Cavefish Don’t Need to See to Find Food

Though they lack eyes, cavefish have other adaptations that help them to survive in their dark habitats. (Photo credit: Martin Shields / Photo Researchers, Inc.)

The fish species Astyanax mexicanus is interesting in that it includes both cave-dwelling and surface-dwelling populations. In Mexico there are 30 separate populations of cavefish. Many of these populations evolved in isolation, which means each population evolved independently of the others. The populations that live in caves lack eyes and body pigment, while the populations that live aboveground have large eyes and are pigmented. Due to these obvious phenotypic differences within the same species, cavefish are a popular subject for evolutionary biologists.

Though in early stages of development cavefish have eyes that begin to grow, at a certain stage programmed cell death, or apoptosis, occurs in the lens and the eyes stop growing. The surrounding skin tissues around the eyes continue to grow, covering over the space where eyes would typically be found. The remains of the undeveloped eye can be found buried within the eyes orbital socket.

However, even without eyes, cavefish still retain the ability to detect changes in light due to the functions of the pineal gland. If a shadow occurs above the fish, they will swim upward to investigate, as it may be a source of food, and without predators in the cave system, they do not fear being eaten. (In direct contrast, surface-dwelling fish typically seek shelter in the presence of a shadow.)

Compared to surface-dwelling fish, cavefish have a larger mouth and jaws and a greater number of tastebuds. Cavefish also have larger and more neuromasts than surface-dwelling fish. Neuromasts are specialized nerve cells that are a part of a fish’s lateral line. In cavefish, these cells are more densely distributed on the fish’s head, particularly in the area where its eyes would be. Cavefish use these sensory organs to detect movement and vibration in their watery environment. The response to vibrations in the water, called vibration attraction behavior, or VAB, is an adaptive behavior. Vibration detection helps cavefish find sources of food in the water, which, without eyes, they would not be able to see. Recent cavefish research conducted by evolutionary biologists indicated that VAB and neuromast abundance coevolved to make up for the loss of vision in cavefish and help the blind fish find food in darkness.

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