Eyes and the brain tissue needed for vision demand about 15 percent of the energy budget of a young Mexican fish, researchers have found. This hard-to-discover 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 fish biologist Damian Moran of Plant and Food Research in Nelson, New Zealand. Brain tissue for vision and eyes 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.
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The energy cost is greater for juvenile fish than for older ones, the researchers say. 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, the researchers calculate.
Some Mexican tetras of this species still have eyes and flourish in waterways above ground. But populations still capable of interbreeding with the sighted fish have colonized caves and over eons lost functioning eyes. There’s “robust debate,” Moran says, over what factors, such as simple disuse or repurposing of genes, drove the change in cavefish.
The idea that energy considerations favor vision loss among cave colonizers has been around, but it took several years to develop the equipment for actually measuring that energy demand, Moran says. In the end, he and his colleagues at Lund University in Sweden developed a fishy version of equipment that keeps organs alive outside their original body. Flushing artificial blood over excised brains and eyes allowed the researchers to compare the demands of vision-related body parts in blind cavefish with demands in tetras with fully functioning eyes. Researchers calculated the energy use based on the difference in oxygen demand.
This cost of vision in this cavefish “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 but was not involved in the study. “This study fills the gap.” He’s surprised, however, that vision doesn’t use more energy. “I would have expected large differences if this is a key driver of regressive evolution,” he says.