima) which are part of the Sakishima Islands in the Ryukyu Archipelago, between September 2020 and October 2021 (Figure 1).

The team focused their studies on the ocellaris clownfish, Amphiprion ocellaris, and their host, the giant carpet anemone, Stichodactyla gigantea, in each study area, which ranged from 0 to 2 meters in water depth. The team filmed the fish associated with anemones and with nearby scleractinian corals (as a control), and divided those fish into three categories based on their stripe patterns: bars (vertical stripes) (Figure 2a), stripes (horizontal stripes) (Figure 2b) and others (Figure 2c). Fishes with both bars and stripes, such as butterflyfish, were classified as ‘others’.

Why might so many fishes that are not hosted by an anemone be found in association with them?

“Although anemonefish aggressively defend their host anemone, a host anemone can still be a temporary home for other fish seeking refuge”, Dr Hayashi said in a statement.

The team observed that fish sheltering in anemones had either spots or horizontal stripes, but only the resident ocellaris clownfish had vertical bars. Why?

“We hypothesized that the anemonefish were playing gatekeeper. It was possible they were excluding fish based on color patterns”, Dr Hayashi proposed.

To investigate this hypothesis, the team documented aggressive behaviors by the resident ocellaris clownfish family by dangling plastic fish toys close to their anemone host. These toys were painted with either vertical or horizontal white stripes, and the reactions of the clownfishes on each study site were recorded as aggressive when they either chased or bit the toy decoys.

Dr Hayashi and her collaborators found that no fish with vertical bars sheltered in anemones, whereas fishes with vertical bars did shelter in the surrounding corals (Figure 3). The team documented that resident anemonefish responded aggressively when presented with fish toys with vertical bars. Further, the duration of clownfish aggressive behaviors toward fish toy models with vertical bars was much longer than towards the models with horizontal stripes.

Dr Hayashi’s analyses revealed that only 39% of the study anemones were used as a shelter by fish other than anemonefish, whilst 100% of the corals were used as a shelter by fish of other species. (This may also be due to these fishes’ lack of tolerance for anemone venom.) But it is probably more likely due to the aggressive behavior of the resident anemonefish. Therefore, Dr Hayashi and her collaborators hypothesize that the reason for this difference was the aggressive behavior of anemonefish, which defend host anemones as their territory by actively chasing away many intruding fish, indicating that only fish species without vertical bar patterns may access host anemones.

“It is likely that the fish community around host anemones is affected by the differences in the frequency of aggressive behavior of anemonefish towards intruders with certain color patterns”, Dr Hayashi explained.

These results suggest that the fish community around host sea anemones is strongly affected by the visual preferences of anemonefish. In order to further explore these findings, Dr Hayashi says future comparisons of aggressive behavior of anemonefish towards a variety of color patterns at different sites are needed.

For example, anemonefish are known to recognize the same species by visual cues (ref), it is possible that they can identify the numbers of white bars and alter their behaviour based on such information.

“Elucidating the relationship between color pattern recognition ability and aggression displayed by anemonefish will provide useful insights on the mechanisms of coexistence employed by coral reef fish, and the roles that their patterns play”, Dr Hayashi elaborated. “In regions with coral reefs, conserving species and their habitats is crucial, which is possible if we understand the interactions between these inhabitant species.”

The researchers aren’t sure yet how color patterns are recognized and what basic rules control the inter- and intra-specific signaling. However, they did note that horizontal stripes are often associated with shoaling in cichlids, zebrafish and rainbowfish (ref). Furthermore, horizontal stripes could be a visual signal of interspecific cooperation, such as cleaning behavior.

“As all the cleanerfish in the study site have horizontal stripes, it was not possible to confirm whether the anemonefish recognized the cleanerfish species or judged them solely on the basis of their stripe patterns, which remains an issue to be examined in the future”, Dr Hiyashi and her co-authors write. “It is also necessary to focus on how cleanerfish behave in response to aggressive behaviour by anemonefish.”

Dr Hiyashi and her co-authors firmly believe that future investigations into how the rules of color patterns influence fish behavior can help us understand the community structure as well as protect biodiversity near coral reefs from climate change.


Kina Hayashi, Katsunori Tachihara, James Davis Reimer and Vincent Laudet (2022). Colour patterns influence symbiosis and competition in the anemonefish–host anemone symbiosis system, Proceedings of the Royal Society B: Biological Sciences 289:20221576 | doi:10.1098/rspb.2022.1576

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