Stockholm University Study Reveals Why Defensive Weapons Prevent the Evolution of Eyespots in Skates and Rays

Stockholm University researchers discover that eyespots in rays only evolve when species lack venomous stings or electric organs.

By: AXL Media

Published: Apr 25, 2026, 6:29 AM EDT

Source: Information for this report was sourced from EurekAlert!

The Evolutionary Logic of Marine Camouflage

A massive evolutionary study of cartilaginous fishes has finally decoded why some rays carry dramatic "fake eyes" while others remain visually unremarkable. Researchers from Stockholm University examined more than 580 species, representing over 90% of all known skates and rays, to map the history of their anti-predator defenses. The findings indicate that these bold circular patterns are not aesthetic accidents but are strategically tied to a species' full defensive toolkit. According to lead author Madicken Åkerman, eyespots emerge only under specific ecological conditions as a single solution within an ongoing evolutionary arms race.

Weapons Systems Versus Visual Deterrents

The research highlights a significant trade-off between mechanical weaponry and visual signaling. Species equipped with powerful electric organs or venomous tail spines rarely develop conspicuous markings, as their physical lethality provides sufficient protection from predators like sharks and marine mammals. Conversely, smaller-bodied species that lack such robust hardware are far more likely to evolve eyespots. Senior author John Fitzpatrick noted that if a species already possesses a strong mechanical or electrical defense, the evolutionary cost of maintaining a complex visual warning signal is unnecessary.

The Depth Factor in Signal Effectiveness

Habitat depth plays a decisive role in whether a species retains or loses its markings. The study found that eyespots and bold spots are predominantly found in species inhabiting shallow waters less than 200 meters deep, where sunlight is sufficient for visual signals to be effective. In the dark, high-pressure environments of the deep ocean, the benefit of a visual deterrent vanishes. Consequently, the researchers observed that markings are frequently lost in deep-water lineages because the metabolic and survival costs of being conspicuous outweigh the nonexistent defensive benefits in low-light conditions.