‘Devil Worm’ Takes Animal Life to New Depths

It came from the deep, a mile below the Earth’s surface, in a place where only bacteria were thought to exist.

It’s Halicephalobus mephisto, a new species of roundworm that radically extends the possibilities of animal life on this planet and perhaps on others.

“Our results expand the known metazoan biosphere and demonstrate that deep ecosystems are more complex than previously accepted,” wrote researchers led by biologist Gaetan Borgonie of Belgium’s Ghent University in a June 1 Nature paper. “The ability of multicellular organisms to survive in the subsurface should be considered in the evolution of eukaryotes and the search for life on Mars.”

It’s only been two decades since scientists recognized that any life whatsoever could live hundreds or thousands of feet beneath Earth’s surface, a region of extreme pressure, high temperatures and few nutrients. Now it’s thought that up to one-half of all biological matter exists there, though this newly conventional wisdom holds that subsurface life is strictly the domain of single-celled organisms, not complex animals.

‘If life arose on Mars and it is still there deep underground, then it may have continued to evolve into something more complex than we are willing to entertain today.’

For the last 20 years, Borgonie has studied roundworms, developing what he calls “a healthy respect for their ability to withstand stress.” Various members of the ubiquitous, 28,000-species-strong phylum can live almost without oxygen, in extremely acidic environments, and despite prolonged starvation. When space shuttle Columbia tragically disintegrated upon re-entering Earth’s atmosphere in 2003, roundworms in a canister on its wings survived.

Five years ago, Borgonie started to wonder whether roundworms might live in Earth’s subsurface. Comparing their known physiological limits to subsurface conditions, he reasoned that roundworms should be able to survive there. Few people agreed.

“Everyone thought I was insane risking a career hunting something everybody said they knew could not be,” said Borgonie. But even as grantmakers denied him funding, he met Tullis Onstott, a Princeton University biologist who also suspected that roundworms could live deep.

Borgonie took a sabbatical in 2008, and the pair used money from their savings to travel to South Africa, home to some of the world’s deepest mines. Water recovered from their depths had already revealed such extremophile marvels as the world’s first single-organism ecosystem. Borgonie and Onstott’s team found the world’s first subsurface animals.

The most striking creature was a previously undescribed, 0.05-cm-long roundworm of the Halicephalobus genus, which Borgonie and Onstott dubbed H. mephisto in honor of the German lord of the underworld. Also present were a known roundworm, Plectus aquiatilis, and an as-yet-unidentified specimen. Subsequent tests found they ate subsurface bacteria, thus sealing any question of their origin.
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While their specimens were all found at depths of one mile, water from two miles down returned a “DNA signal,” or genes that belonged to some still-unidentified roundworm. Asked what else could be there, Borgognie said, “My guess is more than we think. If nematodes are there, then some other small invertebrates might be there too.”

As to how H. mephisto and other animals might influence flows of energy and chemicals beneath Earth’s surface, that isn’t yet known, said Borgonie. It’s not even known whether and how life’s subsurface cycles affect life above, though it makes sense that some connection exists. “We’re only scratching the surface,” said Borgonie. “What is sure is that the nematodes we found do eat bacteria. As such they will affect the turnover of the microbial community, and that is completely new.”

According to Borgonie, subsurface roundworms should be found all over the world, including far below the ocean floor, where some scientists think Earth’s life originated. The implications may even extend to other worlds, where researchers generally assume that conditions will be so extreme as to preclude all but single-celled life.

“Harsh conditions do not automatically preclude complexity,” said Borgognie. “If life arose on Mars and it is still there deep underground, then it may have continued to evolve into something more complex than we are willing to entertain today.”

Image: Halicephalobus mephisto. (Ghent University).