Cave fish have obvious adaptations such as missing eyes and pale colors that show how they evolved over millennia in a dark, underground world.
Now researchers from 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 from the UC College of Arts and Sciences have found that Mexican cave fish produce more hemoglobin through much larger red blood cells than those of surface fish. Hemoglobin helps the body transport oxygen and carbon dioxide between the cells and organs of a fish and its gills.
The study was published in the journal Nature Scientific reports. It shows how much there is still to learn about animals that has intrigued biologists for 200 years.
“I’ve been fascinated by these fish for a long time,” said UC associate professor Joshua Gross.
Cavefish has evolved in caves all over the world. The species the UC biologists examined, Astyanax mexicanus, diverged just 20,000 years ago from the surface fish still found in nearby streams in Mexico’s Sierra de El Abra.
Cave fish are pale pink and almost translucent compared to their surface silver counterparts. While cave fish have the faintest outline of residual eye sockets, surface tetras have huge round eyes that give them a perpetually startled expression.
Despite their many obvious physical differences, the two fish are considered by many to be members of the same species, Gross said.
“Unlike Charles Darwin’s finches in the Galapagos which are separated at the species level, cave fish and surface fish are considered members of the same species and can interbreed,” he said.
This makes it a good model system for biologists to study evolutionary and genetic adaptations, Gross said.
Gross and his students have learned a lot about these puzzling fish over the years. They discovered that the fish’s skull is asymmetrical, which could be an adaptation for navigating a world without visual cues. And they identified the gene responsible for the fish’s ghostly pale color. This is the same gene responsible for red hair color in humans.
Scientists elsewhere have reported that cave fish sleep less than surface fish.
For the latest study, Gross and UC biology students Jessica Friedman and Tyler Boggs, the study’s lead author, looked at hemoglobin in the blood of cavefish to see if that might explain how they survive the low oxygen environment of deep underground caves. The UC study looked at cavefish from three Mexican cave populations called Chica, Tinaja, and Pachón.
While fast-moving surface streams are saturated with oxygen, cavefish live in deep caverns where standing water remains undisturbed for long periods of time. Studies have shown that some of these stagnant pools contain much less dissolved oxygen than surface waters.
“They move around all the time, but they have little access to nutrition,” Boggs said. “It’s a paradox. They spend all that energy. Where is she from ?
Blood samples revealed that cave fish had more hemoglobin than surface fish. The UC researchers speculated that the cave fish must have a higher hematocrit – a clinical measure of the relative contribution of red blood cells to whole blood.
These researchers expected to find more red blood cells in cavefish, “but they were pretty much the same,” Gross said. “We couldn’t figure out what was going on.”
UC biologists examined the red blood cells of both fish and found that those of the cave fish were much larger in comparison.
“This size difference largely explains the differences in hematocrit,” Gross said. “We know very little about the mechanism of cell size in evolution, so this finding is something we could capitalize on to better understand how animals evolve with high hemoglobin capacity.”
Gross said the elevated hemoglobin could allow cavefish to feed longer in an oxygen-poor environment. Cave fish often have to work harder to find the limited food available in caves.
Boggs said scientists are very interested in how fish get oxygen from water. Due to climate change and human development, marine systems are increasingly experiencing ecological disasters such as red tides, algal blooms that create low oxygen environments that often lead to mass fish kills.
“There’s a lot of ecological relevance here,” he said. “It happens in freshwater environments, saltwater environments. Researchers are trying to bring attention to this terrible problem.”