On the surface there is nothing special about the Atlantic molly fish (Poecilia mexicana). The small, iridescent pearly-coloured fish is widespread across Central America, where it usually lives in freshwater streams.
But when biologists stumbled across a community of mollies living in a cave with water enriched with hydrogen sulphide they were baffled. They are the only known mollies to live in a cave, and the only known example of a fish being able to survive in such a poisonous environment.
Michi Tobler from Kansas State University in the US has spent decades researching the fish. Tobler believes that, in order to survive, the mollies have had to change both their behaviour and their genes.
“Fish avoid taking in too much hydrogen sulphide by breathing directly at the water’s surface,” says Tobler. “This compensatory behaviour, which is called aquatic surface respiration, increases their ability to acquire oxygen in the hypoxic [oxygen-depleted] environment, and it likely also minimises hydrogen sulphide uptake by the body.”
As well as limiting the amount of toxin that enters their body, the fish are also able to detoxify the hydrogen sulphide once it has entered their system.
Hydrogen sulphide is deadly because it turns off energy production in cells, by interfering with specific proteins. To stop their cells from shutting down, the fish switch to anaerobic metabolism, which is an alternative but much less efficient of producing energy that does not involve oxygen.
When Tobler and his team analysed the cave molly’s DNA they found that, compared to their freshwater cousins, the fish have increased their expression of genes that code for proteins that break down hydrogen sulphide into nontoxic forms and excrete it.
photo: Michi Tobler