Blind Cave Fish Traded Eyesight for Energy

Researchers from Lund University in Sweden have shown that well-developed eyes come at a surprising cost to other organ systems.

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.

These Fish Were Made For Walking: Photos

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.

Biofluorescent Fish Light Up The Deep: Photos

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.

This week came distressing news from marine biologists, who reported that there were

only a few dozen

spotted handfish left in a survey of the fish's only known habitat in waters off Tasmania. Spotted handfish (shown above) are small, bottom-dwelling fish that would rather 'walk' on their pectoral and pelvic fins than swim. Their distinctive mode of locomotion alone is worth hoping they can be saved (there is talk now of ramping up a captive breeding program). The critically endangered fish aren't the only ones that enjoy a nice walk, though. Let's sample a few of the other species of fish that like to feel the ground beneath their fins.

Rare Walking Fish Population Down To Just 79

Here's a literal fish out of water. Mudskippers are the broad name given to more than three-dozen species of fish that are amphibious, using their fins to motor (or wriggle, really) around on land. Different sizes of mudskipper will venture on land to varying degrees, with the largest ones spending up to 90 percent of their time on land, on the uppermost part of mud flats that only become submerged at high tide. In water, they breathe through gills, like any other fish, but on land they draw oxygen from outsized gill chambers they fill with water before coming ashore. They can also breathe through their skin, but they have to stay wet or very moist to pull that off. They're found in freshwater tidal mud flats in tropic, subtropic and temperate areas of the Indo-Pacific region and Atlantic coast of Africa.

Nudibranches Stand Out In Any Ocean: Photos

Frogfish won't venture onto land any time soon, but they do move across the sea bottom on their fins. They don't generally move much, however, preferring to wait idly until suitable prey comes into view. They dine on crustaceans, other fish, and even --


-- other frogfish.

Biofluorescent Fish Light Up The Deep: Photos

Epaulette sharks, a species of longtailed carpet shark, are night owls that hang out in shallow waters such as tidal pools and coral reefs. Often times, in lieu of swimming, they will do a kind of walk, using their fins to push themselves along the sea floor.

Close Encounter With A Great White Shark: Photos

Flying gurnards, despite their enlarged pectoral fins, can't actually go airborne or even glide for just a bit, but they


use their pelvic fins to "walk" along the bottom while they search out meals of crustaceans and small fish. They live in tropical to warm temperate waters on either side of the Atlantic, from southernmost South America to New England.

Crazy-Looking Microbes From The Ocean: Photos

Flattened in shape, bottom-dwelling


, sometimes called batfish, can also stroll along the sea floor, using an awkward gait. This red batfish will rest on the sea floor and scoot along just skimming the bottom.

The Quirky Beauty Of Sea Horses: Photos

This little guy is a mangrove rivulus, an amphibious fish that lives in the Atlantic waters from Florida all the way down to Brazil. It's only about 3 inches long (75 millimeters) and it can live for a bit more than two


out of water! It manages this trick by breathing through its skin, while it hides out in tidy places such as fallen logs. And it gets even cooler. On land this little fish fashions long jumps by doing a kind of backflip, tossing its head back over its body in the direction of its tail.

LEGO-Like Ocean Reef Shelters Sea Life: Photos