How do blind cavefish survive their low-oxygen environment?
Remarkable fish have a hidden evolutionary adaptation -- big red blood
cells

Date:
March 11, 2022
Source:
University of Cincinnati
Summary:
Cavefish have obvious adaptations such as missing eyes and pale
colors that demonstrate how they evolved over millennia in a dark,
subterranean world. Now researchers say these incredible fish
have an equally remarkable physiology that helps them cope with
a low-oxygen environment that would kill other species.



FULL STORY ========================================================================== Cavefish have obvious adaptations such as missing eyes and pale colors
that demonstrate how they evolved over millennia in a dark, subterranean
world.


==========================================================================
Now researchers at 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 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 study was published in the Nature journal Scientific Reports. It demonstrates how much more there is to learn about animals that have
intrigued biologists for 200 years.

"I've been fascinated by these fish for a long time," UC associate
professor Joshua Gross said.

Cavefish evolved in caverns around the world. The species UC biologists examined, Astyanax mexicanus, diverged as recently as 20,000 years ago
from surface fish still found in nearby streams in Sierra de El Abra,
Mexico.



========================================================================== Cavefish are pale pink and nearly translucent compared to their silvery counterparts on the surface. While cavefish have the faintest outline
of vestigial eye sockets, the surface tetras have enormous round eyes
that give them a perpetually surprised 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 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.

That makes them 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 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 elsewhere have reported that cavefish sleep less 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 Pacho'n.

While fast-moving surface streams are saturated with oxygen, cavefish
live in deep caverns where standing water lies undisturbed for long
periods. Studies have found that some of these standing pools have far
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're expending all this energy. Where
does it come from?" Blood samples revealed that cavefish have more
hemoglobin than surface fish. 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 examined the red blood cells of both
fish and found that those of cavefish are much larger by comparison.

"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 said the elevated hemoglobin might allow cavefish to forage longer
in the low-oxygen environment. Cavefish often have to work harder to
find limited food available in the caves.

Boggs said scientists are very interested in how fish draw oxygen from the water. Because of climate change and human development, marine systems
are seeing more ecological disasters such as red tides, algae blooms
that create low-oxygen environments that often lead to massive fish kills.

"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."

========================================================================== Story Source: Materials provided by University_of_Cincinnati. Original
written by Michael Miller. Note: Content may be edited for style and
length.


========================================================================== Related Multimedia:
* Researchers_and_Mexican_blind_cavefish ========================================================================== Journal Reference:
1. Tyler E. Boggs, Jessica S. Friedman, Joshua B. Gross. Alterations to
cavefish red blood cells provide evidence of adaptation to reduced
subterranean oxygen. Scientific Reports, 2022; 12 (1) DOI: 10.1038/
s41598-022-07619-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220311182516.htm

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