From butterflies to peacocks, bold circular "eyespots" are among nature's most eye-catching patterns. But why do they appear in some animals and not others? A new study of skates and rays finally provides an answer – and it lies in the full range of defences an animal has at its disposal.
In a new study examining more than 580 species – over 90% of all known skates and rays – researchers from Stockholm University have mapped the evolutionary history of conspicuous markings across this ancient group of cartilaginous fishes.
By analysing multiple anti-predator defences together, rather than studying eyespots in isolation, they were able to explain why such dramatic visual signals appear in some groups but are completely absent in others.
“Our results show that you have to look at the full range of options for avoiding predators. Eyespots evolve only under certain ecological and defensive conditions. They are one solution among many in the evolutionary arms race between predator and prey,” says lead author Madicken Åkerman.
Skates and rays face a wide range of predators, including sharks, marine mammals and large fish. Some species defend themselves with powerful electric organs or venomous spines. Others rely more heavily on camouflage, burying themselves in sand on the ocean floor.
The researchers found that species equipped with such robust defences rarely evolved conspicuous markings. In contrast, smaller-bodied species without those weapons were much more likely to develop bold spots or eyespots – particularly if they lived in well-lit, shallow waters less than 200 metres deep.
“Eyespots are far from random. They tend to evolve in species that lack strong physical defences, such as venomous tail stings or electric shocks, and that live in bright, shallow waters where visual signals are effective,” says senior author John Fitzpatrick.
“Evolution seems to favour different defensive toolkits. If you already have a strong mechanical or electrical defence, you don't also need a visual warning signal,” says Madicken Åkerman.
The study also uncovered a surprising evolutionary pattern: eyespots almost never evolved directly. Instead, species typically first gained simpler markings – such as bold spots – which later refined into the classic concentric-ring eyespots seen in some skates. In evolutionary terms, gaining simple markings was about 100 times more likely than gaining eyespots outright.
“It appears to be a stepwise process. Other markings come first, and over time they refine into eyespots,” says John Fitzpatrick.
Yet conspicuous markings are also frequently lost. This makes sense when considering the trade-off at play. In deep, dark waters where little light penetrates, a visual signal cannot be seen – and therefore offers no protection. Under those conditions, the cost of being conspicuous outweighs the benefit – and the markings disappear.
Nature
Observational study
Animals
The evolution of eyespots in skates and rays
24-Apr-2026