August 28, 2025

After the predator ate the crab, its stomach acid went to work, digestive juices slowly dissolving a coating on the multivitamin-sized transmitter affixed to the prey.

Soon, the transmitter signal changed, telling researchers it was resting in a larger animal’s belly.

A two-meter-long underwater glider, called Shackleton, intercepted a signal off the coast of Alaska, demonstrating to researchers that transmitter tags placed on crabs could effectively indicate when the animals they tracked were preyed upon.

This glider is part of a multi-year effort to learn more about issues facing Alaskan crabs. The research is timely; in 2018, the southeastern Bering Sea snow crab population experienced a 90 percent population collapse, resulting in a two-season closure of the fishery.

“It was the biggest collapse that that stock had ever seen,” says Jared Weems, Crab Research Biologist for the Alaska Department of Fish and Game. “It had huge economic ramifications throughout Alaska.

NOAA scientists found that the snow crab collapse in Alaska’s southeastern Bering Sea was due to borealization, which basically means an ecological shift “from Arctic to sub-Arctic conditions” due to climate change.

Warmer water can increase crab metabolism, making it difficult for them to ingest enough food to survive, while also leading to increases in species such as the Pacific Cod, which is known to prey on snow crabs. These challenges are further compounded by open-water algal blooms, which can harm snow crab habitats by depleting oxygen levels, changing food availability, and potentially introducing harmful toxins into the environment.

While populations have slightly rebounded in the last few years, due to cooler water temperatures and a decrease in marine heatwaves, researchers say that, overall, conditions will continue to worsen for species like the snow crab.

In recent years, other crab species in Alaska, like the Bristol Bay king crab, have also faced declines and fishery closures. Researchers are now using the Shackleton to study several different crab species. This underwater glider is a Teledyne Webb Slocum AUV, weighing 80 kilograms. A lithium battery pack can keep it running for around 90 days on average, covering up to 1,500 km on a single charge.

Researchers in the lab at the University of Alaska, Fairbanks, can remotely pilot the glider, which uses a buoyancy propulsion system that pulls seawater into the nose cone to propel it forward or cause it to change elevation. When the glider is underwater, researchers cannot communicate with it; however, it is programmed to surface every three hours to make contact, establish its GPS location, and transfer data before submerging again.

The glider collects an array of measurements, including water temperature, salinity, solar radiation, dissolved oxygen, depth, and chlorophyll concentration in the water.

“We’re trying to paint a picture of the ecosystem and how the crabs fit into that ecosystem,” said Hank Statscewich, Director of Operations at the University of Alaska Fairbanks Institute of Marine Science’s Oceans Glider Lab.

Statscewich explained that what’s particularly powerful about using gliders to study these animals is the ability to identify the types of temperatures they prefer. The data could also indicate whether the area has high chlorophyll levels or whether animals are migrating to regions that have recently experienced a spring bloom. “It’s a whole biological suite of measurements that we’re collecting from a single vehicle,” he said

Researchers originally used Shackleton in 2021 to study Pacific herring but soon realized it could also be applied to other species, such as crabs, and to learn about crab movement, predation, and habitat utilization. The glider’s passive acoustic receiver can also detect tags affixed to crabs that are up to a kilometer away, recording the acoustic pulse of each tag it detects, identified by a unique serial number.

According to Statscewich, using underwater gliders provides a much clearer perspective on the types of habitats crabs use and how frequently they are preyed upon. He added that it is “a much more cost-effective way than sending people out on ships to do what might be called semi-invasive studies where they trawl along the bottom, collect crabs, and then count them and put them back down.”

A 2022 pilot project in the Kodiak area successfully tagged 35 Tanner crabs, which led researchers to launch Shackleton in the Bering Sea in 2023.. That season, although a leak ended the mission, the team worked with a nearby research vessel to recover the glider. They gained valuable data, including information that showed a single crab moved 100 miles in just a few short months.

After repairs, Shackleton was again launched in 2024. That year’s effort focused on tagging 30 juvenile king crabs and the successful test of sensors to detect when a crab was eaten. Researchers aim to build on this research by deploying more gliders and tagging more crabs to track and study. Their latest proposal involves gliders that feature an optics package, which allows them to capture images of the sea floor, share the deep-sea scene and inform researchers about important information beyond what sensors can detect. These new features can provide researchers with images that show information such as habitat densities on the seafloor, how crabs congregate, the presence of other species, and specific habitat features.

As researchers utilize tools like the Shackleton glider and more advanced underwater devices, such as gliders equipped with imaging systems, they will continue to gather data that can help them learn more about how crabs and other wildlife are responding to a changing climate. Having multiple gliders will also allow researchers to operate them in tandem and utilize survey grids and other more complex study methods. These clues can help them understand everything from where animals gather to what habitats they prefer to how long it takes them to end up in the belly of a predator.