Ghostly gene helps blind cave fish hunt better in the dark Free

If you live where there's no light, there is one expensive tissue that you can do without and that's precisely what cave-dwelling Mexican tetras have done. After becoming trapped in caves beneath ground hundreds of thousands of years ago, the fish lost their eyes, devoting more energy to other senses that were more useful in this dark. Their appearance changed also: they lost their sparkly sheen to become more ghostly, thanks to a mutation in a gene, oculocutaneous albinism II (oca2), which causes the fish to become albino. But scientists know that changes in specific genes can also affect apparently unrelated characteristics. As the larvae of the blind cavefish hunt in a different way from their surface-dwelling cousins, Stefan Choy and Johanna Kowalko from Lehigh University, USA, with colleagues from institutions across the USA, wondered whether loss of the oca2 gene might have something to do with how the larval cave fish feed. And they had the perfect opportunity to find out. By disabling the oca2 gene in surface-dwelling Mexican tetras, creating albino surface tetras, they could reveal how loss of the gene affected the ability of the larval fish to hunt.

Offering the albino surface larvae a snack of brine shrimp, the team realised that instead of facing their prey head on as they close in on their prey in the light, the albino surface larvae came in from the side, more like their blind cavefish cousins. Then, the team turned off the light and this time, the albino surface larvae were every bit as successful, while sighted surface dwellers struggled to catch their brine shrimp dinners. ‘These data suggest that albino surface fish may use methods for feeding that do not depend on eyesight’, says Kowalko. So how were the albino surface fish hunting like cave dwellers, even though there was only one gene different between them and regular surface fish? Had loss of the oca2 gene affected their vision?

First, the team checked the albino larvae's sight by showing them a movie of moving lines, which the youngsters followed, just like normal surface larvae; the albino surface larvae's vision was perfectly fine. Next, the team checked whether the albino surface larvae rely on sensory cells along their bodies that detect water movements which might guide their hunt for brine shrimp. But when the team inactivated these sensory cells, the albino larvae still used the same sideways hunting strategy, suggesting that they don't require the side sensory cells to hunt, although they may still contribute to them locating brine shrimp when hunting by vision.

The team then wondered whether being albino was somehow a disadvantage when feeding at the surface, but provided advantages when hunting in caves. This time, they pitched the albino larvae against their pigmented siblings in the light while they both chased brine shrimp. Tallying up the larvae's successes, the team realised that the albinos fared worse when hunting in the light, consuming fewer brine shrimp than the shiny surface larvae. However, when the researchers raised the albino and shiny fish in the dark, the tables were turned; the albino fish were more successful hunters than the pigmented surface larvae when the lights were out.

The loss of the oca2 gene which produces albino cavefish provides them with an advantage when hunting in dark caves and the team points out that fish that have lost the oca2 gene also sleep less, suggesting that the consequence for cave fish of losing this gene extend way beyond the fish's ghostly appearance.