Eyelessness has evolved many times in the animal kingdom. Creatures that dwell in the dark no longer need visual perception. Over time, their eyes become vestigial – a process called regressive evolution. Without light, the peepers bestowed by their ancestors are superfluous.
The Mexican tetra (Astyanax mexicanus) offers insight into the loss of vision. Populations that inhabit surface waters have fully functioning eyes. However, those inhabiting cave ecosystems have eyes that have been severely reduced. They are effectively blind.
Some 35 populations of blind Mexican tetras have been identified – all derived from surface populations that have adapted to the pitch black of cave life.
A study published in April 2026 on the pre-print server BioArxiv documents a novel ecosystem in which Mexican tetras with and without eyes coexist.
The researchers focused on a population of the tetra in Caballo Moro, one of the caves riddling the Sierra de Guatemala mountains of Tamaulipas, Mexico. There, a collapse of the surface rock has created a window onto an underground lake, exposing a portion of the water to light while leaving the rest in darkness. This has led to the evolution of two distinct populations – one with eyes and one without. Both are distinct from the ancestral surface population.
The tetras with eyes resemble their surface-dwelling relatives.
“At first glance one wouldn’t be able to tell the difference. However, physiologically and behaviorally they are more like cavefish. Which they genetically are,” says co-author Nicolas Rohner, an evolutionary geneticist at the University of Münster.
- This blind, poop-eating cavefish from Mexico tastes with its head. Scientists just figured out why
- "Like an ‘aircraft’ hovering in the middle of the water." Ghostly cavefish found beneath the ground in China
When the surface rock collapsed, surface fish were washed in at some point. They interbred with the cave dwellers, leading to a hybrid population. The fish that retain the eyes of their surface parents prefer the light and can see perfectly well. The others remain in the shadows, happily blind.
“The eyed cavefish in this cave are still aggressive and territorial. Because they have eyes they are better at defending their territory compared to the blind ones,” Rohner says. “That is one of the reasons that segregation remains.”
- What is the world's largest freshwater fish?
- 16 weirdest fish in the world
- 11 of the strangest eyes on the planet
The researchers used this population to study the evolution of eyeless in the species as a whole.
“The fact that they are the result from a longtime hybridisation event allows us to map the regions in the genome that are responsible for them having eyes because recombination has been shuffling the genomes of surface and cavefish over many generations,” Rohner explains.
They identified a gene called cx50 encoding for a lens protein. Disrupting this gene in lab tests conducted on surface fish led to eye defects. Mutations in this same gene lead to eye diseases in humans and have led to the development of reduced eyes in subterranean mammals too.
Top image: R Kellermeyer et al. 11 April 2026, BioArxiv
More amazing stories about fish
