A new tadpole that burrows through sand has been unearthed in the Western Ghats of India. Researchers were surprised by this discovery, noting that tadpoles don't normally burrow through sand, nor swallow the material, but this "remarkable tadpole" does.

The new tadpoles belong to the Indian dancing frog family, Micrixalidae. Researchers said they were found in "deep recesses of streambeds, where they live in total darkness until they fully develop into froglets."

This discovery was made by a group of scientists from University of Delhi, University of Peradeniya and Gettysburg College. Genetic analyses confirmed their identity as Micrixalus herrei.

"We provide the first confirmed report of the tadpoles of Indian Dancing frog family," said SD Biju, a professor from the University of Delhi. "These tadpoles probably remained unnoticed all these years because of their fossorial nature, which in itself is a rare occurrence in the amphibian world."

Indian dancing frogs are known for waving their legs while perched on a boulder, in either a sexual or territorial manner. However, tadpoles from this family had remained a mystery to scientists. 

The newly found tadpoles are equipped with muscular eel-like bodies and skin-covered eyes, which facilitate burrowing through gravel beds. And while they lack teeth, they have specialized mouthparts with serrated jaw sheaths that act as a filter to prevent large sand particles from entering the mouth as they move through sand. 

However, researchers found that the tadpoles' gut does contain small sand grains and decaying organic matter, which acts as a nutrient source. 

Examining the tadpoles' external morphology and bones revealed the presence of ribs in very early stages of development. Having ribs provides extra protection for internal organs and is believed to facilitate underground movement. 

"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; this adaptation may provide for greater muscle attachment, helping them wriggle through sand," explained Madhava Meegaskumbura, one of the study researchers from the University of Peradeniya. 

Furthermore, whitish "lime sacs" storing calcium carbonate were found in juvenile frogs, which is fairly uncommon. 

Researchers note that this new tadpole highlights the "uniqueness of amphibians" living in the Western Ghats, which is a mountainous region in southern India, known as a hotspot for biodiversity. 

While little is known about the habitat requirements of these elusive tadpoles, initial observations show they prefer sandy banks under canopy-covered streams.

Researchers hope their findings provide the framework for future studies and offer useful information for improving conversation of these ancient and endemic frogs.

Their study was recently published in the journal PLOS ONE.