cross-posted from: https://lemmygrad.ml/post/12107550
Some rays flash decoy eyes while others never do, as evolution's hidden trade-off comes into focus
Pacific Starry Skate. Credit: Andy Murch.
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 defenses an animal has at its disposal.
In a 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 analyzing multiple anti-predator defenses 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. The work has been published in Nature Ecology & Evolution.
"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.
Mediterranean Rough Skate. Credit: Andy Murch
Rasptail 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 defenses 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 meters deep.
"Eyespots are far from random. They tend to evolve in species that lack strong physical defenses, 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.
John Fitzpatrick at Stockholm University. Credit: Anette Gärdeklint Sylla/Stockholm University
"Evolution seems to favor different defensive toolkits. If you already have a strong mechanical or electrical defense, you don't also need a visual warning signal," says Madicken Åkerman.
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 were 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.