Brittle stars (Ophioleucidae sp.) have important nerves running down their long arms.ROV KIEL 6000 Team / GEOMAR Kiel

Earlier this year, a Belgian company planned to drop a giant underwater Roomba from the side of a ship, in search of treasure. The 27-ton machine was to be deployed into the Clarion-Clipperton Zone (CCZ), where the miles-deep seafloor is spotted with dark, fist-size nodules of manganese and other valuable metals. There, the machine would sweep across the sand and, luck willing, suck up nodules, in a test run for mining operations to come.

Perhaps it was just as well that luck was not willing, and a technical failure scuppered the trial. A team of scientists led by Magdalini Christodoulou of the German Centre for Marine Biodiversity Research has now shown that the bottom of the CCZ is home to a hidden world of strange and ancient creatures that evolved in the dark of the deep and that have only just come to light. If mining companies are allowed to dredge the CCZ’s floor for minerals in the next abyssal gold rush, this unique ecosystem could be threatened.

Christodoulou and her colleagues learned about that ecosystem through six expeditions into the CCZ and two into another potential mining site, near Peru. On each voyage, the team deployed remotely piloted submersibles and viewed the ocean floor through the vehicles’ cameras. And what they saw, largely, were brittle stars.

Related to starfish but less well known, brittle stars are actually more diverse and more abundant. Their basic form is that of a skinny starfish: five slender arms around a distinct central disk. Among the many species of brittle stars found around the world, some look like pinwheels, or pipe cleaners stuck to a drinks coaster, or a quintet of centipedes wriggling out of a hockey puck. Instead of a central brain, they have a ring of nerves that circumscribes the disk and connects to spokes running down the arms. The arms can flash green or blue, and even self-amputate while glowing to distract predators. If that happens, brittle stars can instantly adjust their movements to compensate for the missing limb—an ability that has inspired the design of adaptable robots.

They can be surprisingly mobile, zipping around, digging, and even fighting. They move on hydraulic tube feet on the underside of their arms, which also taste with each step. In the middle of those arms is the somewhat disturbing mouth, which consists of five triangular, tooth-edged plates and which also doubles as an anus. Brittle stars usually feed by scavenging or sieving food from the water, but larger ones can grab shrimp and squid. And as the starfish expert Christopher Mah has noted, one species can even trap fish by raising its disk off the ground, waiting for its prey to swim underneath, twisting its body to close the arms into a helical prison, and gradually lowering its mouth onto the incarcerated animal.

Against their own predators, they defend themselves with armor. When dead and dried, they fall into pieces—hence the name. But in life, their arms are covered in sturdy plates of calcite. To these, some brittle stars add thorns; sometimes the plates are covered in tiny, hair-thin bumps. Scientists once took these bumps for lenses that focus light onto underlying light sensors, turning the entire animal into a compound eye. Recent studies have disproved that idea: The light sensors lie between the bumps, not beneath them. The animal’s whole body still acts as a rudimentary eye, but the bumps aren’t involved and their purpose is still unknown. They do darken during the day as pigment rises to their tops, so perhaps they act like an array of tiny sunglasses.

While most brittle stars reproduce by ejecting sperm and eggs into the water, some can also reproduce by tearing themselves in two. Others raise their young inside themselves, packing them into the central disk like socks in a well-organized drawer.

These varied lifestyles have served them well. Brittle stars have been around for almost half a billion years, and have survived several mass extinctions. Today they live throughout the oceans, in coral reefs, in Antarctica, on hydrothermal vents, on the bodies of jellyfish, on other brittle stars, and even in the abyss—the layer of ocean that’s deeper than 2.5 miles (or 4,000 meters). That underworld can be “brittle star paradise,” writes Tim O’Hara from Museums Victoria in Australia, who was part of the new study.

In its surveys, the team found 42 species, most of which had never been seen before. Several belong to entirely new lineages, and perhaps a new family—a group of brittle stars that’s only distantly related to those that have been cataloged. “It’s thought that in the abyssal plain you don’t get a lot of biodiversity, but there are all these species that look very different,” Christodoulou says. “They’re beautiful.”

For perspective, about 2,000 brittle-star species have been identified, and the team increased that tally by about 2 percent, by exploring a football field’s worth of seafloor. What else awaits? “That the relatively small number of samples collected to date, representing an extremely minute fraction of the CCZ and Peru Basin, should reveal so much diversity within the brittle stars is exciting,” says Cindy Lee Van Dover, a deep-sea biologist at Duke University. That variety will doubtless be mirrored in other animal groups that are less visible.

Three of the new brittle-star lineages that the team discovered don’t even exist in surface waters, and have been living and evolving in the deep for more than 70 million years. This contradicts the long-standing idea that the deepest ocean is simply an evolutionary sink—a retirement home for shallower species. The brittle stars reveal that it’s also a place where new species are born, independently of what occurs on the surface. The abyss is both cradle and crucible.

Most of the brittle stars were found among or upon the CCZ’s manganese nodules, perhaps because they are “the only hard substrate on which that kind of animal can live,” says Sabine Stöhr, a brittle-star expert at the Swedish Museum of Natural History. The nodules “create a more varied environment, which in turn allows a more varied fauna to evolve.” It is unfortunate, then, that those same life-creating nodules have attracted the attention of prospectors.

In international waters, all deep-sea mining must be licensed by the International Seabed Authority, the Jamaica-based body that is writing a code for mineral exploitation in the high seas. No such operations have begun, but 29 exploratory licenses have been issued to state-sponsored companies that want to trial mining technology like the failed nodule-collecting robot. There’s so much wealth on the ocean floor—including minerals necessary for making cellphones and other modern technologies—that Christodoulou thinks mining is all but inevitable. “We can’t prevent it, but we can try to do it in the most sustainable way,” she says.

At minimum, that means better understanding what exists in the deep, and what stands to be destroyed. Food is scarce so far from the ocean’s surface, so “deep-sea species often have low population densities, and are easily eradicated,” Stöhr says. Life may be diverse down there, but it is slow to grow. In a recent study, Ann Vanreusel of Ghent University in Belgium looked at nodule-rich parts of the CCZ that were subject to experimental mining simulations up to four decades ago and found that they have thus far failed to recover.

“Building sufficient knowledge to gain some reasonable certainty about the environmental impacts of mining will take much longer than it will take to develop mining tools,” Van Dover adds. It doesn’t help that the abyss is hard and expensive to reach, and that few scientists are trained to study it. Van Dover and others have called for the International Seabed Authority to proactively set up no-mining zones in 30 to 50 percent of the prospective areas, in the same way that fishing is verboten in marine reserves. “If we get the ecology wrong or if we misjudge the environmental impacts of mining, we may not be able to fix the harm,” she says. “A precautionary approach is essential.”

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