At first glance, the blind cave fish is an example of evolution seemingly moving backward.
Over time, a handful of Mexican tetra (Astyanax mexicanus) living at deep depths have gradually lost not only their eyesight, but also their eyes, while their surface-dwelling counterparts have maintained their vision. Dubbed the "blind cave fish", the eyeless creature also lost much of its pigmentation, growing to sport a body of fleshy pink scales.
According to new research out of Sweden's Lund University, however, the blind cave fish's lost vision is actually a major step forward in adapting the fish to its new environment.
Researchers conclude that a highly developed visual system can suck up to 15% of an animal's "total energy budget". For a fish living at deep, dark depths with an irregular food supply, that expenditure simply isn't worth it.
"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", study lead author Damian Moran explains in a news release.
Instead, the blind cave fish has come to rely upon a finely tuned sense of smell and a keen sensitivity to changes in water pressure.
Scientists revealed last year that the fish has also ditched its circadian rhythm as an energy-saving measure.
"These cave fish are living in an environment without light, without the circadian presence of food or predators, they've got nothing to get ready for, so it looks like they've just chopped away this increase in anticipation for the day," Plant and Food Research New Zealand scientist Dr. Damian Moran explained when his research was published.
The reduction of traits over time is known as "regressive evolution", according to a 2007 study from New York University.
Article originally appeared on Discovery's Discovrd blog.