Rasptail Skate. Credit: Andy Murch.
A new study of skates and rays has finally provided an answer to why some have ‘fake eyes’ and others don’t – it’s down to defence.
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?
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.
Lead author Madicken Åkerman said: “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,”
Mediterranean Rough Skate. Credit: Andy Murch.
Different species, different toolkits
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.
Senior author John Fitzpatrick said: “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.”
Åkerman added: “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.”
Pacific Starry Skate. Credit: Andy Murch.
A stepwise process
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.
Fitzpatrick said: “It appears to be a stepwise process. Other markings come first, and over time they refine into eyespots.”
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. The study is published in Nature.
