Tadpoles Found That Spend Life Underground, Eating Sand

By Douglas Main On 3/31/16 at 8:58 AM EDT

Tadpole 2 — Tadpoles of the species Micrixalus herrei, the Kallar dancing frog, have eel-like bodies that help them burrow into sand and gravel. SD Biju

When the serpent tempted Eve to eat the forbidden apple in the Bible, God cursed the animal: "Upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life." And while snakes don't eat dirt, scientists have just found a tadpole that does, spending all its time underground consuming sand and bits of organic matter.

But these youthful frogs don't spend their whole life sucking sand. After several months, the tadpoles develop into adult Kallar dancing frogs, colorful creatures known for waving their legs artfully during mating displays.

Prior to this finding, described in a paper published March 30 in the journal PLOS One, this family of frogs was the only one for which the tadpoles hadn't been found and described, says study co-author S.D. Biju, at the University of Delhi.

The study authors discovered the tadpoles in the Western Ghats, an extremely biodiverse mountain range in southern India. They searched for and found them by digging in the sandy bottoms of streams. It was no easy task: The first time they were spotted, the tadpoles "each wriggled back into the gravel bed upon exposure, defying capture," the scientists wrote in the study.

Other bizarre features of the tadpoles include skin-covered eyes, a powerful eel-like tail for propelling the creatures through sediment, and ribs, which haven't been observed in other burrowing frogs, says co-author Madhava Meegaskumbura, at the University of Peradeniya. They also possess specialized mouthparts, with "serrated jaw sheaths [that] act as a filter to prevent large sand particles from entering the mouth," Biju says.

So great were the anatomical differences between larvae and adult, the researchers had to use a genetic sequencing technology called DNA barcoding to demonstrate that these tadpoles were one and the same species as adult Kallar dancing frogs (Micrixalus herrei).

The shift from tadpole to adult isn't exactly a form of reincarnation, as would befit a frog in a Hindu region, but it isn't that far off.

James Hanken, a herpetologist at Harvard University who wasn't involved in this paper, says the disparity between the adults and "bizarre-looking" tadpoles is quite striking. "There's nothing in the adult that would lead you to suspect the larval stage is so weird," he says.