Fish Species Survives Regular Poisoning From Local People

A school of cave molly fish in Southern Mexico seems to be on its way to becoming a distinct species. The fish, which live in toxic waters, have developed several remarkable adaptations that helped them survive their surroundings.

According to BBC, the cave where these mollies live -- called Cueva del Azufre (Sulfur Cave) -- is poisoned with hydrogen sulphide. Natural oil deposits and volcanic activities added to the enrichment of these poisonous gases. As a result, they become dense and leak into the air inside the cave. What's even more remarkable is that they are somehow surviving a religious ritual that involves poisoning them on purpose.

Once a year, the indigenous Zoque people ask their gods for a bountiful season by heading to the caves and tossing leaf bundles with paste made from the mashed-up root of the Barbasco plant -- an anesthetic that according to Discover Magazine, proves toxic for the fishes. When the fish get stunned from the poison and go belly up to the surface, they are considered by the Zoque people as gifts from the underworld. Then they scoop up the fish and bring them home for supper.

It is also interesting to note that that fish from different chambers of the cave are distinct from each other. Scientists believe that they are adapting to each of their own unique environment -- since each pool differs in darkness as well as toxicity levels of hydrogen sulphide gas.

The high toxicity of the water somehow helped the fish develop a resistance to toxins -- over time, more and more fish have become immuned to the barbasco posion. In fact, they have evolved so much that they have become genetically distinct from the rest of the species -- even those living just meters away from the cave.

There is a reason why these fish don't interact with each other though: near the water surface are giant water bugs that prey on the mollies with their spear-like mouth. They hunt cave and surface mollies alike, and it was found that the different mollies are more akin to fighting the predators while in their own environment, which is why it is sufficient that they are kept away from each other.