All the Way Down
It has to be said, nematodes are not among the most loved of organisms. For the most part, the only nematodes that get any press are either the developmental model animal Caenorhabditis elegans or the small proportion of species that affect us economically as parasites of ourselves or our food sources. There are not thriving clubs of amateur nematologists, there are no news-groups where nematode spotters eagerly spread the news of their latest desmoscolecid sighting. So it suggests something out of the ordinary may be going on when a new nematode species makes its debut in the pages of Nature, as one did today (Borgonie et al. 2011).
Appearance-wise, Halicephalobus mephisto is fairly ordinary (most nematodes are). It does have a higher-than-usual temperature resistance, being able to live in temperatures up to 41°C, but this is not extreme. Nor is the new species' ability to tolerate low oxygen concentrations, a common ability among minute animals such as nematodes and tardigrades. What is unusual about H. mephisto is where it was found: some 1.3 km beneath the surface of the Earth. Halicephalobus mephisto was recovered from fracture water (that is, water that was sitting in a fracture within the rock) in South Africa that had been isolated from the rest of the world for somewhere between 3000 and 12000 years, before being broken into by the Beatrix gold mine.
The presence of living organisms at this depth was not unexpected: bacteria had been found in fracture water previously. Nevertheless, this is the first time that a multicellular animal has been found at this depth. A samples taken in another mine at a depth of 0.9 km also recovered nematodes: one belonging to a previously known species, Plectus aquatilis, and one belong to an unidentifiable species of the family Monhysteridae. A sample taken at a depth of 3.6 km in a third mine did not recover any specimens, but did allow the recovery of nematode DNA suggesting their possible presence. Samples from the soil surrounding the boreholes from which the water samples were taken, as well as samples of the water used in drilling the mines themselves, were tested to establish that the nematodes were indeed from the original fracture water and not recent contaminants, but these control samples were nematode-free.
Halicephalobus mephisto lives a life completely isolated from the surface world, presumably feeding on the bacterial biofilms growing along the edge of the fracture. It would not be abundant: in the Beatrix mine sample, 6480 litres of water were filtered but only a single nematode was recovered (thankfully, the parthenogenetic nematode was successfully raised and bred in the lab, providing the necessary specimens for the species description). But it provides further support for the principal that where there is liquid water, there is life.