The Mexican tetra (Astyanax mexicanus) is a fascinating species. It has a “regular” form that lives in the waters of the Rio Grande and several other rivers in Mexico and Texas, as well as a “cave” form. The cavefish live in complete darkness and have almost no use for their eyes — so over thousands of years of evolution, they’ve basically become blind (although they’re still the same species).

While much of the focus has been on these regressive traits, recent research has shown that cavefish have also gained some constructive evolutionary features: specifically, the expansion of taste buds outside their mouth. This adaptation provides a compelling glimpse into how organisms can evolve new sensory capabilities in response to extreme environmental pressures.

Use it or lose it

It’s well known that if a species doesn’t use a particular sense, it starts to lose it through evolution. A good example of this happens in caves, where plenty of cave animals (not just fish) lose their sight and skin pigmentation. But it’s not so clear when and how this happens.

“Regression, such as the loss of eyesight and pigmentation, is a well-studied phenomenon, but the biological bases of constructive features are less well understood,” says the article’s senior author UC professor and biologist Joshua Gross, whose laboratory is dedicated to the study of evolution and development of cave-dwelling vertebrates.

The extraoral (outside of the mouth) taste buds are believed to be a counter-adaptation to the cave environment. These taste buds are believed to have evolved as an adaptive response to the cavefish’s unique environment, where food is scarce and traditional sensory modalities like vision are rendered useless.

The researchers found that the development of these extraoral taste buds is a gradual process that occurs well into adulthood. Unlike the rapid development seen in other sensory organs, the appearance of these taste buds in cavefish begins around five months post-fertilization and continues to expand in both number and distribution until at least 18 months.

This slow but steady expansion was observed in two geographically distinct cavefish populations, Pachón and Tinaja, suggesting a common evolutionary pressure driving this adaptation.

“Knowing just how many doors this opened for future research involving taste buds and taste development was a truly rewarding aspect of this research; especially considering how long these fish live,” says co-author Daniel Berning who earned his master’s degree in biology at UC in 2022, while working on the study in Dr. Gross’ lab.

But why exactly would the fish have evolved this adaptation?

The role of taste in a dark world

The answer likely lies in their harsh, food-scarce environment. Cavefish primarily feed on bat guano and other organic matter that drifts into the caves, which means that detecting these sparse and scattered food sources is crucial for survival. The expansion of taste buds on the head likely enhances the cavefish’s ability to detect and identify potential food sources in their dark habitat.

Moreover, the timing of taste bud expansion coincides with a dietary shift from live foods to bat guano as the fish mature. This suggests that the development of extraoral taste buds may be closely linked to the cavefish’s changing dietary needs over its lifetime. By expanding their taste-sensing capabilities, these fish can better locate and evaluate the nutritional content of the food available in their environment.

“While the precise function of this late expansion on to the head is unknown, the appearance of extraoral taste buds coincides with a dietary shift from live-foods to bat guano, suggesting an adaptive mechanism to detect nutrition in food-starved caves,” the researchers explain in the study

Evolution beyond the cave

While much remains to be discovered about the specific functions and adaptive significance of these extraoral taste buds, this study provides a crucial first step in unraveling the mysteries of sensory evolution in cave-dwelling species. Future research may explore how these taste buds interact with other sensory systems in cavefish and whether similar adaptations can be found in other organisms living in extreme environments.

Ultimately, the evolution of taste buds in cavefish is not just a quirky example of adaptation in an obscure species; it has broader implications for understanding how sensory systems evolve in all animals, including humans. The principles uncovered in this study—such as the role of environmental pressures in shaping sensory capabilities and the genetic architecture underlying these traits—can be applied to other systems, providing insights into how our own senses might have evolved in response to different challenges.

The study was published in Nature.