"HYDROTHERMAL VENTS are the planet’s exhaust pipes. Kilometres below the ocean surface, they relentlessly belch out searing hot water rife with harsh chemicals from beneath Earth’s crust. When they were first discovered in 1977, nobody expected these inhospitable sites to bear signs of life. And yet, thriving alongside these vents were colonies of tubeworms, mussels and clams entirely new to science. It is hard to think of an environment that could be more hostile. Now, however, new work is revealing evidence that these animals are raising their young in just such a place: the fractured rocks underneath the vents themselves.
The tubeworms found at hydrothermal sites are unlike almost all other animals on Earth, in that they do not consume other organisms for food. Instead, they get essential nutrients from bacteria that live within them. These bacteria, in turn, live off the chemicals released by the vents. This unorthodox lifestyle (known as chemoautotrophy) results in tubeworms having neither mouths nor guts.
Tubeworms have an additional idiosyncrasy. For years it was assumed that they were distributing themselves to new vents by releasing their larvae into ocean currents that would sweep them away to locations new. The problem with this theory, however, is that tubeworm larvae have never been detected in open water. Aware of this, Sabine Gollner at the Royal Netherlands Institute for Sea Research and Monika Bright at the University of Vienna wondered if the larvae might, instead, be going underground. It was a radical notion which required a radical test: breaking open the crust beneath a hydrothermal vent.
Working with a team of marine biologists and deep-sea technical experts, Drs Gollner and Bright headed out to the volcanically active East Pacific Rise, far to the west of South America. Working from the research vessel Falkor Too, the team sent a remotely operated vehicle (ROV) down 2,500 metres to a hydrothermal site known as Fava Flow Suburbs, where chemoautotrophic worms abound, to investigate the crust.
Drs Gollner and Bright report in Nature Communications this week that the area below the vent crust was teeming with a variety of complex animals, not only the hardy bacteria and viruses they had expected to find. They found carnivorous polychaete worms and heat-tolerant limpets that seem to have slipped between the cracks in the seafloor to colonise the cavities underneath. There were also tubeworms.
Although the team did not find tubeworm larvae, the adult worms they found beneath the crust were living in clusters of individuals of the same size, and therefore age. This suggests they were drawn into the cavities by cool water currents as groups of larvae before settling down when the temperature and chemical conditions are right. Precisely what those conditions might be remains unclear, but measurements collected by the ROV indicated that the cavities boast sulphide concentrations that are much higher (and more toxic) than those above the crust, oxygen levels that are much lower than those above and temperatures of around 25°C (well above the 2°C of the ocean floor).
The discovery reveals that the strange ecosystems documented around vents are by no means limited to the surface of the ocean crust. As Dr Gollner aptly puts it, “The study of subseafloor animal life has only just begun.”" [1]
1. Hunting tubeworms. The Economist; London Vol. 453, Iss. 9419, (Oct 19, 2024): 76.
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