Now researchers at the University of Cincinnati say these incredible fish have an equally remarkable physiology that helps them cope with a low-oxygen environment that would kill other species.

Biologists in UC's College of Arts and Sciences found that Mexican cavefish produce more hemoglobin through red blood cells that are much larger compared to those of surface-dwelling fish. Hemoglobin helps the body transport oxygen and carbon dioxide between a fish's cells and organs and its gills.

The study was published in the Nature journal Scientific Reports. It demonstrates how much more there is to learn about animals that have intrigued biologists for 200 years.

"I've been fascinated by these fish for a long time," UC associate professor Joshua Gross said.

Cavefish evolved in caverns around the world. The species UC biologists examined, Astyanax mexicanus, diverged as recently as 20,000 years ago from surface fish still found in nearby streams in Sierra de El Abra, Mexico.

Cavefish are pale pink and nearly translucent compared to their silvery counterparts on the surface. While cavefish have the faintest outline of vestigial eye sockets, the surface tetras have enormous round eyes that give them a perpetually surprised expression.