The Bestiary

One Fish, Two Fish, Stunned Fish, Clear Fish

How Two Species of Mexican Cave Fish Evolved

October 5, 2010

There are creatures that lurk in the watery depths of Mexican caves, never knowing the warmth of sunlight on their scaly skin. They are species of Mexican cavefish, and they are popping up as evolutionary research subjects in a variety of studies.

An article in New Scientist examined the recent evolution of a group of cavefish colloquially known as cave mollies, which live in caves with high sulphur content. Every year, the indigenous Zoque tribe include the fish that inhabit one particular cave in a ritual appeal to the gods for rain. Mesoamerican cultures believe that caves are the dwellings of the gods, spirits, and ancestors. The Zoque stun the cave mollies with an anesthetic derived from the Barbasco plant by crushing the plant and dunking it into the water.

Scientists from Oklahoma State University examined whether cavefish from the ritual cave and populations in other caves react differently to the Barbasco anesthetic. They found that cavefish in the ritual cave had evolved a tolerance for the drug, requiring higher doses of the toxin before they were knocked out completely.

Peering even deeper into the evolutionary history of cavefish, a team from the University of Maryland examined the evolution of the cavefish Astyanax mexicanus in a recent Current Biology paper.

Astyanax mexicanus still has an extant ancestor, which lives outside of caves and has the pigment and eyes that the cavefish lack. Since the researchers had an “ancestor” to compare with the cavefish, they were able to identify distinct evolutionary changes between the two. Most significantly, they discovered that although cavefish lost their eyes, they evolved an enhanced sensitivity to vibrations, allowing them to capture prey even in the dark. Because few predators threaten Astyanax mexicanus, any stirring in the water is more likely to be a meal than a danger and, as a result, the fish tend to approach movement of any kind.

Though the two different species share little more than gills, a love of the dark, and living in Mexico (excellent food, who could blame them), they are both great examples of how evolution can happen, even in short periods of time. Astyanax mexicanus shows how the environment can select for the enhancement of certain senses and behaviors, while the evolution of the cave mollies demonstrates that human activities can have a substantial impact on the evolution of wildlife around us.

About the Author

Mary Beth Griggs

Mary Beth is a native of Charlotte, North Carolina and has a B.S. in Geology and Archaeology from Tufts University. As a direct result of her undergraduate majors, she has an unhealthy interest in old rocks and natural hazards. She has a blog on those subjects at


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