Hagfish fossils reveal stepwise eye simplification before near-total vision loss

June 5, 2026 feature Hagfish fossils reveal stepwise eye simplification before near-total vision loss Ingrid Fadelli Author Lisa Lock Scientific Editor Robert Egan Associate Editor Many animals, including humans, rely on their eyes to detect changes in their surroundings. The eyes of vertebrates, animals with a backbone or a similar supporting structure, contain a transparent structure (i.e., the lens) that focuses incoming light onto a layer of light-sensitive cells, known as the retina. Cells in the retina then convert light into signals that are sent to the brain. Hagfish, unusual eel-like fish that have a skull but no vertebral column, are known to possess highly simplified eyes. Hagfish eyes do not contain a lens or visible pigmentation and are covered by soft tissue, which greatly limits the animals' ability to capture detailed visual information. Researchers at University of Wisconsin-Milwaukee and the Lauer Foundation for Paleontology recently carried out a study exploring how hagfish eyes evolved over millions of years. Their observations, outlined in a paper published in Biology Letters, suggest that the eyes of these eel-like fish gradually lost their vision over time, as their eyes became increasingly simplified. "The hagfish eye is highly reduced, lacking pigment and a lens, and covered by soft tissue," wrote Victoria E. McCoy, Jason D. Pardo and their colleagues in their paper. "The timing and mode of this loss of complexity remain unknown. We present high-resolution anatomical data on the fossilized eyes of three stem hagfishes that form a transitional series towards the highly vestigialized eye of modern hagfishes." Fossilized hagfish eyes as a window into evolution To map the evolution of hagfish eyes in greater detail, McCoy, Pardo and their colleagues examined fossilized eyes belonging to specimens of three distinct hagfish species, which are now extinct. They studied these fossilized eyes using high-resolution imaging techniques and running anatomical analyses. The team specifically looked at the size and shape of the eyes, the extent to which they were covered by soft tissue, the presence or absence of lenses and the distribution of melanosomes (i.e., pigment containing structures). They then compared this data, looking for differences between the three species they considered. "All [three species] have small eyes covered in soft tissue and containing only spherical melanosomes," wrote the researchers. "Lenses are present in the more stemward hagfishes but absent in the more crownward hagfish." The results of the team's analyses suggest that the eyes of hagfish gradually became smaller and "simpler" over time. While all the extinct species they focused on had small eyes covered by soft tissue, like modern hagfish, earlier fossilized eyes were slightly bigger and contained lenses. This suggests that they were likely able to form some images. "Reduction of the eyes occurred gradually across the Paleozoic, with an initial stage of size reduction and loss of cylindrical melanosomes from the retinal pigmented epithelium, an intermediate stage with loss of image-focus capability and finally a near-complete loss of vision (the hagfish crown group)," wrote the authors. "The initial and intermediate stages of this process likely occurred in nearshore environments prior to the Permian colonization of the continental slope by hagfish." Insight into the gradual simplification of sensory organs Based on their observations, the researchers propose that the simplification of hagfish eyes occurred in multiple stages, gradually leading to the near blindness of existing species. Firstly, the eyes of hagfish shrunk in size and lost some of their pigmented tissue. Subsequently, the animals lost their ability to properly focus images, as their eyes no longer included a lens. Finally, their eyes became very similar to those observed in modern hagfish and lost almost all their function. This recent study provides an interesting example of how in some cases, sensory organs can gradually become simplified and less functional over time, as opposed to becoming more complex and sophisticated. In some instances, like in the case of hagfish, this simplification can be advantageous or could reflect a growing reliance on other sensory organs. Written for you by our author Ingrid Fadelli, edited by Lisa Lock, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. Publication details Victoria E. McCoy et al, Stepwise loss of complexity in hagfish eyes prior to deep sea colonization, Biology Letters (2026). DOI: 10.1098/rsbl.2026.0127 Journal information: Biology Letters © 2026 Science X Network