Cavefish Survives in Deep Caverns for Long Periods, How Can It Endure Low-Oxygen Environment?

Researchers at the University of Cincinnati recently said cavefish have equally outstanding physiology that's helping them cope with a low-oxygen environment that would kill other species.

As specified in a EurekAlert! report, these incredible fish have evident adaptations like missing eyes and pale colors, demonstrating their manner of evolution "over millennia in a dark subterranean world."

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

This study demonstrates how much more there is to discover about animals that have captured the interest of biologists for two centuries. According to Joshua Gross, UC associate professor, he has been fascinated by cavefish for a long time.

Cavefish

Essentially, cavefish evolved in caverns all over the world. The species Astyanax mexicanus or Mexican tetra, examined by UC biologists, diverged as recently as 20,000 years ago from surface fish still found in neighboring streams in Sierra de El Abra, Mexico.

A similar Eurasia Review report describes cavefish as "pale pink and almost translucent" fish compared to their silvery counterparts on the surface.

While cavefish possess the faintest outline of vestigial eye sockets, the surface tetras have huge round eyes, giving them a permanently surprised expression.

Despite their numerous evident physical differences, the two fish are considered by many to be part of the same species, explained Gross.

He added, "Unlike Charles Darwin's finches in the Galapagos" that are separated at the species levels, both the surface fish and cavefish are considered belonging to the same species and can interbreed.

Such characteristics make them a good model system for biologists to examine evolutionary and genetic adaptations, elaborated Gross.

Surviving Low-Oxygen Environment

For this new study, published in the Scientific Reports journal, Gross and biology students at UC Jessica Friedman and Tyler Boggs, the study's lead author, investigated hemoglobin in cavefish blood to determine if it might explain how they endure a low-oxygen environment of deep underground caves.

The UC research studied cavefish from three Mexican cave populations, including Chica, Tinaja, and Pachon. While fast-moving surface streams are flooded with oxygen, cavefish live in deep caverns where standing water is undisrupted for long periods.

Studies have shown that some of these standing pools have less dissolved oxygen surface compared to surface water. Boggs explained these fish move around all the time, although they have "little access to nutrition." It's a paradox, adding that they are expending all the energy.

Blood samples showed that cavefish have more hemoglobin compared to surface fish. Researchers at UC assumed that the former must have a higher hematocrit, a clinical measure of the relative contribution of red blood cells in whole blood.

Red Blood Cells Studied

The researchers were expecting to find more red blood cells in the cavefish, but Gross said they were virtually the same. They could not understand what was going on.

UC biologists studied the red blood cells of both fish and discovered that cavefish are much larger when compared.

The professor explained that the difference in size largely explains the variances in hematocrit. He added, very little is known about the cell size's mechanism in evolution. Therefore, this finding is something to capitalize on to understand how animals are evolving elevated hemoglobin capacity.

Related information about cavefish is shown on BlueWorldTV's YouTube video below:

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