Research Highlights: Blind cavefishes have larger red blood cells to survive in a low-oxygen habitat

By H. Zell – Own work, CC BY-SA 3.0,

Blind cavefishes have larger red blood cells to survive in a low-oxygen habitat

  • Animals that thrive in extreme environments can be used to study adaptive evolution in response to different pressures.
  • One example of these pressures is reduced oxygen levels.
  • Environments with low oxygen are commonly found in subterranean and high-altitude regions.
  • Animals living in caves must also deal with starvation and the dark environment, both of which have been thoroughly studied as an important factor driving the evolution of traits related to caves.
  • Hypoxia, the state in which oxygen is lacking at the tissue level, does not receive much attention as an environmental pressure.
  • Researchers examined adaptive characteristics evolving in Mexican tetra, also known as the blind cavefish.
  • Mexican tetra is notable for having no eyes or pigment.[1]
  • Mexican tetra has two forms, surface-dwelling, and cave-dwelling.
  • Additionally, researchers also identified other responses to hypoxia with the help of many natural and independently-colonized cave populations together with closely-related surface animals of the same species.
  • Researchers focused on a very important oxygen-carrier molecule called hemoglobin.
  • Researchers discovered that numerous cave populations had higher hemoglobin concentration which was proportional to the increase in red blood cell size of the cave-dwelling form compared to the surface-dwelling fish.
  • Interestingly, both cave and surface-dwelling fishes had similar concentrations of red blood cells which suggest that higher hemoglobin levels were not due to the rise of red blood cell count.
  • Researchers speculated that the larger-sized red blood cells in cavefish contain more hemoglobin.
  • The study reinforces the idea that cavefish have adapted to low oxygen environments through changes in both red blood cell size and hemoglobin production.


Boggs, T.E., Friedman, J.S. & Gross, J.B. Alterations to cavefish red blood cells provide evidence of adaptation to reduced subterranean oxygen. Sci Rep 123735 (2022).

[1]  Keene, A.; Yoshizawa, M.; McGaugh, S. (2016). Biology and Evolution of the Mexican Cavefish. pp. 68–69, 77–87. ISBN978-0-12-802148-4

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