New paper: Drivers of eyespot evolution in coral reef fishes

The twinspot goby (Signigobius biocellatus) is a wearer of prominent eyespots on coral reefs. Many fishes show these patterns - but how did they evolve?
Photo: R. Streit

Coral reefs are well known for their exceptional diversity of various colours and patterns. Each unique colouration provides certain fitness costs and benefits to its bearers. Thus, understanding the factors that shape these costs and benefits has remained a key interest to researchers for decades, and was the focus of PhD student Christopher Hemingson’s Doctoral Thesis.

“Colouration is a fascinating trait to study – it can tell us so much about how an organism behaves and interacts within its environment” - Christopher Hemingson

However, studying colouration is not as straightforward as it initially seems. Photographs provide a quick and effective way to collect colour data, however, to be comparable, the images must be taken in a standardised manner. This is easy to do for smaller studies, but becomes problematic when your goal is to study the colouration of reef fishes at a global scale.

Fig. 1 The global distribution of coral reef fishes with eyespots. While the overall taxonomic and phylogenetic richness of coral reef fish species varies substantially among regions, the proportions of species with and without eyespots are remarkably similar. The total number of species surveyed in each location are listed and have been used to scale each chart accordingly. Black: eyespot‐bearing; grey: eyespot‐lacking. Throughout all figures, these colours will represent eyespot‐bearing and eyespot‐lacking species, respectively.

In a recently published paper in the journal Evolution, Christopher got around this limitation. Along with co-authors Ale Siqueira, Peter Cowman and David Bellwood, Christopher studied the evolutionary drivers of a specific colour pattern across the global reef fish community: the eyespot. The eyespot is a discrete colour pattern that strongly mimics a vertebrate eye. Due to this resemblance, this marking is very effective at reducing predation.

Chris and colleagues set out to investigate what factors shaped the evolution of eyespots in coral reef fishes; essentially the rules that govern their appearance. They surveyed over 2,500 species of reef fishes (approximately 42% of all reef fish species) and recorded the presence or absence of eyespots, where they were located on the body, as well as the ecology of the species.

Fig. 2 The number of species, by family, with eyespots. Only families with >1 species with an eyespot have been plotted. The percentage of species with eyespots is represented by the large number above each bar. The total number of species surveyed in each family is given in parentheses. Note this is not the total number of species within each family. Three families make up 57.1% of all species with eyespots: Labridae (wrasses), Pomacentridae (damselfishes), and Chaetodontidae (butterflyfishes).

“The evolution of eyespots appears to be very phylogenetically conserved. Quite simply, when you plot the presence of eyespots on the tree of life, you can see clearly that they are extremely common in some groups of fishes but not others” - Peter Cowman

Fig. 3 The evolutionary history of eyespots within coral reef fishes. The internal branches have been painted according to which trait state was estimated to have the higher probability at each node. The concentric rings represent 25 million‐year increments. The full phylogeny with the exact character probabilities at each node (obtained from the 1000 averaged SIMMAPS) is available in the supporting information Fig. S3).

Hemingson and colleagues looked specifically into small damselfishes and related the presence of eyespots to their feeding ecology. Fishes that feed on plankton (small plants and animals that live suspended in the water) experience greater predation risk due to the exposed nature of where they feed. Although eyespots are effective at deterring predation, planktivorous species almost never had this marking.

“It’s really interesting. Even though these fishes occupy highly exposed habitats which are inherently riskier, they don’t utilise eyespots to deter predators. This tells us that eyespots are not a universal solution to predation; they are only effective in certain contexts” - Christopher Hemingson

Finally, they created a heatmap of where eyespots specifically occur on a fish’s body. Using innovative new mapping tools, they found two locations where eyespots are most likely to occur. The first is towards the back of the dorsal fin and the second is right behind the eye. Interestingly, the eyespots location appears to be tightly correlated to a fish’s activity on the reef. Active swimming fishes most often have an eyespot on the fin, whereas cryptic and benthic-dwelling species most often have the eyespot near the eye.

“This suggests that eyespots may have different functions depending on the location of where it occurs on the body. Eyespots on the fins are likely to direct attacks away from the head, whereas eyespots near the real eye may be used for intimidation” - David Bellwood.

Fig. 6 The total heatmap for all species (A). Eyespot heatmaps for active (B) and cryptobenthic (C) coral reef fishes. The colour changes from red to yellow reflect the frequency of occurrence of eyespots in that location. Yellow has been scaled to represent the location of highest eyespot occurrence in each heatmap. To the right are examples of species within each category: (B) Pomacentrus vaiuli and (C) Labrisomus nuchipinnis. Photo credit: Jeffery T. Williams, Smithsonian Institute.

Christopher believes that this study was an important first look into the broad evolutionary drivers that shape colouration in coral reef fishes. He strongly believes that as resources become more publicly available (like databases of standardised photographs), researchers will be able to more fully investigate what factors make coral reefs such colourful and enigmatic ecosystems.

To find more about this work and Christopher’s other research on fish colouration have a look at his profile here. To find a full copy of this paper, click here - or get in touch with Christopher via his profile.