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.
Researchers at the UC’s College of Arts and Sciences discovered that Mexican cavefish produce more hemoglobin than other surface-dwelling fish. Hemoglobin is responsible for transporting oxygen and carbon dioxide between cells, organs, and gills of fish.
The Nature journal published the study. Scientific Reports. It shows how much there is to know about animals that have fascinated biologists for over 200 years.
Joshua Gross, an associate professor at UC, said that he has been fascinated by the fish for a long period.
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 may have the faintest vestige of vestigial eyes sockets, but surface tetras have huge round eyes that give them an ever-changing expression.
Gross stated that despite their obvious differences, many consider the two fish to be one species.
He said that unlike Charles Darwin’s finches in Galapagos, which are separated at species level, both cavefish and surface fish can interbreed.
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 discovered that the skull of the fish is asymmetrical. This could be an adaptation to navigate in a world without visual cues. They also identified the gene that causes the fish’s ghostly pale color. It’s also responsible for red hair color in humans.
Scientists from other countries have reported that cavefish sleep more than surface fish.
Gross and Tyler Boggs (UC biology students) examined hemoglobin levels in cavefish blood to determine if it could explain how they survived the low-oxygen environment deep underground caves. The UC study examined cavefish from three populations in Mexican caves called Chica, Tinaja and Pachón.
Cavefish live in caverns that have standing water for long periods. Surface streams are saturated with oxygen. Studies have shown that standing water pools can have significantly less dissolved oxygen then surface waters.
Boggs said that although they move around a lot, they are not able to get enough nutrition. It’s a paradox. They are expending all their energy. “Where does it all come from?”
The hemoglobin content of cavefish was higher than that of surface fish in blood samples. Researchers at UC assumed that cavefish would have a higher hematocrit, which is a clinical measurement of the relative contribution to red blood cells in whole body blood.
Gross stated that the researchers expected to find more red cells in cavefish. “We couldn’t figure out why.”
UC biologists compared the red blood cells of both fish to find that cavefish have larger red blood cells.
Gross said that the differences in hemoglobin levels are explained largely by the differences between hematocrits. “We don’t have much information about the mechanism of cell sizes in evolution so this finding is a great opportunity to gain insight into animal development of high hemoglobin capacities.”
Gross stated that the cavefish may be able to survive longer in low oxygen environments because of their high hemoglobin. 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.
He stated that there is a lot to be said about the ecological implications of this. “It’s happening both in freshwater environments and saltwater environments. Researchers are trying raise awareness to this horrible issue.
