Blind Yet Light-Sensitive: The Paradox of Mexican Cavefish

Meet the Mexican blind cavefish, a fascinating creature that resides in the profound depths of Mexico's intricate cave systems, shrouded in perpetual darkness. Despite being devoid of sight, this remarkable fish still possesses some uncanny ability to detect light.

This intriguing discovery was made by University College London marine biologist Inga Steindal and her team. Given that these cavefish seem not to rely on an internal daily cycle and some barely sleep, the researchers were eager to discover if their bodies could still regulate cyclically over time. Most creatures on Earth possess an internal clock known as the circadian rhythm, which uses light levels to align our bodies with the day-night cycles of our planet. This rhythm then governs various biological processes that influence our behavior, such as our hunger cycles prompting us to eat and our melatonin levels controlling our alertness and sleepiness. The Mexican blind cavefish, scientifically known as Astyanax mexicanus, inhabit a network of over 30 isolated caves, each having adapted independently to the dark. Their bodies have developed an extraordinary sensitivity to vibrations, enabling them to detect changes in water currents for navigation, compensating for their limited or non-existent vision.

What's even more fascinating is that these adaptations occurred even though all the fish from each cave evolved from the same species with fully functioning eyes. The research team took tissue samples from blind cavefish from three isolated caves and their surface ancestor embryos, testing the cells under different conditions. They observed the activation of several molecular clock mechanisms when the cells were exposed to light, even in the cavefish cells. "Non-visual light detection is retained at a fundamental cell-based level," the researchers reveal, although the cavefish cells did not respond as vigorously as those from surface fish.

While there were similarities among the fish from the different caves compared to their surface relatives, there were also differences confirming that their biological clock mutations each evolved independently through different molecular mechanisms. "We have provided proof that despite being blind, cells from the Mexican blind cavefish can detect light and entrain their clocks to a light/dark cycle," concludes Steindal and her team. Their hope is that these cell cultures will shed more light on circadian rhythms and provide an easier way to study animal adaptations to dark environments. This groundbreaking research was published in the Proceedings of the Royal Society B.