Why Do Some Rays Have ‘Fake Eyes’ While Others Don’t?

From the vibrant wings of butterflies to the iridescent feathers of peacocks, eyespots have long fascinated biologists and lay observers alike due to their striking and sometimes dramatic appearance. These circular markings serve as visual signals, often thought to deter predators or attract mates. Yet, one pressing question has persisted: why do such conspicuous patterns evolve in some species and not in others? A groundbreaking new study focusing on an extensive group of cartilaginous fishes—skates and rays—has yielded significant insights into this evolutionary enigma by exploring the interplay between multiple defensive strategies rather than focusing on eyespots alone.

The research, conducted by scientists at Stockholm University, examined over 580 species, encompassing more than 90 percent of all known skates and rays. This comprehensive dataset enabled the team to reconstruct the evolutionary history of these animals’ conspicuous markings in unprecedented detail. Their findings underscore the importance of considering an animal’s entire suite of defenses when trying to understand the evolutionary pressures shaping these visual traits. By integrating ecological context, environmental factors, and other defensive mechanisms, the study reveals why dramatic visual features such as eyespots appear selectively and evolve only under certain conditions.

Unlike previous studies that isolated eyespots as singular evolutionary phenomena, this research examines how these markings coexist and trade off with other anti-predator adaptations. Skates and rays navigate a perilous world inhabited by formidable predators, including sharks, marine mammals, and predatory fishes. Evolutionary biology teaches us that survival tactics rarely rely on a single solution. Some species are equipped with potent mechanical defenses such as venomous spines or electrical organs capable of delivering shocks to potential attackers. These weapons represent a highly effective deterrent that reduces the necessity for conspicuous signaling. By contrast, species lacking such robust physical defenses are more inclined to develop vivid markings as a complementary or alternative strategy.

One of the study’s pivotal revelations is the strong correlation between the density and type of visual markings and the animal’s habitat. Species dwelling in shallow, well-lit waters—typically less than 200 meters in depth—are far more prone to exhibit eyespots or bold spots. Light availability is a crucial factor since the efficacy of visual cues depends directly on the environmental conditions in which they operate. In darker, deeper waters where sunlight penetration is minimal, visual signals lose their utility, and selection pressures favor cryptic coloration and other forms of camouflage, leading to the absence or loss of conspicuous markings. The researchers conclude that in well-illuminated habitats, where vision-based signaling can effectively communicate danger or unpalatability, eyespots provide a valuable addition to the defensive arsenal.

Lead researcher Madicken Åkerman eloquently summarizes the findings: eyespots do not emerge randomly but are tightly linked to ecological and defensive variables that shape their adaptive value. Their presence reflects a trade-off within evolutionary “toolkits” available to a species. If an organism already possesses a strong mechanical or chemical defense, such as a venomous sting or an electrical shock, investing energy and evolutionary resources into visual warning signals offers marginal benefit. In such cases, the exclusivity of potent physical defenses suppresses the need for prominent coloration. In contrast, smaller-bodied skates and rays that lack these formidable features often resort to visual signals as a frontline deterrent, a testament to the multiplicity of evolutionary solutions to predation threats.

The evolutionary pathway leading to the development of eyespots does not follow a straightforward trajectory. Contrary to intuition, these complex concentric-ring markings rarely appear abruptly in evolutionary time. Instead, eyespots typically arise through a gradual refinement of simpler markings—bold spots or other conspicuous dots. Statistical modeling shows that the probability of initially gaining simple markings outstrips that of directly evolving fully formed eyespots by an estimated factor of one hundred. This suggests a stepwise evolutionary process whereby incremental changes in pigmentation patterns accumulate over generations, eventually culminating in the elaborate eyespots that are ecologically advantageous under specific conditions.

Senior co-author John Fitzpatrick emphasizes this gradational evolution as indicative of adaptive fine-tuning. Gradual enhancement of visual signals allows species to balance the costs and benefits associated with conspicuousness over time. Larger, more complex eyespots may offer superior deterrence by mimicking the eyes of predators or by creating illusions that confuse attackers, but they also carry risks. Conspicuous markings increase visibility to predators and may come at an energetic cost. Thus, natural selection carefully mediates their appearance, favoring eyespots only when the benefits in predator deterrence outweigh the risks inherent in being more visible.

Furthermore, the study highlights that these conspicuous markings are not permanent fixtures in an evolutionary timeline but are subject to frequent loss. When environmental conditions change—particularly when species transition into deeper, darker habitats—the selective pressures favoring eyespots diminish. Since the visual signals become ineffective without adequate light, and indeed may even increase predation risk by making the organism more noticeable to predators with enhanced sensory modalities other than vision, the markings tend to fade or disappear altogether. This dynamic pattern underscores the reversible nature of some evolutionary traits, dictated by fluctuating ecological demands.

In addition to ecological factors, body size appears to influence the propensity for developing eyespots. Smaller species, which often lack the means for powerful physical defense, lean more heavily on visual deterrence, while larger species with formidable mechanical or electrical defenses have reduced need for such signals. This interplay of size, defense mechanism, habitat, and environmental light availability shapes a complex adaptive landscape in which multiple selective factors operate concurrently.

This research offers profound implications for understanding the evolutionary biology of visual signaling beyond skates and rays. The nuanced insights into how diverse defensive systems integrate and trade off against each other provide a fresh perspective on the evolution of warning coloration in marine and terrestrial animals alike. By advancing the paradigm that eyespots are not standalone features but components of multifaceted defense strategies influenced by environmental and physiological contexts, this study paves the way for further investigation into the evolutionary ecology of visual communication.

The use of comprehensive phylogenetic analysis combined with broad ecological data enhances the robustness of the findings, showcasing the power of multidisciplinary approaches to evolutionary questions. Moreover, the findings invite renewed attention to the evolutionary arms race between predator and prey, revealing that adaptation is a balance of multiple strategies rather than a singular optimization. Ultimately, this study exemplifies how integrating detailed natural history, morphology, and environmental variables can disentangle complex evolutionary processes.