Swimming robot spies on creatures of the ‘twilight zone’

The so-called twilight zone (aka the mesopelagic zone) spans between the depth at which just one per cent of light reaches and the depth where there is no light at all, hundreds of metres beneath the surface of the ocean. The region is host to a diverse range of flora and fauna uniquely adapted for low-light conditions, including giant squid, anglerfish and bioluminescent jellyfish. Understanding this unusual ecosystem is essential for modelling how carbon dioxide is transported from the atmosphere to the deep sea, as well as for modelling the impact of twilight zone fisheries on the wider ocean ecosystem.

Mesobot expands the ability of scientists to observe creatures – many of which are very fragile – in their natural habitat without causing the disturbance that would be associated with, for instance, research submarines. Developed at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts, Mesobot is capable of tracking and recording high-resolution images of slow-moving and fragile zooplankton, gelatinous animals and small particles in the region.

"Mesobot was conceived to complement and fill important gaps not served by existing technologies and platforms," said senior WHOI scientist Dr Dana Yoerger. "We expect that Mesobot will ermerge as a vital tool for observing midwater organisms for extended periods, as well as rapidly identifying species observed from vessel biosonars. Because Mesobot can survey, track and record compelling imagery, we hope to reveal previously unknown behaviours; species interactions; morphological structures, and the use of bioluminescence."

Yoerger led a study which outlines the robot's performance in autonomously tracking two gelatinous marine creatures during a research cruise in Monterey Bay in 2019. High-definition video revealed a "dinner plate" jellyfish ramming a siphonophore, which narrowly escaped its venomous tentacles. Mesobot also recorded a 30-minute video of a giant larvacean, which appeared motionless but was actually riding internal waves that can rise and fall six metres.

These observations represent the first time that a self-guided robot has tracked these small, clear creatures as they move through the water column like a "parcel of water", Yoerger explained.

Kakani Katija, an engineer from Monterey Bay Aquarium Research Institute, added: "Mesobot has the potential to change how we observe animals moving through space and time in a way that we've never been able to do before. As we continue to develop and improve on the vehicle, we hope to observe many other mysterious and captivating animals in the midwaters of the ocean, including the construction and disposal of carbon-rich giant larvacean 'snot palaces'."

The robot is packaged in a hydrodynamic yellow case and fitted with a suite of oceanographic and acoustic survey sensors. It can either be steered remotely via a fibre-optic cable tethered to a ship; be released from its tether to follow pre-programmed missions, or autonomously track a target at depths up to 1km. This autonomous capability will one day enable Mesobot to follow a target animal for over 24 hours without human intervention, the researchers said, which is enough time to observe its migration from the midwater twilight zone to the surface and back.

Future studies with Mesobot could provide researchers with useful insights into twilight zone creatures' behaviour during diel vertical migration (known as "the greatest migration on Earth" due to the vast number and diversity of creatures undertaking it each night).

"By leveraging the data we've collected using Mesobot, and other data that we've been curating for 30-plus years at [Monterey Bay], we hope to integrate smarter algorithms on the vehicle that uses artificial intelligence to discover, continuously track and observe enigmatic animals and other objects in the deep sea," Kakani said.