More hemoglobin and more blood cells, discovering how blind fish survived

Blind cavefish have unique evolutionary characteristics such as their loss of eyes, and their faint coloration that reflects how they evolved over thousands of years in the dark undersea world.

And recently, a research study conducted by scientists from the University of Cincinnati in the United States - and published in the journal Scientific Reports on March 8 - indicated that these amazing fish have a distinctive physiological configuration that helps them to adapt in The environments are low in oxygen, which are conditions that are capable of killing other types of fish.

According to the press release published by the university in response to the study, the red blood cells of blind cave fish are larger in size compared to fish that live near the surface. It also produces more hemoglobin, which helps the body transport oxygen and carbon dioxide between the cells, organs, and gills of the fish.

This unique fish has intrigued scientists for 200 years. "I've been fascinated by these fish for a long time," notes Joshua Gross, associate professor at the University of Cincinnati and lead of the study.

These fish live in dark caves. The species that the scientists examined - the blind Mexican cave fish Astyanax mexicanus - diverged evolutionarily from pelagic fish 20,000 years ago.

Distinctive aspects and features

The blind cave fish are characterized by their pale pink color and almost translucent appearance, compared to their silver-colored counterparts that are found near the surface. These fish also have a faint atrophic eye socket, which differs from their pelagic relatives, which have round and wide eyes.

As the report published on the Eurek Alert website points out, despite the obvious morphological differences between these two fish, they belong to the same species. This is what Gross points out, saying that "cave fish and pelagic fish belong to the same species, so they can reproduce."

Over the years, scientists have uncovered a lot about these bewildering fish, finding that the fish's skull is asymmetrical, which may be an adaptation that helps the fish navigate a world devoid of any visual cues.

They also identified the gene responsible for the fish's pale color, and the same gene responsible for red hair color in humans. Scientists have noticed that cave fish approach their prey - or any unusual signals - using the right side of their body, while they approach their familiar counterparts during social interaction using the left side. Scientists also indicated that these fish sleep less than pelagic fish.

Cell size variance

Surface water currents are saturated with oxygen, but deep water, especially stagnant water in caves where blind cave fish live, has a low oxygen content.

In this study, scientists examined the percentage of hemoglobin in the blood of the cave fish in order to identify the reason that explains its survival despite the low percentage of oxygen in the deep caves. The study examined 3 groups of cave fish collected from different Mexican caves.

It is known that these fish are constantly moving, but they have little food. Where does this energy come from while traveling?

The study revealed that cave fish contain a higher percentage of hemoglobin than surface fish, noting that both contain the same percentage of red blood cells. However, the red blood cells in the cave fish were larger in size.

"We don't know much about the role that cell size plays in evolution," Gross says. "So this discovery will enable us to understand how animals have evolved the capacity to hold a higher proportion of hemoglobin."

Gross concludes that this high hemoglobin "allows the cave fish to live longer in a low-oxygen environment. However, they have to do more to get the limited amount of food that might be available in these caves."