For the general public, the expression “twilight zone” is the original title of a famous American television series, The Fourth Dimension. But among researchers in marine biology, it designates an area of the seabed where daylight can no longer penetrate enough for photosynthesis to take place. Located between 200 and 1,000 meters below the surface, it is known in French as the mesopelagic zone (or aphotic zone, “deprived of light”). Hence the name given to a 225 kg underwater robot responsible for exploring these regions, Mesobot.
Designed by a team of researchers at Monterey Aquarium, Stanford University, both in California, and the Woods Hole Institute of Oceanography in Massachusetts, this compact craft was featured in the June issue. 2021 review Robotics Sciences.
An overall vertical displacement
The ambition is to follow and film the marine fauna in its migration from the dark seabed towards the surface, when night comes, and, conversely, from the surface towards the depths when the day arrives. This vertical path corresponds to a defense mechanism: during the day, animals (fish, crustaceans, zooplankton) dive where light no longer penetrates, in order to protect themselves from predators.
The Mesobot is equipped with several technologies to support this overall movement. First, algorithms for detecting objects, in particular gas bubbles when the animals come up, and for analyzing movement that allow him to know where to go.
Concretely, the robot follows the animals according to the information captured by its cameras. Its computer vision system is based on the principle of “blob detection”: the identification of an area, in the image, which is distinguished from the rest by a shape, a color, a movement, and which testifies to the presence of an organism.
If this shape goes out of scope, a coordinate calculation with respect to the robot’s position allows the latter to find the animal. However, the system does not take into account shapes that move too fast. In the event that several life forms pass in front of the camera, “The algorithm chooses its“ targets ”according to these various criteria and is able to continue to follow the same“ blob ””, says Dana Yoerger of the Woods Hole Oceanographic Institute.
Red light illumination
Then red light LEDs illuminate objects in the dark. Marine animals are not sensitive to this light frequency which, therefore, does not bother them. Last element, the propulsion system. Here too, the challenge is to avoid frightening the fauna with disturbances of the aquatic environment that could make people believe in the arrival of a predator. Six low-power propellers (60 watts) orchestrate the Mesobot’s maneuvers, ensuring slow movements on three axes (which is why the craft does not follow animals that move very fast).
For the rest, in addition to its cameras, the device “embeds conventional oceanographic sensors to measure the water temperature, its salinity, an optical backscatter sensor to assess its clarity, fluorometers of different wavelengths (to detect the presence of algae, Editor’s note)”, continues the researcher. The data thus collected are intended to be analyzed in a second time in the laboratory, and not by the robot in real time.
The very first tests of the device date back to 2019. They took place with a tennis ball in a pond at the Monterey aquarium. Since then, a life-size experiment has been carried out in the waters of Monterey Bay, at a depth of 200 m. The robot is submerged, connected to a fiber optic cable to the aquarium’s research vessel, and then, initially, remotely piloted. Once targets have been identified, it can switch to stand-alone mode.
The Mesobot was thus able to follow and film a small organism called Copelata. He also observed a Solmissus jellyfish hunting a siphonophore, a zooplankton organism. And immortalized how the prey was finally able to escape.