Feisty Fish and Squid Fight by Dazzling Attacking Seals

A female elephant seal equipped with two data-logging tags. (Photo by Martín Ignacio Brogger)

(CN) — Scientists have recorded a unique game of cat and mouse that exists between prey — squid and lantern fish — and their predator Southern elephant seals, documenting each of their tactics for success in the murky waters of the Southern Ocean.

In a study published Tuesday in the Journal of Experimental Biology, co-authors Pauline Goulet from the University of St. Andrews, United Kingdom, and Mark Johnson discuss their findings on the role of bioluminescence in this predator-prey dynamic.

After nursing their pups for weeks on the beach shores, Southern elephant seal mothers are itching to dive back in the ocean and hunt for squid and lantern fish, but this team of scientists wanted to know exactly how the seals locate their prey in the inky depths. Goulet and Johnson suspected that it might have something to do with the ability of many deep-sea creatures to produce an eerie bioluminescent glow as a form of self-defense.

“Bioluminescent organisms are the main source of light (80%) in waters deeper than 500 meters (1,600 feet),” Goulet said in a statement accompanying the study.

Animals from these depths produce different types of light for two reasons: a continuous dim glow to function as camouflage, and bright, dazzling flashes to distract predators. The authors pondered whether the predatory elephant seals were able to capitalize on bioluminescent creatures revealing themselves, or if the prey was successful in dazzling their attackers long enough to escape.

Goulet, Johnson and Christophe Guinet from the Centre d’Etudes Biologiques de Chizé, France, formulated a plan to witness how the dynamic plays out under the waves, and were able to catch the elephant seals in pursuit. They discovered the bioluminescent squid and fish flash at predatory elephant seals when under attack to disorientate the assailant and buy enough time to slip away unharmed.

These seals have been tagged in the past to track behavior and migration patterns as they correlate to environmental conditions such as temperature, salinity and depth. The first step in this study involved Goulet and Johnson designing a tag that could detect and log the seals’ movements during the hunt, as well as record any flashes of light set off when the mammals encountered their flashy prey. 

“Because the bioluminescent flashes are so short, typically less than a second, the tags required a very fast light sensor,” Goulet said.

After assembling the tags, Guinet traveled to the Kerguelen Islands in the Southern Ocean and with the help of colleagues attached the tags and GPS trackers to five lucky elephant seal mothers. Soon after, Johnson joined Guinet and other team members in Argentina to tag two more seals.

“There is always one person on watch for other seals when you are equipping them, because you are completely focused on what you are doing and unaware of an aggressive individual coming to bite you,” Guinet said.

The seals returned two months later, and after retrieving four tags the team discovered most of the group had trekked more than 1,800 miles deep into ocean regions known to be teeming with fish —except for one brave Argentinian seal. This seal circumnavigated Cape Horn, traveling more than 1,400 miles before it reached fish off the coast of Chile. 

The team analyzed the seals’ documented maneuvers for months, as well as more than 2,000 recorded bioluminescent flashes over depths ranging from 259 to 2,300 feet. From the results, Goulet and Johnson realized that the animals were flashing to scare off 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.

Furthermore, they found the seals were able to quickly snap up the prey that didn’t light up fast enough, and struggled more to catch the ones that dazzled them unexpectedly. One seal, however, demonstrated exceptional hunting skills by tricking its prey into giving themselves away by subtly twitching its head, triggering a revealing flash.

The study authors said bioluminescent fish flash defensively to startle the elephant seals but noted the predators can also learn to exploit this trait. In future studies, Goulet and Johnson hope to return to the Southern Ocean to identify which species the seals hunt based on the animals’ distinctive flashes.

%d bloggers like this: