Cavefish are able to show their evolution over millennia by displaying obvious adaptations like missing eyes and pale colors.
Researchers at the University of Cincinnati now believe these fish have a remarkable physiology that allows them to survive in low oxygen environments 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 Nature journal published the study. Scientific Reports. It shows how much more there is about animals than has been known for 200 years.
“I’ve been fascinated by these fish for a long time,” UC associate professor Joshua Gross said.
Cavefish evolved in caves all over the globe. Astyanax mexicanus was the species that UC biologists studied. It diverged from surface fish found in nearby streams in Sierra de El Abra in Mexico as recently as 20,000 year ago.
Cavefish are nearly transparent and pale pink compared to the silvery counterparts they share on the surface. Cavefish have the smallest outline of vestigial eye sockets. However, surface tetras are larger and have large round eyes that give them a perpetually amazed expression.
Gross stated that despite their obvious differences, many consider the two fish to be one species.
“Unlike Charles Darwin’s finches in the Galapagos that are separated at the species level, both the cavefish and surface fish are considered members of the same species and can interbreed,” he said.
Gross said that this makes them an ideal model system for biologists to study genetic and evolutionary adaptations.
Gross and his students learned a lot over the years about these puzzling fish. They found that the fish’s skull is asymmetrical, which could be an adaptation for navigating in a world with no visual cues. And they identified the gene responsible for the fish’s ghostly pallid color. It’s the same gene responsible for red hair color in people.
Scientists have also reported that cavefish sleep less well than surface fish.
For the latest study, Gross and UC biology students Jessica Friedman and Tyler Boggs, the study’s lead author, examined hemoglobin in cavefish blood to see if it might explain how they survive the low-oxygen environment of deep underground caves. The UC study examined cavefish from three populations in Mexican caves called Chica, Tinaja and Pachón.
Cavefish live in underground caverns, where the water is unaffected for long periods of time. Some of these standing pools have lower levels of dissolved oxygen that surface waters, according studies.
“They move around all the time, but they have little access to nutrition,” Boggs said. “It’s a paradox. They’re expending all this energy. Where does it come from?”
Cavefish have higher hemoglobin levels than surface fish, according to blood samples. UC researchers assumed that cavefish must have a higher hematocrit — a clinical measure of the relative contribution of red blood cells in whole blood.
These researchers expected to find more red blood cells in cavefish, “But they were virtually the same,” Gross said. “We couldn’t figure out what was going on.”
UC biologists compared red blood cells from both fish and discovered that the ones of cavefish were larger.
“That 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 gain insight into how animals evolve elevated hemoglobin capacity.”
Gross suggested that cavefish might be able to forage longer in low-oxygen environments due to their higher hemoglobin levels. Cavefish have to work harder to find the limited food they can find in caves.
Boggs stated that scientists are fascinated by how fish get oxygen from the water. Due to climate change and human evolution, marine systems are experiencing more ecological disasters, such as red tides and algal blooms that cause low oxygen levels that can often lead to massive fish deaths.
“There is a lot of ecological relevance here,” he said. “It’s happening in freshwater environments, saltwater environments. Researchers are trying to call attention to this awful issue.”
