In the permanent twilight of the mesopelagic, a silent predator hunts.

The enigmatic giant squid is rarely observed in its natural habitat. In the first videos of their kind, unveiled in 2021, marine scientists caught its hunting behavior in the wild – revealing for the first time how these monsters of the deep stalk and attack their prey.

Although the crushing pressures and darkness of the oceanic depths are hostile to us air-breathing humans, we've slowly but surely been learning more about them, thanks to the wonders of robotic technology. Most of our underwater vehicles, however, are best suited to studying slow or immobile organisms.

For giant squid, the bright lights mounted on underwater vehicles can be uncomfortable for their sensitive, low-light eyes, which can grow to the size of dinner plates; the sound and vibration can also scare off more mobile animals. And, of course, bringing giant squid to the surface won't record their behavior in their natural environment.

That's why a team of researchers led by Nathan Robinson of the Oceanographic Foundation in Spain devised a different solution: a passive deep-sea platform, equipped with a camera. Because giant squid eyes are optimized to see shorter-wavelength blue light, they used longer-wavelength red lighting that won't annoy them, in order to see the animals on video.

Finally, they added bait: a fake jellyfish, called E-jelly, equipped with lights that mimic the blue flashing bioluminescence emitted by an atolla jellyfish (Atolla wyvillei) in distress. Although giant squid aren't known to eat jellyfish specifically, they may be attracted to the distress lights of these atolla jellyfish – they might mean that the jellyfish is under attack by something the squid does want to eat.

All that remained, then, was to wait. And it paid off: At depths between 557 and 950 meters (1,827 and 3,117 feet) in the Gulf of Mexico and in Exuma Sound near the Bahamas, the team's platform recorded several encounters with large squid.

The first encounters were in 2004 and 2005 with two large animals that may have been Promachoteuthis sloani, at a mantle length of 1.0 meters – a species only previously known from small juveniles.

The team continued to update their platform, and captured Pholidoteuthis adami, with a mantle length of 0.5 meters, in 2013. In 2019, they finally filmed Architeuthis dux, the giant squid itself, clocking in at a mantle length of 1.7 meters (that's excluding the tentacles).

Interestingly, the encounters suggest strongly that the squid are visual hunters, ignoring olfactory bait that had been placed nearby in favor of visual signals.

The giant squid's hunting behavior was perhaps the most fascinating. It tracked the platform for around six minutes prior to attacking, suggesting that it was stalking its prey before moving in for the kill.

This contradicts the supposition that giant squid are ambush predators, as previously put forward in several papers. Rather, the animal appears to be an active and engaged hunter that uses visual cues (and its humongous eyes, down there in the dark) to find a meal.

Each of the encounters, few as they were, also provided new information about the range and distribution of the species observed.

This suggests that passive platforms may be extremely useful tools for observing these elusive creatures, especially if refined and optimized for specific encounters, the researchers said.

"We recommend that future studies assess the value of using low-light systems or optical lures in a more scientifically-robust manner," they wrote in their 2021 paper.

"For example, while the bioluminescence-mimicking E-Jelly appears to be an effective tool for attracting cephalopod species, future studies could assess whether lures of differing intensities, colors, or light patterns vary in their capacity to attract various taxa of deep-sea cephalopods."

The research was published in Deep Sea Research Part I: Oceanographic Research Papers.

A version of this article was first published in May 2021.