Why do cave fish lose their eyes? A Darwinian mystery unfolds in the dark

Natural History, June, 2005 by Luis Espinasa, Monika Espinasa

Could the larger size enhance the blind fish's sense of smell? It is very likely, and we view the trade-off as a beautiful example of a pleiotropic effect. A mutation causes eye loss in cave fish, which consequently deforms the skull. In turn, the deformation enhances olfaction. Natural selection is not acting on cave-fish eyes; it is acting instead to increase the fish's sense of smell.

Olfaction is probably not the end of the stow; other effects have surely been at work in the evolution of the blind cave fish of the world. According to one of our favorite theories, there is a kind of pleiotropic trade-off of neurological connections in the brain. After all, the neurons of many non-troglobitic organisms, such as cats and mice, have a certain inherent plasticity, which may depend on age. In newborn kittens, for instance, neurons of the visual cortex are activated and connected to both eyes. As development progresses, however, some neurons become sequestered, forming networks that can process information only from either the right eye or the left eye. If one eye is removed, or simply covered with an eye patch, more neurons join the network of the eye being used. In short, more neurons are applied to the most frequently used networks, making the brain more efficient.

Could something similar be happening in cave fish? Electrodes inserted into the visual cortex of eyeless cave fish show that their "visual" neurons actively respond to tactile stimuli, not visual ones. Do eyeless fish in caves develop a more efficient neural network than eyed fish riving in the dark? We think the answer is yes, because functional eyes in darkness do not stop sending information to the brain: "I detect no light. I detect no right. I still detect no light." If there are no receptors, however, the neurons can be fully recruited and sequestered by other systems, making the brain as a whole work more efficiently.

There is evidence that blind people develop above-average abilities in specific tasks related to hearing or to touching. There is also anecdotal evidence that sighted people have keener nonvisual senses when they cannot see--remember how the nonvisual senses seem to come alive in a cave when the lights go out? Would blind people reorganize their neural networks better than eyed people, if both groups began living in the dark or inside a cave? That is a question our students, and readers, will have to ponder.