The eel-like tadpoles burrow into the sand. (SD Biju)

When you think of tadpoles, you probably don't think of things wriggling through sand. But that's exactly where Micrixalus herrei tadpoles live: According to a new study, the little guys burrow down into the sand and spend infancy in the dark.

The newly-discovered tadpoles are described in a study published Wednesday in PLOS ONE. The "eel-like", muscular babies have skin-covered eyes, which allow them to wiggle through gravel. They also have serrated jaw sheaths that researchers believe allow them to ingest sand without taking in large particles.

The species belongs to a family known as the Indian Dancing frogs – so named for the way they wave their legs when mating or staking out territory – and this is actually the first time scientists have found tadpoles from that family (save for one report in 1924, which the authors argue was sorely lacking). Now it makes sense that these little guys have stayed mysterious for the 125 years since their family was first discovered.

But because Micrixalus herrei lays its eggs in the sand, the researchers from University of Delhi, University of Peradeniya and Gettysburg College went looking in the grit. "Eight tadpoles were observed within an hour of digging," the authors wrote in the study. "Each wriggled back into the gravel bed upon exposure, defying capture." Indeed, whenever a tadpole was exposed, it used "vigorous, eel-like movements" to try to get back underground.

"They were even observed burrowing into wet sand in the absence of free water, confirming their active and potentially obligate burrowing nature," the study states. "They were never seen free-swimming in open water or resting at the bottom of the stream."

It's rare behavior in the amphibian world. It may help explain another unusual characteristic of the family: "Only four families of frogs are reported to have ribs, but we show that at least some of Micrixalidae also have ribs, even as tadpoles," study author Madhava Meegaskumbura of the University of Peradeniya said in a statement. "This adaptation may provide for greater muscle attachment, helping them wriggle through sand."

The study reports that three other species of tadpole from the family have been found "at similar sites" and are being investigated now – suggesting that this sand-burrowing may be typical for Micrixalidae.

Why some species of frog evolve for this subterranean is a fascinating question for scientists to answer. And understanding how it affects their habitat needs could help us keep them from disappearing as forests and rivers change.

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