How do cave fish survive in an oxygen-deficient environment?

For thousands of years, cave fishes have continuously evolved and adapted in a dark underground world that is as obvious as having no eyes and pale colors. Now researchers at the University of Cincinnati say these fish have an equally amazing physiology that helps them cope with environments so low in oxygen that it can kill other species.

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

Population of blind cave fish Astyanax mexicanus.

The study, published in Nature Journal Scientific Reports, shows that there is so much more to learn about animals that have intrigued biologists for 200 years.

'I have been fascinated by these fish for a long time,' said Professor Joshua Gross.

UC biologists examined the species Astyanax mexicanus, which split about 20,000 years ago from the surface fish still found in nearby streams in the Sierra de El Abra, Mexico.

Cave fish are pale pink and almost translucent compared to the silvery fishes on the surface. While cave fish have the faintest vestibular orbital contour, surface tetrapods have large round eyes that give them a perpetually surprised expression.

"Despite many obvious physical differences, the two fish are considered by many to be members of the same species," says Gross.

He said: 'Unlike Charles Darwin's finches of the Galapagos which are segregated at the species level, both cavefish and groundfish are considered members of the same species and can interbreed.

That makes them a good model system for biologists studying genetic evolution and adaptation," says Gross.

Gross and his students have learned a lot about this puzzling fish over the years. They found that the fish's skull was asymmetrical, possibly an adaptation for navigating in a world without visual cues. And they identified the gene responsible for the fish's ghostly pale blue color. It's the same gene responsible for red hair color in humans.

Scientists elsewhere have reported that cave fish sleep less than fish that live on the surface. For the latest study, Jessica Friedman and Tyler Boggs, biology students at Gross and UC and lead author of the study, examined hemoglobin in the blood of animal fish to see if it could explain how they survive. in the low oxygen environment of deep underground caves or not. The UC study examined cave fish from three populations in caves in Mexico known as Chica, Tinaja and Pachón.

Whereas fast-moving surface currents are saturated with oxygen, cavefish live in deep burrows where water stands undisturbed for long periods of time. Studies have found that some of these standing lakes have much less dissolved oxygen than surface water.

"They move everywhere, but they have very little access to nutrition," says Boggs. Blood samples showed that cave fish had more hemoglobin than surface fish. The UC researchers assumed that cave fish must have a higher hematocrit - a clinical measure of the relative contribution of red blood cells in whole blood."

The researchers expected to find more red blood cells in cave fish, "But they're mostly the same," says Gross. "We couldn't understand what was happening."

UC biologists examined the red blood cells of both fish and found that the red blood cells of the cave fish were much larger than in comparison.

'That difference in size explains much of the difference in hematocrit,' Gross said. "We know very little about the mechanism of cell size in evolution, so this finding is something we can leverage to gain insight into how animals evolved to enhance their capacity." hemoglobin".

Gross says the elevated hemoglobin may allow cave fish to forage longer in low-oxygen environments. Cave fish often have to work harder to forage for limited food in the burrow.

Boggs says scientists are very interested in how fish draw oxygen from the water. Due to climate change and human development, marine systems are witnessing many ecological disasters such as red tides, algal blooms that create low oxygen environments that often lead to mass fish deaths.

'There's a lot of ecological relevance here,' he said. It is happening in freshwater environments, saltwater environments. Researchers are trying to call attention to this terrible problem."