Blind cave fish lost eyes by unexpected evolutionary process

12 October 2017

By Michael Le Page

We’ve found out why a Mexican cavefish has no eyes – and the surprising answer is likely to be seized upon by those who think the standard view of evolution needs revising.

Over the past few million years, blind forms of the Mexican tetra (Astyanax mexicanus) have evolved in caves. Maintaining eyes and the visual parts of the brain uses lots of energy, so the loss of eyes is a big advantage for animals living in the dark. Instead the cavefish “see” by sucking.

It was assumed that these fish became blind because mutations disabled key genes involved in eye development. This has been shown to be the case for some other underground species that have lost their eyes.

But Aniket Gore of the US’s National Institute of Child Health and Human Development and colleagues haven’t found any disabling changes in the DNA sequences of eye development genes in the cavefish.

Instead, the genes have been switched off by the addition of chemical tags called methyl groups. This is what is known as an epigenetic, rather than genetic, change.

“Although a central role for DNA methylation in development and disease has been well-documented, our results suggest that epigenetic processes can play an equally important role in adaptive evolution,” the team writes.

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How do blind cavefish find their way? The answer could be in their bones.

A study has found asymmetry in the cranial bones of Mexican cavefish 

Date: May 24, 2017
Source: University of Cincinnati

Imagine living in perpetual darkness in an alien world where you have to find food quickly by touch or starve for months at a time.

The limestone caverns of Mexico's Sierra del Abra Tanchipa rainforest contain deep cisterns cloaked in utter blackness. This is where researchers at the University of Cincinnati traveled to find a little fish (Astyanax mexicanus) that has evolved to feast or endure famine entombed hundreds of feet below the ground.

"They have been able to invade this really extreme environment. They are exposed to darkness their entire life yet they're able to survive and thrive," said Amanda Powers, a UC graduate student and lead author of a study on blind cavefish published in May in the journal PLOS One.

"They've evolved changes to their metabolism and skull structure. They've enhanced their sensory systems. And they can survive in an environment where not many animals could," she said.

Mexican cavefish are bizarre, not merely blind but born with eyes that regress until they are completely lost as adults. The bones of their once-round eye orbits have collapsed. In place of eyes, their empty sockets store fat deposits that are covered in the same silvery, nearly translucent scales as the rest of their pale, unpigmented bodies.

The UC study examined one biological adaptation that might help to explain how these fish navigate and find food without benefit of sight -- asymmetry. Researchers examined juvenile and adult cavefish to understand how their skulls change during their lives.

Most fish are symmetrical -- their left and right sides are virtually identical and streamlined to provide the most efficient locomotion in the water.



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Possible Genetic Link Discovered Between Cavefish And Human Facial Asymmetries

April 5, 2014

redOrbit Staff & Wire Reports – Your Universe Online

A newly-identified genetic association with facial asymmetry in ancient cavefish could shed new light into mysteries surrounding conditions such as cleft palate or hemifacial microsomia in humans, according to research appearing in a recent edition of the peer-reviewed journal Genetics.

Joshua Gross, an assistant professor in the University of Cincinnati’s Department of Biological Sciences, and his colleagues compared the craniofacial features of the Astyanax mexicanus, an eyeless fish that has lived in the dark caves of Mexico’s Sierra de El Abra region for millions of years, with related fish living in Mexico, Texas and New Mexico.

The cavefish are said to be extremely sensitive to sound and vibration, and while they have no eyes, they do possess bony features in their eye regions that are similar to their sighted, surface-dwelling relatives. Those similarities allowed Gross and his fellow investigators to compare traits in both the cavefish and the surface-dwellers.

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New Cavefish Found in Sinkhole, Named for Fever

Researchers have discovered a new species of cavefish within a sinkhole in southwestern Madagascar. Shortly after swimming in the sinkhole's waters to find the fish, the scientists were afflicted with a "strange and debilitating" sickness dubbed "sinkhole fever," for which the species is named, according to the Guardian. The new species is Typhlerotris mararybe — "Mararybe" means "big sickness" in a local dialect.

The fish is small, with a length of about 1.5 inches (38 millimeters). Like many cavefish it lacks eyes and has "well-developed sensory canals and pores on its head," the Guardian reports.

Locals often visit this sinkhole and regard it as sacred, coming to offer prayers. While locals were familiar with several related cavefish species, they hadn't encountered this new animal, even though the people often descend to the sinkhole's waters via tree roots, reports the Guardian.

The fish is described in a study published in the journal American Museum Novitates. 

http://www.livescience.com/38825-new-cavefish-found-in-sinkhole-named-for-fever.html

Blind Cavefish Also Hard of Hearing

Joseph Castro, LiveScience Contributor

Date: 27 March 2013 Time: 03:21 PM ET

After generations of living underground and in perpetual darkness, species typically lose their eyesight. Scientists have observed this phenomenon in a range of creatures, including moles, shrimp, flatworms and fish. But new research shows that compared with their surface-dwelling relatives, at least two species of amblyopsid cavefish are partially deaf, in addition to being blind.

"The first hypothesis we had was that these fish that lost their sight should have an increase in hearing capabilities," said lead researcher Daphne Soares, a sensory neuroscientist at the University of Maryland. "It was a big surprise when we found they were a little deaf, and it took a while to come up with an understanding of what was happening."

In humans, the loss of sight is sometimes accompanied by better hearing, at least in people who became blind early in life. But until now, research looking at blind fish species hasn't shown the same phenomenon. For example, the cave and surface forms of theMexican tetra (Astyanax mexicanus) hear equally well; same thing goes for the molly Poecilia mexicana.

Continued:  http://www.livescience.com/28235-cavefish-hearing-loss.html