Subterranean Fishes of the World

5730

2005

Muse Magazine (for children)

8-14

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In dozens of caves in Mexico, in deep, ink-dark pools where sunlight never shines, live fish called Astyanax. They are not much to look at—simple tetras, one to five inches long, similar in most ways to fish swimming in streams throughout Mexico and Central America. But there is one important difference: cave-dwelling Astyanax have no eyes. If you dissect them, you can find tiny remnants of eyes buried deep in their heads. On the surface, though, all you see is smooth, silvery skin. By studying the geology of the caves and comparing the fish there with their cousins on the surface, biologists estimate that different groups of Astyanax have been living in the dark for tens of thousands of years. Their normal-eyed ancestors reached the caves by swimming down streams that plunged underground, and they never found their way back to the surface. They are not alone. In caves around the world, more than a hundred species of fish of various shapes and sizes have lost some or all of their sight. So have scores of other cave-dwelling animals, including crustaceans, insects, salamanders, and spiders. As biologists, we find such troglobitic (cave-dwelling) creatures fascinating—and perplexing. With or without eyes, a fish can see exactly what you can in the perpetual darkness of a cave: nothing. But why should they lose their eyes? Nobody knows for sure, but the possible answers reveal a lot about evolution. And little Astyanax is playing a central role in helping scientists solve the mystery.

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