There's a fish that lives in the pools of a cave called Cueva del Azufre in the town of Tabasco, Mexico, that is somehow able to thrive even though its habitat is full of toxins.
Hydrogen sulphide, a toxic chemical, seeps into the water from oil deposits and nearby volcanic activity. The compound also permeates the air, making it toxic for humans to breathe, reports The BBC.
These mysterious "cave molly" fish live in water that is 50 times more toxic than what's considered fatal for aquatic life. How can they survive?
One Kansas State University professor, Michi Tobler, has spent decades researching these molly fish. He has concluded that in order for them to survive in the poisonous water of Cueva del Azufre, they've actually had to change their genes, as well as their behavior.
From his research, Tobler believes that the mollies breathe right at the water's surface to avoid taking in too much of the sulphide.
"This compensatory behavior, which is called aquatic surface respiration, increases their ability to acquire oxygen in the hypoxic [oxygen-depleted] environment, and it likely also minimizes hydrogen sulphide uptake by the body," he told The BBC.
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The cave mollies have had to adapt to their environment in other ways as well. They're in almost total darkness inside the cave, so rather than relying on their eyes for detection, they use a hypersensitive pressure detector that runs along the sides of their bodies.
Regular mollies that live in the light are well known for their iridescent skin, but the cave mollies have lost this distinctive coloring because it's a waste of energy in such a dark environment.
Besides their already dark and toxic home, the cave mollies also have to contend with a ritual poisoning that happens every year. The Zoque people of southern Mexico drop leaves with a paste made from barbasco root into the water where the mollies live in an effort to ask the gods for rain.
Shortly after, many fish start floating to the surface, where the Zoque capture them and take them home to eat. The barbasco contains rotenone, which is poisonous to fish, but strangely not all the mollies are affected. Some have built up a resistance to it.
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So if the cave mollies are subjected to such a tortuous existence, why wouldn't they just swim outside the cave and re-build their lives with the other regular mollies that live in the light?
Unfortunately, if the cave mollies leave their dark home, they are susceptible to a fierce predator waiting for them outside. Huge water bugs, called Belostoma, stay just near the mouth of the cave, waiting to attack the mollies with their spear-like mouthparts.
The Belostoma attack regular mollies and cave mollies alike, but Tobler found in his research that each type of fish is much more likely to escape an attack in their native environment, which is why the cave mollies stay put, despite their unfortunate living conditions.
Scientists can't figure out why the cave mollies seem to be the only species we know of that has managed to adapt to such harsh conditions. But whatever the reason, the cave molly just might be the toughest fish in the world.
Top Photo: Coppia di Black Molly (Poecilia sphenops, Poecilia mexicana)