Mexican Cavefish Must Expand Large Energy Cost For Visual System

New findings published in Science Advances show that Mexican cavefish require between 5 to 15 percent of their total energy budget for well-equipped eyes.

A team of researchers at Lund University in Sweden conducted measurements and calculated the actual cost of well-developed vision in the animals. They examined how the creature loses its visual system via regression.

"Our measurements in the Mexican cavefish show that the visual system requires between 5% and 15% of the animal's total energy budget, depending on the age of the fish. This is a tremendously high cost! Over evolution, this morph lost both eyes and visual cortex, without a doubt because of the unsustainable energy cost of maintaining a sensory system that no longer had any significance," says Damian Moran, one of the researchers behind the study, in a news release. 

"Animals with large and well-developed eyes, necessary for their survival, pay a high price for them. As all animals have a strictly limited energy budget, a major investment in the visual system only occurs at a cost to other organ systems," says Eric Warrant, researcher in Functional Zoology at Lund University.

This fish is clearly different to the surface-dwelling variant, known as a morph, of the same species. The surface-dwelling morph has large eyes, but also far greater access to food, which the cave-dwelling morph lacks. The cavefish lives in a very dark, nutrient-poor environment and has no use for eyes.

The researchers were surprised that the visual system of Mexican cavefish required such a large proportion of the fishes' total energy budget; the cost was much higher than expected.

The new findings may lead to a better understanding of selective pressure in evolution, i.e. what causes the same species to develop in different ways depending on their environment.

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