Jasminca Behrmann-Godel

How to make a cave fish in just a few thousand years

How long does it take for a cave fish to evolve from an open-water swimmer? Only a few thousand years, according to a new study. Scientists used to think that ice age glaciers covering northern Europe had prevented fish from colonizing the continents’ caves. Such species were thought to live no farther north than Pennsylvania’s Nippenose Valley. But a new cave-dwelling fish discovered in southern Germany 2 years ago is turning that assumption on its head. The pale, tiny fish with long, whiskerlike barbs sprouting from its head (above) is a new species of loach, as yet unnamed. It’s also the first cave fish to be found in Europe, 760 kilometers farther north than those in Pennsylvania. Until 12,000 years ago, Europe and its caves were buried beneath glacial ice, which blocked any connection between above- and underground waterways. But as the glaciers retreated, sinkholes and springs formed around Germany’s upper Danube, connecting the river to extensive caves and streams 250 kilometers below. Some fish made their way in, becoming smaller, with pale scaleless bodies, large nostrils, and tiny eyes—all adaptations for living in the dark, the scientists report in today’s issue of Current Biology. Based on their genetic analysis, the scientists say the cave loach is a close relative of the darkly mottled stone loach, which is twice the size of the cave fish, and still swims in the sunny, open waters of the Danube River.