Blind Cavefish Grow Taste Buds Across Their Faces

In limestone caves beneath a forest in Mexico, inky pools shelter unusual eyeless fish whose faces are covered in taste buds.

These fish, called blind cavefish, hatch with vestigial eyes. In certain populations, however, even these remnants disappear during development. Once the fish reach adulthood, their vacant eye sockets contain fatty tissue and are concealed beneath scales.

A new US study suggests that this is not the only striking facial change cavefish undergo as they mature. The researchers found that newly hatched fish have taste buds much like those of their surface-living relatives. Before long, though, additional taste buds start emerging in other locations, including on the face and chin.

Blind cavefish are not a separate species, despite their unusual looks. Like their close relatives in nearby surface streams and rivers, they are Mexican tetra (Astyanax mexicanus).

Where cavefish are pale and lack eyes, surface tetra have silver-coloured scales and prominent round eyes. Nevertheless, the two are genetically alike enough to interbreed readily and produce fertile young.

Earlier research indicates that cave forms probably evolved repeatedly through convergent evolution. Surface fish appear to have entered several caves, creating isolated populations that subsequently developed comparable adaptations to similarly lightless environments.

Their lack of eyes is also a case of regressive evolution: the loss of a complex trait, which is common among animals that live in caves. Numerous cave-dwelling creatures likewise develop enhanced non-visual senses, although the precise way this happens remains less clearly understood.

"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 senior author Joshua Gross, a biologist at the University of Cincinnati.

Blind cavefish taste buds and facial development

Scientists first documented extra taste buds on the heads and chins of blind cavefish during the 1960s. Yet, according to Gross and his colleagues, that original finding was not followed by research into the genetic or developmental processes that make this possible.

In the new research, the team set out to determine when these extra taste buds emerge during a cavefish's lifetime. They examined two distinct populations from the Pachón and Tinaja caves in the Sierra de El Abra region of north-eastern Mexico, where cavefish are known to show this characteristic.

For the first four months of life, the observations revealed little difference in taste buds between surface fish and cave morphs. At roughly five months old, however, the cavefish underwent a marked change.

Their taste buds then increased in both number and range, extending from the mouth across the head and chin. The change continued into adulthood, and taste-bud totals were still increasing in some fish beyond 18 months of age.

The changing diet of Mexican tetra cavefish

The researchers note that these cavefish are known to survive well beyond 18 months, both in the wild and in captivity. As they get older, they may therefore continue accumulating taste buds across their faces. As well as illuminating these enigmatic fish, the findings provide wider insight into evolution and the development of sensory organs.

"Knowing just how many doors this opened for future research involving taste bud and taste development was a truly rewarding aspect of this research, especially considering how long these fish live," says the study's first author, biologist Daniel Berning from the University of Cincinnati.

The rapid increase in taste buds takes place during a stage of life when cavefish are also moving towards different food sources. In some populations, it coincides with a switch to feeding on bat guano, although the same development occurs in fish from caves that have no bat populations.

A larger number of taste buds ought to provide cavefish with a stronger sense of taste, which is likely to be adaptive, but much remains unknown.

"It remains unclear what is the precise functional and adaptive relevance of this augmented taste system," Gross says.

Research on cavefish taste is continuing, with new work exploring how the fish react to different flavours.

The study was published in Communications Biology.