Blind Fish Retain Light-Sensing Abilities in Mexican Cave System, Challenging Circadian Rhythm Understanding

Researchers, led by marine biologist Inga Steindal from University College London, discovered that blind fish living in a Mexican cave system can still sense light despite the perpetual darkness.

The study, titled "Blind Fish Have Cells That See Light" published in Proceedings of the Royal Society B, aimed to investigate whether these cavefish, which no longer rely on an internal daily cycle and exhibit minimal sleep, maintain the ability to regulate their bodily functions cyclically.

Non-Visual Light Detection Retained in Blind Cavefish

Humans and most animals possess an internal clock known as the circadian rhythm, which utilizes light levels to synchronize our bodies with the natural day and night cycles. This rhythm regulates various biological processes that influence our behavior, such as hunger cycles and alertness levels governed by melatonin.

The Mexican blind cavefish, specifically the Astyanax mexicanus species, inhabit a network of more than 30 isolated caves where they have independently adapted to the darkness, as per Science Alert's report.

Despite their limited or complete lack of sight, these cavefish have developed heightened sensitivity to vibrations, enabling them to sense changes in water currents for navigation. Remarkably, some individuals have even lost their eyes altogether.

Interestingly, these cave-dwelling fish evolved from a common ancestor with functional eyes that still reside in surface waters within the El Albra region of Mexico and parts of the Southwestern US.

The separation between the cave and surface populations occurred between 10,000 and 10 million years ago, depending on the cave. Despite their distinct environments, they remain the same species capable of interbreeding.

Studying cavefish is challenging due to their rarity and poor adaptability to captive environments. However, blind A. mexicanus has displayed an unexpected ability to adopt a light/dark cycle when kept in laboratory conditions.

Researchers studied tissue samples from blind cavefish in three isolated caves, as well as from surface-dwelling ancestor embryos. They exposed these cells to different conditions and discovered the activation of several molecular clock mechanisms when exposed to light, even in the cavefish cells.

Although the cavefish cells exhibited a weaker response compared to their surface-dwelling counterparts, the study demonstrates that non-visual light detection is still present at a cellular level.

The findings confirm that blind cavefish cells can detect light and synchronize their internal clocks to a light/dark cycle, despite their lack of visual perception. It also highlights that these biological clock mutations evolved independently in different caves through distinct molecular mechanisms. The researchers anticipate that studying these cell cultures will provide valuable insights into circadian rhythms and facilitate a better understanding of animal adaptations to dark environments.

Cave Fish Evolved To Lose Their Sense of Sight

Throughout millions of years, species of fish that once possessed sight have undergone evolutionary changes. According to the American Museum of Natural History, this resulted in many cave fish species becoming blind or nearly blind, with some completely losing their eyes, while adapting to survive and thrive in dark environments.

The blindness of cave fishes is attributed to genetic changes affecting the development of their eyes, which are inherited and passed down to subsequent generations, resulting in blind offspring. However, understanding how an entire species of fish became blind raises questions about the role of natural selection in this process.

Evolution operates through natural selection, where advantageous traits increase an animal's reproductive success. In the case of cave fishes, it remains unclear how blindness confers an advantage in the dark, leading to the evolutionary development of a blind species.