Losing sight

Natural History, Sept, 2005 by Steve Miller, Glen D. Dillon

Luis and Monika Espinasa's article, "Why Do Cave Fish Lose Their Eyes" [6/05], brought to mind a zoology instructor who offered the following alternative explanation: In any fish population, mutations give rise to fish with defective, degenerate, or absent eyes. Outside caves, such fish are at a severe disadvantage, and so the mutations are eliminated. In the cave environment, however, fish without eyes might enjoy an advantage, because eyes are delicate organs and susceptible to injury. Eyes would be at risk as fish collided with their surroundings in caves.

Steve Miller

Santa Fe, New Mexico

The Espinasas assume that cave fish "lost" their eyes. But did they? Are eyeless fish the outcome of regressive evolution? Couldn't developmental plasticity--in which identical genotypes develop differently, depending on their environment--also account for the phenomenon?

Glen D. Dillon

Pahrump, Nevada

LUIS AND MONIKA ESPINASA REPLY: Steve Miller suggests that an alternative driving force behind the selection for eye loss and blindness in cave fish could be related to the prevention of injuries that may cause infectious diseases. Although multiple factors probably play a role, our experience does not support this hypothesis in the case of Astyanax. Both the eyed and the blind morph of Astyanax inhabit a cave whose water is contaminated by the droppings of a large community of bats. Yet even in this unhygienic cave, eyed fish survive quite well and are not out-competed by the blind morph, presumably because of the abundance of food.

Glen D. Dillon's question--would eyelessness occur in fish if they were hatched and raised in complete darkness?--has been addressed with Astyanax in the laboratory. The answer was no. Those results confirm that blindness in cave fish is genetic, the product of regressive evolution.