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

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

Sometimes, when we lead students or beginning cave explorers into their first "wild" cave, we turn out all our lights at our first rest stop. For many people, the experience of total darkness is stunning, and even a bit overwhelming. Wave a hand in front of your face, and you see nothing. Yet other senses seem to come alive. If the cave is wet, you can hear the drip, drip, drip of water across the chamber. You can smell the earth and damp air as the cave "breathes." As you grope for the comforting switch on your electric headlamp--or the flint lighter of your carbide lamp--your spatial sense, or proprioception, becomes keen and alert. If you leave the lights off for a few minutes, you might even begin to "see" how, with patience, you could learn a lot about your surroundings--and perhaps move about (if there are no deep pits nearby!) without the sense of sight at all.

Many species, of course, have lived in the total darkness of the underground for millennia, functioning perfectly well without vision. In fact, scores of troglobitic (cave-living) animals as diverse as crustaceans, insects, salamanders, and spiders have lost their eyes in the process. More than a hundred species offish living permanently in caves around the world are blind or have some degree of eye degeneration. To a biologist, those facts are fascinating--and deeply perplexing. With eyes and without eyes, a fish can see no more than you can in the perpetual darkness of a cave. But why lose them? What's more, since so many species of cave-dwelling fish, not at all closely related, have lost their eyes, the phenomenon seems to be neither an accident devolution nor an isolated event.

The loss or degradation of a trait through time is known as regressive evolution. But why is loss so fascinating, and so important to understand? Textbooks tend to focus instead on "constructive" evolution and the development of new or modified structures. A student of biology is bound to learn about the development of such novelties as legs in amphibians, hair and mammary glands in mammals, and the large, complex brain in higher primates. What the student probably does not realize is that for every new development, in all likelihood, something was sacrificed. The gills, scales, and tails that were lost by the ancestors of amphibians, mammals, and higher primates are just a few cases in point.

So why do cave fish lose their eyes? We ask the reader to stop for a moment and ponder this riddle. Whenever we pose the question to our students in introductory biology classes, one of them invariably responds: "It's obvious, isn't it? Caves are dark. Cave fish don't use their eyes because they can't see in the dark."

Oops, wrong. That would be a Lamarckian answer. Poor Lamarck! The French naturalist Jean Baptiste Pierre Antoine de Monet Lamarck, a pioneer of evolutionary thinking in the late eighteenth and early nineteenth centuries, is often ridiculed for his theories of use and disuse and his belief in the inheritance of acquired characteristics. In academia, being called a Lamarckian is less than flattering. But if you, like our students, came to a Lamarckian conclusion, you're in very good company. The question of eyelessness in cave fish is, and was, so challenging that it baffled even Darwin. The co-founder of the theory of evolution by natural selection gave an explanation that was, in the end, remarkably Lamarckian. Even today, the question has no definitive answer--though several fascinating lines of research suggest a genuinely Darwinian resolution is within sight. But the complexity and sophistication of the efforts that have been needed to reach this resolution show how, at times, a simple question can open up penetrating new leads into the labyrinth of evolutionary theory.

When biology textbooks describe the mechanism Lamarck proposed for evolution, they unfailingly invoke his explanation of neck length in giraffes: Giraffes have long necks, Lamarck maintained, because their forebears were continually stretching to reach the highest leaves in trees. The desire to reach higher leaves led to longer necks, and later on, the giraffes' offspring inherited that physical trait. Similarly, Lamarck believed, unused organs shrivel until they disappear. In short, use it or lose it.

Darwin told the story quite differently. A Darwinian explanation assumes that neck length has always varied among the individuals in a giraffe population. In the past, the giraffes with the longest necks reached the highest leaves, which were more abundant, and may have held more nutrients than the lower leaves, but were inaccessible to shorter-necked giraffes. Overall, then, the longest-necked animals were the best-fed members of the population. Better nutrition translated into longer or healthier lives, and so longer-necked giraffes produced more offspring than shorter-necked giraffes. With time, differential rates of survival and reproduction skewed the giraffe population toward animals with elongated necks. The key to the mechanism of Darwinian evolution is natural selection.