For most of history, humans have assumed that this hostile environment would be a wasteland. But we couldn't have been more wrong.
A global survey published this week, the first of its kind, found an astonishing range of biodiversity in the lightless zone more than 2,000 meters beneath the surface. Using a class of sea stars comprising more than 2,000 species as a benchmark, researchers mapped diversity across the alien realm of the deep ocean, noting where stars were sparse and where they seemed to thrive.
They found that creatures of the deep exist in defiance of the rules that govern life everywhere else on Earth — but that doesn't make them invulnerable to the planet's various threats.
"This is why we wanted to establish this baseline," said Derek Tittensor, one of the authors of the report, which was published Wednesday in Nature. "There are a lot of reasons why we should put some effort into conserving deep sea biodiversity ... and this begins to give us a guide to the places we might want to protect."
As Tittensor, a marine biologist at Dalhousie University and the United Nations Environment Program World Conservation Monitoring Center, explains it, the rules for life on and around land are relatively well-known. Species richness in those areas depends on light, warmth and access to water, which is why the most biodiverse ecosystems tend to cluster around the equator. Bathed in year-round sunlight and drenched in reliable rain, the lush rain forests and colorful coral reefs of the tropics are home to roughly half of all life on land and 25 percent of marine species.
But if you swim away from the sunlit reefs, to the edge of the continental shelf and down the steep slope that leads to the deep ocean floor, you'll find that life abides by an entirely different set of constraints. Down there, light is nothing but a long-lost memory. Warmth is a moot point, since everywhere in the deep ocean is invariably, intensely cold. And water — well, water is practically the only resource that isn't in scant supply.
Instead, the most powerful indicator of diversity in the deep is food.
Most of it comes from "marine snow," a dusting of organic detritus that drifts down from creatures living in the waters above. Only an estimated 1 percent of all material from the upper ocean ever makes it to the 2,000 meter to 6,500 meter zone researchers deem the deep sea, according to Tittensor. But the material that does helps sustain almost all life in the dark waters below (with the exception of the bacteria that live off chemicals from hydrothermal vents).
The highest levels of marine snow were found in areas near the edges of continents and at higher latitudes, giving rise to deep ocean biodiversity hot spots in locations far from the richest places on land and in the shallow seas. The widest array of brittle and basket stars — hardy relatives of starfish that are some of the dominant creatures in deep-sea ecosystems — existed in the deep ocean between Europe and Greenland and off the coast of New Zealand.
"It's a fundamentally different pattern," Tittensor said. "The force that structures the distribution of life ... is this food that's raining down."
The study offers yet another example of how life in the deep adapts itself to take the best advantage of the rare resources the ocean provides. A whale fall in an otherwise barren part of the ocean can support dozens, even hundreds of species for more than a decade after the massive mammal's demise. Hydrothermal vents — those fissures in the ocean crust that spew hot, chemical-laced water from deep inside the Earth — support hundreds of species found nowhere else on the planet. Bioluminescent creatures glow against the black, taking advantage of the deep ocean's lightlessness in their efforts to pursue mates or lure prey.
"There are all these bizarre forms of life attracted to these unique habitats, taking advantage of these ecological niches," Tittensor said. The strangeness of their environment isn't a burden to these animals — it's a boon.
Despite its remoteness, the deepest part of the ocean is vital to humans, too. It's one of the Earth's essential carbon sinks — a place on the planet that absorbs more carbon from the atmosphere than it releases. It supports life that exists nowhere else, including potential sources of medicine. And it's taught us about the many, varied ways living things can adapt and thrive, a lesson that has implications for exploration on Earth and beyond it.
But like much of the planet, the deep ocean is threatened, Tittensor said. Trawlers scrape the sea floor in search of fish, disturbing cold water corals and the creatures that surround them. The acidification of the oceans — a byproduct of the excess carbon dioxide in the atmosphere — dissolves the fragile calcium carbonate shells of creatures that live far from the surface. And researchers are still sorting out the ramifications of the relatively new industry of deep sea mining, though they're unlikely to be all good.
Tittensor hopes the map of sea star diversity can give an indication of what places in the deep ocean are most in need of protection.
"It's just an extraordinary part of the planet that we live on," he said. Dark and distant though it may be, "it's important that we don't forget about it."