American biologists finally understand how these cute fish, measuring little more than an inch can survive in extreme environments.

This fish can survive in highly toxic, acidic water. Photo courtesy of Michael Tobler

Don’t let their looks deceive you: the Atlantic mollies in southern Mexico are as hardcore as it gets. They can live not only in tropical freshwater, but also in brackish water, and acidic, volcanically influenced springs. Most fish wouldn’t last for a day in these types of waters.

“These fish are very extreme,” said Joanna Kelley, a genome scientist in the School of Biological Sciences at Washington State University. “Ordinary fish, when you put them in that water, are belly up in about a minute.”

She and her colleagues realized that only the molly survives in hydrogen sulfide water and wanted to learn why; they compared the genes expressed in three sets of hydrogen sulfide-tolerant fish and freshwater fish.

“In the freshwater system, there are 30-plus species of fish,” said Kelley. “In the sulfidic springs there’s the molly.”

The results were spectacular and surprising. They were expecting the fish to have developed a filtering system or somehow managed to avoid the substance, but the reality was quite different – it’s all in their genes.

“It’s not that they’re keeping the hydrogen sulfide out,” said Kelley. “It’s not that they are necessarily turning on some other unrelated genes. It’s really that the genes that have been previously implicated in hydrogen sulfide detoxification are turned on or turned up. That’s really the exciting part.”

Studying these fish could reveal more than just how awesome they are. Extreme environments are key in biological studies because they offer examples of extreme evolution. The fish could help address questions in evolutionary development, aging research, and evolutionary ecology. The work of Kelley and her colleagues could very well have bio-medical applications.

“For a variety of reasons, extreme environments are a great place to study evolutionary processes,” said Kelley. “We know the selective pressure. In this case, the selective pressure is hydrogen sulfide. So we can study evolutionary processes in extreme environments in a way that we can’t when we don’t know what the selective pressure is or there are multiple subtle selective pressures.”