This Robotic Spies on Creatures within the Ocean’s ‘Twilight Zone’


The grandest migration on Earth isn’t the journey of some herbivore in Africa or a bird in the sky, but the vertical movement of whole ecosystems in the open ocean. All kinds of animals, from fish to crustaceans, hang out in the depths during the day, where the darkness provides protection from predators. At night, they migrate up to the shallows to forage. Then they swim back down again when the sun rises—a great big conveyor belt of biomass.

But now a spy swims among them: Mesobot. Today in the journal Science Robotics, a team of engineers and oceanographers describes how they got a new autonomous underwater vehicle to lock onto movements of organisms and follow them around the ocean’s “twilight zone,” a chronically understudied band between 650 feet and 3,200 feet deep, which scientists also refer to as mid-water. Thanks to some clever engineering, the researchers did so without flustering these highly sensitive animals, making Mesobot a groundbreaking new tool for oceanographers.

“It’s super cool from an engineering standpoint,” says Northeastern University roboticist Hanumant Singh, who develops ocean robots but wasn’t involved in this research. “It’s really an amazing piece of work, in terms of looking at an area that’s unexplored in the ocean.”

Mesobot looks like a giant yellow-and-black AirPods case, only it’s rather more waterproof and weighs 550 pounds. It can operate with a fiber-optic tether attached to a research vessel at the surface, or it can swim around freely.

Video: Erik Olsen/Woods Hole Oceanographic Institution

Mesobot’s first bit of clever engineering is its propulsion system—large, slow-moving propellers that create low-velocity jets. “Why are we so concerned about disturbing the water?” asks Dana Yoerger, a senior scientist at the Woods Hole Oceanographic Institution and lead author on the paper. “Most mid-water animals are extremely sensitive to any hydrodynamic disturbance. Because usually, that’s something coming to eat them.” If you’re disturbing these animals, you’re not observing their natural behaviors. (Unless you’re curious about what annoys them.)

The second clever trick ensures that Mesobot doesn’t bother its subjects by blasting them with light. Well, at least not white light. Yoerger and his team opted for a red beam, because it doesn’t penetrate seawater well. “Evolution doesn’t waste a lot of capability on stuff that doesn’t work very well, so most animals are blind to red light,” says Yoerger. That’s why when you see bioluminescent critters popping off in the deep sea, they’re blue or green. “We use red,” Yoerger continues, “even though red is pretty lousy, because it doesn’t go very far. But it doesn’t spook the animals as much. And that’s pretty well documented. So it’s a trade-off: You need a lot of light, you need a sensitive camera, and then you can work in the red.”