A species of blind cavefish from Mexico has a profusion of tastebuds on its head and chin. Why? It's all to do with what it eats, according to a new study published in the journal Communications Biology
Scientists from the University of Cincinnati say that an appetite for bat poop may be the cause for this unusual way of tasting.
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Over thousands of years, cavefish – a generic term for freshwater and brackish water fish that have adapted to life in underground habitats – have become blind.
Unlike their counterparts that still live above ground, cavefish are pink and nearly translucent, with the faintest outline of eye sockets still remaining. The surface fish, meanwhile, still have their enormous round eyes.
With this loss of vision, some species of cavefish have developed new ways of getting by in the darkness.
In the case of one species from Mexico, Astyanax mexicanus, this is expressed in the unusual positioning of taste buds on the head and chin, explains Joshua Gross, biologist and senior author of the study.
“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 Gross, whose laboratory is dedicated to the study of evolution and development of cave-dwelling vertebrates.
Scientists have known since the 1960s that certain populations of blind cavefish had taste buds beyond the oral cavity, however the reason for this unusual trait was never explored.
Keen to find out more, Gross and his team studied two separate populations of Astyanax mexicanus, known to live in the Pachón and Tinaja caves in northwestern Mexico.
They found that when the cavefish are young (from birth up to five months) the number of taste buds is similar to that observed on the surface fish, but after five months they start to become more abundant, appearing in smatterings on the head and chin well into adulthood (up to 18 months).
The researchers found that the timing of taste bud appearance was similar for the two cavefish populations, however there were differences between the density and timing of expansion.
Gross says differences correlate with the time the cavefish stop eating live foods for sustenance and start to pursue other food sources. "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.
"Heightened detection to guano may therefore be adaptive to cavefish competing for nutrition in the cave environment."
The team, who admit there is still a lot they don't understand, say the next step is to begin new studies that focus on taste, by exposing the fish to different flavours such as sour, sweet and bitter.
Find out more about the study: The spatiotemporal and genetic architecture of extraoral taste buds in Astyanax cavefish
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