Scientists used underwater drone swarms to solve the mystery of plankton mating

Each plankton is microscopic and can’t navigate on its own; together, they move in swirls and eddies caused by ocean currents.
Each plankton is microscopic and can’t navigate on its own; together, they move in swirls and eddies caused by ocean currents.
Image: NOAA
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Swarms of airborne drones can already dance in unison, so it was only a matter of time before marine scientists tried to have them swim in formation underwater.

A group of University of California-San Diego researchers, led by Jules Jaffe, built a set of 16 mini-autonomous underwater explorers, or M-AUEs, and dropped them in the water a few miles off the coast of California. The robots were told to do one thing: stay at a depth of about 33 feet. They were allowed to drift laterally in any direction, in order to track the motion of water below the surface.

Pressure sensors let the robots maintain their depth, which they managed to do within about 3 feet. To measure their position the team deployed five “pingers”—beacons that surrounded the area the robots were in. The beacons could use GPS to track their own positions, and when they pinged the M-AUEs in the water, using sound (not unlike sonar used in submarines) the swarm of robots could time those signals and calculate where they were. The UCSD team came up with this sort of convoluted system because GPS signals don’t penetrate well into the water.

By maintaining their depth, the robots were able to track what are known as “internal waves.” These waves are longer, shallower, and slower than surface waves, and usually invisible to the human eye—though you can sometimes look at a calm ocean and see “slicks” of smoother water that indicate internal waves, says study co-author Peter Franks, also of UCSD.

Internal waves occur because the ocean has differing densities at different depths. Surface waves happen for the same reason, but the difference between the density of air and water is a factor of 1,000, whereas the change in density from for example, 10 meters down to 50 meters is a factor of 1/1000.

Despite their subtlety, internal waves are important—especially to plankton, the tiny creatures that are one of the main parts of the bottom of the marine food chain. The internal waves seem to be the way that plankton move around to find each other.

Plankton can’t really swim the way fish do (many species are single-celled). Yet they also have to find each other—they might be tiny, but they still have to mate. “One of the big problems is how do they find each other to have sex in ocean,” says Franks. Most plankton, after all, don’t have anything like a brain, so they don’t know much about their surroundings. “Picture two people blindfolded on a football field trying to find each other.”

The robots seem to have solved that mystery. As the machines drifted at depth, they encountered the internal waves, rode over the crests, and clustered in the troughs.  Franks says it’s likely the plankton do the same thing—which allows them to get together.

The proof-of-concept, published Jan. 24 in the journal Nature Communications, opens up a lot of other kinds of data gathering, says Franks. If they added chemical sensors to the M-AUEs, for example, they could measure nitrates in the water, which would tell scientists how nutrients get transported between the sunlit layer of ocean and the depths. A light sensor could help them track the phosphorescent creatures as they migrate through the water column.

And allowing them to drift for months (instead of hours as in this experiment) might teach us a lot about where plankton go throughout their lives. This could be important for the shellfish industry: “One of the new [fisheries] management strategies is marine protected areas,” Franks says, which is where you raise fish or shellfish in a managed area of the open sea. The problem, Franks says, is that lobsters, clams, and mussels all spend part of their lives as plankton, and during the “planktonic life stage, they’re drifters. They then have to find a habitat, and that’s a huge control over how many adults you have. So we want to see the process by which those larvae come back to the shore.” If we know where the plankton go, then that gives us a better handle on how many make it back, and that in turn helps refine estimates of how fast they renew after harvesting.