Closer inspection of the giant larvacean, seen here in an undersea video taken by a remotely operated vehicle, could reveal clues about how the deep sea sequesters carbon. Image Credit: © 2017 MBARI

In the deeps off Monteray Bay, Calif., a multitude of delicate, translucent creatures called giant larvaceans have a potentially big role in trapping carbon. Now, for the first time, scientists have engineered a deep-sea robot to study these creatures in their natural habitat. The results, which could offer clues about how the deep ocean stores carbon long-term, were published today in Science Advances (1).

The animals are called ‘giant’ because at 10 cm, they are at least twice as large as normal larvaceans. These tadpole-like animals secrete mucous, which they use as raw material to build a ‘house’ to live in. Their homes can be more than a meter long. Because of their size and seeming omnipresence in the Pacific and Atlantic Oceans, scientists believe these animals play an outsize role in the cycling of carbon through the planet.

In 2005, scientist Bruce Robison and his colleagues at the Monterey Bay Aquarium Research Institute (MBARI) found that giant larvaceans deliver large amounts of carbon to the sea floor. But studying these fragile creatures any further proved challenging since they did not function normally in the laboratory.

So Kakani Katija, a bioengineer at MBARI, and her colleagues built an instrument to study the animals in the deep sea. In 2015, she and her colleagues deployed the DeepPIV, an instrument mounted on a remotely operated vehicle that hovers unobtrusively near the giant mucous balls. It shoots out a plane of laser that reveals a cross-section of the animal. A camera captures the movement of particles through the house, which allowed the scientists to estimate filtration rates. The scientists found that giant larvaceans in the Monterey Bay could graze the entire water column between 100 to 300 meters depth in just 13 days.

“If they indeed clear the water around them of particles every few weeks, they will have a big impact [on carbon sequestration],” says Dennis Hansell, an ocean biogeochemist at the University of Miami, who was not affiliated with the study. The technology is multipurpose and could be used to study fluid motion effects on small-scale processes in organisms such as sponges and deep-sea corals, Katija says.

A variety of tiny organisms that live in the oceans are efficient trappers of carbon. Phytoplankton live in the topmost layers and use sunlight and carbon dioxide to photosynthesize and generate organic carbon. Further in the depths live zooplankton, such as giant larvaceans, that feed on the phytoplankton and eject carbon-rich fecal pellets that fall to the sea floor. About 11 gigatons of carbon (equal to the yearly emissions from 8.5 billion cars) sinks into the ocean every year and is sequestered for decades or even millennia. The entire process is called the biological carbon pump.

Giant larvaceans are key members of that carbon pump. Their house is actually two layers of mucous membranes that trap sediment from the surrounding water. The outer layer traps coarse sediment while the inner filter traps finer particles that the organism eats. The filters clog after about 24 hours, at which point the organism leaves the carbon-rich house, which sinks to the ocean floor. DeepPIV allows the scientists to peer inside the house and quantify the filtration process

Data about the giant larvaceans could be used to improve computer models of the biological carbon pump, says Richard Sanders, an ocean biogeochemist at the National Oceanography Centre in the U.K. who works on the biological pump. Ecological models incorporate detailed ecology of the deep-sea organisms, such as giant larvaceans, to simulate the carbon pump and predict how it might change.

The study does not quantify the amount of carbon that giant larvaceans sequester in the ocean, which would allow for a fuller understanding of their contribution to the biological pump, notes Hansell of the University of Miami. But that next step is now doable, he says, in light of the new technology.