For centuries, Zoque, an indigenous tribe of Mexico has a tradition of releasing a leaf-bound paste primarily made of lime and the root of the barbasco plant into a popular cave’s water.

The paste is a natural fish toxin. Now, researchers from Texas A&M University say this has resulted in the fish in the cave ‘Cueva del Azufre’ being resistant to the toxin. They would feed on the fish poisoned earlier till the harvest of the crops.

Dr. Michael Tobler, an evolutionary ecologist at Oklahoma State University, and Dr. Gil Rosenthal, a biology professor at Texas A&M, conducted a research and discovered that these fish have developed resistance to the plant's toxin and also managed to pass on the tolerant genes to future generations. This enables them to survive the plant’s poisonous toxins. Their research was published in the online journal "Biology Letters."

The cave-dwelling fish, Atlantic molly and its ability to survive in the toxic sulfur environment of Cueva del Azufre has been under research since 2004. Tobler and Rosenthal wanted to investigate the effects of this ceremony on the fish, their habitat and evolution. "We wanted to do a lab experiment where we exposed fish from different parts of the creek to barbasco," Tobler said. "Some of these fish had been more exposed than others."

The researchers collected specimens from different areas of the cave exposed to the barbasco toxin. They also went on to collect sample from areas upstream.

They found that the mollies annually exposed to the barbasco indeed were more resistant than the fish further upstream. "The cool thing is that this ceremony has gone on a long time and that the fish responded to it evolutionarily," Tobler said. "Lots of species couldn't live with these changes. It highlights how nature is affected by human activity."

"We tend to have this wonderful Pocahontas idea that before Europeans came in, everything was pristine and in harmony with nature and that all of the changes in our environment have been post-industrialization," he explained. "No. People have been changing the environment forever."

“Once a species has become genetically adapted to human presence, it is not very easy to suddenly reverse,” Rosenthal added.

"We need to understand what the impact really is on these fish rather than eliminate the ceremony completely," Tobler said. "We want to hopefully find a balance between the cultural practices of these people and the ecosystem."