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Loss of vision saved cavefish energy: in dark, eyesight can be costly, oxygen measurements show.

Eyes and the brain tissue needed for vision demand about 15 percent of the energy budget of a young Mexican fish, researchers say. This percentage supports the idea that energy cost-cutting helps explain how cavefish go blind.

That 15 percent represents a notable energy demand for a 1-gram juvenile Mexican tetra fish (Astyanax mexicanus) at rest, says Damian Moran of Plant and Food Research in Nelson, New Zealand. Vision could therefore become a liability in food-sparse caves, where no sunlight supports energy-catchers such as plants, Moran and colleagues argue September 11 in Science Advances.

The cost is greater for juvenile fish than for older ones. As fish grow, their bodies enlarge more than their brains do. By the time a Mexican tetra reaches 8.5 grams, vision demands only about 5 percent of its total resting energy budget.

Some Mexican tetras of this species live in aboveground streams and have functioning eyes. There's debate, Moran says, over what factors, such as simple disuse or repurposing of genes, drove vision loss in the populations that colonized caves.

Flushing artificial bloodlike fluid over excised brains and eyes allowed the team to compare the demands of vision-related body parts in blind cavefish with the demands in tetras with fully functioning eyes. The team calculated energy use based on differences in oxygen demand.

This cost of vision "has not been tested directly before, or as elegantly," says William Jeffery, who studies the same species at the University of Maryland in College Park. He's surprised that vision doesn't use more energy. "I would have expected large differences if this is a key driver of regressive evolution."

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Title Annotation:LIFE & EVOLUTION
Author:Milius, Susan
Publication:Science News
Article Type:Brief article
Date:Oct 17, 2015
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