Hungry elephant seals go out on the prowl in search of lantern fish and squid to fill their empty bellies after weeks marooned on beaches nursing their pups, but until recently it wasn’t known how the hunters knew where to find their prey in the inky ocean depths. New research published in the Journal of Experimental Biology set out to answer what gave the prey’s position away by strapping cameras to the seals’ heads to get a seal-eye-view of the pursuit, and the footage is dazzling.
"Bioluminescent organisms are the main source of light (80 percent) in waters deeper than 500 meters," said Pauline Goulet from the University of St Andrews, UK in a statement. Lantern fish and squid can produce two kinds of light, a continual dim glow to act as countershading and vibrant flashes to confuse attackers. The researchers on the study wondered if these bright flashes essentially gave the position of the prey animals away to hungry seals, or if the bright lights actually acted as an effective getaway against elephant seals.
To find out, Goulet and colleagues decided to capture the cat-and-mouse chase between the seals and their prey to see how the fight played out underwater. In order to paint a detailed picture of the events, they needed a tag that could log the seals’ movements as well as recording the flashes of light from the bioluminescent snacks. "Because the bioluminescent flashes are so short, typically less than a second, the tags required a very fast light sensor,” Goulet explained.
They took the kit to the Kerguelen Islands, also known as the Desolation Islands, in the Antarctic and attached it to five elephant seal mothers, making sure to keep an eye out for approaching seals who can be a bit nippy. Two months later, of the five seals, they were able to retrieve tags from four of them and the data analyses revealed that most of the hungry mums had popped off on a 3,000-kilometer (1,864-mile) journey in search of food, with depths ranging from 79 to 719 meters (260-2,360 feet).
After painstakingly analyzing more than 2,000 bioluminescent flashes, the researchers recognized that the prey animals were flashing to spook their attackers. "The prey always emits a flash the second the seal launches an attack, which suggests that the flash is a defensive reaction when the prey realizes it is being attacked," said Goulet.
A flashing squid being pursued by a diving elephant seal. Credit: Akinori Takahashi, The Graduate University for Advanced Studies. Reproduced with permission of The Company of Biologists. Yoshino, K. et al. (2020). Acceleration-triggered animal-borne videos show a dominance of fish in the diet of female northern elephant seals. J. Exp. Biol.
The seals were more successful in eating prey that didn’t flash and had a harder time catching those that did. Amazingly, one seal appeared to perform a false lunge by twitching their head so that the animal would flash and give itself away. The flashing is therefore adaptive in helping the prey to escape but can also be exploited by savvy seals who have clocked on to the performance. Goulet and Johnson next hope to be able to identify exactly which animals the seals are snacking on by decoding their distinctive flashes.