Like the boroughs of New York City or the arrondissements of Paris, deep-sea communities are turning out to have a strong local flavor.
In the waters off Antarctica at the southernmost seafloor vents where hot water percolates from below, piles of hairy crabs swarm in the thousands. In the middle of the Indian Ocean lives a motley collection of creatures never before seen together. And south of Cuba, at the world’s deepest vents, shrimp rule.
Thanks to a recent string of oceanographic expeditions, scientists are learning that there isn’t a stereotypical hydrothermal-vent ecosystem that exists everywhere. Rather, each locale hosts its own eclectic residents, in patterns that may hint at how life spread through the ocean over geological time.
“It allows us to move beyond the picture we’ve had so far, where you go to a new area and you find new and different species,” says Jon Copley, a marine ecologist at the National Oceanography Centre in Southampton, England. “Now we can say what are the relationships, the differences and similarities among all these places. That’s much more useful knowledge.”
The classic picture of a deep-sea vent ecosystem comes from the first glimpses researchers had of these otherworldly realms during the late 1970s. Explorers in deep sea submersibles like ALVIN saw giant tubeworms, mussels and shrimp living off chemical energy provided by warm hydrothermal waters, fueled in turn by the volcanic activity of new seafloor being born.
But near Antarctica, in the only hot-water vents known in the frigid Southern Ocean, tubeworms and mussels are nowhere to be seen. A 2010 expedition found the seafloor crawling with thousands of a newfound species of Kiwa crab, a light-colored creature with long hairy arms. This particular species also has hair on its chest, so much that a graduate student on the cruise nicknamed them “The Hoff” crabs, after hirsute actor David Hasselhoff. “It’s visually really quite astonishing,” says team leader Alex Rogers, a deep-sea biologist at the University of Oxford in England. “You have huge heaps of these crabs.”
Crabs usually die in polar waters because they can’t flush magnesium from their blood in the cold. But these animals survive by scrabbling over one another for the position closest to the 380° Celsius water pouring from the seafloor.
Along with the crabs, the researchers spotted smaller numbers of other creatures, including barnacles, limpets, snails and a predatory seven-armed starfish. The findings appeared January 3 in PLoS Biology.
“There was a feeling that perhaps the Southern Ocean was a root of dispersal for hydrothermal faunas between other large oceans,” says Rogers. “So it was very surprising when it turned out to be a completely distinct vent community.” The extreme cold and seasonal swings in Antarctic waters may exclude animals such as mussels and shrimp, whose larvae need to feed at sea upon hatching, the scientists speculate.
Half a world away but just as puzzling are deep-sea vent animals along the Southwest Indian Ridge in the Indian Ocean. New seafloor crust is being born there at some of the slowest rates in the world, suggesting a relative lack of volcanic activity in the rocks below. But in 2007 a Chinese expedition discovered hydrothermal vents along the ridge at an average density of 2.5 vent fields every 100 kilometers.
“That number is big, about two to three times what scientists expected to find,” says chief cruise scientist Chunhui Tao of the Second Institute of Oceanography in Hangzhou, China. Tao and his colleagues, who describe their survey in the January Geology, think that magma welling up locally may fuel the hydrothermal vents.
Last year, the International Seabed Authority granted China a permit to explore 1,000 kilometers of the Southwest Indian Ridge, including the active vents, for possible mining of metal deposits. Before that could happen, Rogers says, “there was a real feeling that we had to get in and document these vents.”
So last November and December he, Copley and other biologists checked out the “Dragon Vent” there. In some ways, the animals there are similar to those near Antarctica, Copley says. There is a yeti crab, but probably not the same species as the Antarctic one and not present by the thousands. There are some shrimp, but not nearly as many as in the nearby central Indian Ocean. And there are a lot of scaly-footed snails, but not enough to take over the place. “Elsewhere you tend to find one species dominating,” Copley says. “Here we’ve got at least four different types all jostling for it. It’s quite a crossroads.”
The idea of a deep-sea crossroads has inspired recent trips to a third place, the Cayman trough in the Caribbean Sea. Volcanic activity has been going on there for 50 million years, Copley says; 3.1 million years ago, the isthmus of Panama closed nearby, cutting off direct circulation between the Atlantic and Pacific oceans. Exploring the Cayman trough, scientists reasoned, might tell them more about how deep-sea creatures once moved through this watery gateway.
A spring 2010 cruise to the vents yielded a surprise. The Cayman animals most closely resembled not those in the Pacific or at the closest known cold-water seeps 1,500 kilometers away in the Gulf of Mexico, but those at hot-water vents along the mid-Atlantic ridge — a full 4,000 kilometers away. At a hot-water vent 4,970 meters down, the dominant species was a new type of shrimp, the British scientists reported online January 10 in Nature Communications.
American researchers, led by the Woods Hole Oceanographic Institution, left Florida on a research cruise on January 6 to further explore the Cayman vent field.
“A single eight-week cruise can completely change our ideas about how these systems evolved and how species are distributed,” says Rogers. “It really gives you some idea of the level of our knowledge of deep-water ecosystems.”
Back Story – MISSED OPPORTUNITYIn 1976, oceanographers exploring the seafloor near the Galápagos Islands were astonished to spot mussels, anemones and crabs living around deep-sea hot springs, totally isolated from ecosystems in shallower parts of the ocean. The vents’ striking “black smoker” chimneys, like this one near New Zealand, quickly became icons of undersea exploration. But some scientists now say hydrothermal vents could have been discovered in Antarctic waters a decade earlier if scientists had realized what they were seeing. Library research recently unearthed a 1966 photograph, taken 2,377 meters deep along the Southern Ocean’s East Scotia Ridge, that shows animals at what’s now known to be an active hydrothermal vent. Oceanographers at the time “simply didn’t recognize what they were investigating,” says Alex Rogers, a marine biologist at the University of Oxford in England. “If they had, the course of scientific history would have taken a very different route.”