This eyeless cavefish grows extra taste buds on its head

Despite being the same species, there are technically two species of cavefish in northeastern Mexico: those with extremely large eyes and those with no eyes at all. But a lack of visual organs isn’t the only unique physical adaptation found in the “blind cavefish.” They have also developed extra taste buds, just not in their mouths. Now researchers are beginning to understand when and why this evolutionary adaptation occurs.

Biologists learned it first that blind cavefish in Mexico developed extra taste buds on their heads and chins in 1967. Since then, however, scientists have spent little time investigating the evolutionary trait. As described in a study published Aug. 6 in the journal Communication Biologya team from Ohio University, Cincinnati, recently traveled to Mexico to learn about the evolutionary journey of blind cavefish (and their taste buds).

“Regression, such as loss of vision and pigmentation, is a well-studied phenomenon, but the biological basis of structural features is less well understood,” Joshua Gross, senior author and professor of biology at UC Cincinnati, said in an August 15 associated university profile.

To better understand these small, semi-transparent fish, Gross and his team focused on two separate populations of Astyanax Mexicanu in the Mexican caves Pachón and Tinaja. After studying their developmental cycles from birth, researchers found that blind cavefish possessed a similar number of taste buds as their surface brethren until they were about five months old. At this point, the eyeless varieties began to grow additional taste buds over their heads and chins through maturation at 18 months, and then well into adulthood. Since the lifespan of cavefish can be five or more years, Gross theorizes that blind cavefish may develop even further over time. The extra taste buds give these cavefish a much more precise sense of taste than their eyed varieties found outside caves.

[Related: How the archerfish evolved to shoot insects.]

The appearance of external taste buds also appears to be related to the transition of cavefish from a diet of living food sources to other options – mainly bat guano (also called poop), given the scarce food options in caves.

“Despite the complexity of this feature, it appears that most taste buds on the head are primarily controlled by just two regions of the genome,” says Gross.

The new discoveries could potentially provide new avenues to study how vertebrates developed their specific sensory organs. For now, however, researchers are still unsure what adaptive or functional relevance the extra taste buds provide to the blind cavefish. To continue this line of research, Gross and his team are now studying how the fish responds to different tastes, such as sweet, sour and bitter tastes.