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.

Subject of Research: Animals
Article Title: The evolution of eyespots in skates and rays
News Publication Date: 24-Apr-2026
Web References: https://doi.org/10.1038/s41559-026-03059-5
Image Credits: Anette Gärdeklint Sylla/Stockholm University
Keywords: eyespots, skates, rays, evolutionary biology, visual signals, anti-predator defense, camouflage, marine biology, phenotype evolution, predator-prey interaction, animal coloration, stepwise evolution

Diatoms, unicellular algae recognized for their intricate silica shells, engage in a fascinating relationship with cyanobacteria of the genus Richelia. These bacteria have the extraordinary ability to fix atmospheric nitrogen (N₂), converting inert nitrogen gas into biologically usable forms that nourish their photosynthetic hosts. This nitrogen fixation is crucial in oligotrophic waters where essential nutrients are scarce, thus sustaining productivity within these harsh marine environments.

The symbiotic association between Richelia cyanobacteria and their diatom hosts exists along a continuum of integration. Some Richelia species reside externally, adhering to the diatom cell wall, while others occupy spaces between the diatom’s frustule—the characteristic silica shell—and the inner cellular membranes. The most intimate interactions occur when Richelia live completely inside the host cell. This gradient of physical association uniquely captures distinct evolutionary stages of symbiont integration, allowing researchers to analyze genome reduction and functional adaptation across the symbiotic spectrum.

Professor Rachel Foster, a co-author from Stockholm University, highlights that as these cyanobacteria become increasingly reliant on their hosts, they undergo a process of genome streamlining. Redundant genes, whose functions overlap with those of the host, tend to be lost over time. This gene loss is accompanied by an increase in genetic integration and interdependence, reflecting the transition from a facultative symbiont to a fully endosymbiotic lifestyle.

The team applied comprehensive comparative genomics to investigate these transformations. Led by postdoctoral researcher Dr. Vesna Grujcic, the analysis mapped gene content changes across different Richelia strains. This pangenomic approach distinguished the “core genome”—genes conserved across all strains—from accessory genes that vary. The study revealed clear patterns: genes involved in independent survival and certain metabolic pathways diminish as the symbiont becomes more embedded within the host. This genomic paring down provides a rare stepwise glimpse into the evolutionary finesse by which a free-living bacterium transitions into a host-dependent organelle-like entity.

Beyond mere gene loss, the study examined the landscapes between genes known as intergenic spacers, and the proliferation of pseudogenes—formerly functional genes that have accumulated debilitating mutations. Maliheh Mehrshad, collaborating in the study, emphasized that not only does the genome shrink in size, but the quality of coding sequences evolves. Non-coding DNA regions and the prevalence of pseudogenes serve as molecular signposts, chronicling the pace and trajectory of genome reduction driven by symbiotic commitments.

An additional layer of complexity was unveiled regarding the role of mobile genetic elements, often referred to as “jumping genes.” Theo Vigil-Stenman, formerly a postdoctoral researcher at Stockholm University, meticulously characterized insertion sequences and transposons—a type of DNA element that can relocate within the genome. These sequences were found to inflate the genome size of certain partially integrated symbionts, particularly those nestled between the diatom’s outer shell and inner membrane, despite functionally streamlined metabolic pathways.

This genomic inflation by mobile elements counters the simplistic assumption that genome size always correlates with symbiont integration level. The presence of abundant insertion sequences suggests that transposable elements actively shape symbiont genomic architecture, possibly influencing gene loss patterns and symbiotic evolution. Understanding these dynamics reveals the nuanced and multifaceted genomic remodeling symbionts undergo during their gradual encroachment into host cellular territory.

The research team advocates for the diatom-Richelia partnerships as a powerful model system to study symbiont genome evolution in real time. Unlike many obligate symbioses locked into highly derived states, these associations exist simultaneously across a spectrum of integration stages. This natural laboratory permits exploration of fundamental questions about the origins of endosymbiosis, genome reduction mechanisms, and the genetic underpinnings of host dependency.

While this study has illuminated critical aspects of the symbiont journey towards endosymbiosis, numerous mysteries remain. For example, the evolutionary consequences of living in symbiosis on the diatom host genomes themselves are yet to be fully unraveled. Additionally, investigators wonder how insights gleaned from these natural nitrogen-fixing partnerships can inform synthetic biology efforts to engineer nitrogen-fixing capabilities in crops—potentially revolutionizing sustainable agriculture by reducing dependency on chemical fertilizers.

The ability of Richelia to convert atmospheric nitrogen into a bioavailable form for their hosts is a compelling example of microbial cooperation with ecological and biotechnological significance. By dissecting the stepwise genome evolution that accompanies this symbiotic lifestyle, the research sets the stage for harnessing symbiont genetics to meet global challenges such as food security and environmental sustainability.

The article detailing these findings, titled “Stepwise genome evolution from a facultative symbiont to an endosymbiont in the N₂-fixing diatom-Richelia symbioses,” is accessible in Current Biology. This landmark work exemplifies how cutting-edge genomics paired with symbiosis biology can unravel complex evolutionary narratives, tracing microbial partnerships that have shaped life in the oceans for millions of years.

Subject of Research: Cells

Article Title: Stepwise genome evolution from a facultative symbiont to an endosymbiont in the N₂-fixing diatom-Richelia symbioses

News Publication Date: 29-Aug-2025

Web References:
https://www.cell.com/current-biology/fulltext/S0960-9822(25)01034-6

References:
DOI: 10.1016/j.cub.2025.08.003

Image Credits:
Images by Dr. Vesna Grujcic.

Keywords:
Symbiosis, Epifluorescence microscopy, Microbial genetics, Microorganisms, Algae, Diatoms, Bacteria, Cyanobacteria, Nitrogen fixation, Nitrogen fixing bacteria, Mobile genetic elements