Deep-sea critters confuse elephant seals with flashes | Polarjournal
Southern elephant seals spend about a third of the year on the beaches of sub-Antarctic islands to give birth to their cubs and wait for the fur change. After the deprivation-rich time, they go on the hunt for lantern fish and squid in the depths of the Southern Ocean. Photo: Julia Hager

Deep-sea hunters such as the Southern elephant seals (Mirounga leonina) have no easy time to track down their prey in the dark depths. Less than one percent of the light penetrates into their hunting grounds. The glow and flashing of the bioluminescent deep-sea inhabitants seems to be a welcome help. Researchers at the University of St. Andrews, UK, have been trying to find out whether this really is the case.

Southern elephant seals are native to the cold waters of the southern hemisphere. They spend about eight months of the year on the open sea, where they go hunting for lantern fish and squid between 200 and 1000 meters deep, in the mesopelagial. Only the breeding season and the time during the fur change they spend in colonies on sub-Antarctic islands.

Scientists Pauline Goulet and Mark Johnson of the University of St. Andrews hypothesized that the mysterious glow produced by many creatures in the deep sea helps elephant seals tracking down their prey. “Bioluminescent organisms are the main source of light (80) in waters deeper than 500 meters,” says Goulet. These animals produce two forms of light: a continuous, faint glow to camouflage from below against the surface of the water, and dazzling flashes, possibly to distract predators.
Goulet and Johnson wondered if the hunting seals could benefit from the lightning storm of the bioluminescent animals. Or do their prey use the flashes of light to dazzle the attackers and make time for the escape?

Southern elephant seals find their prey mainly in the mesopelagial between 200 and 1000 meters depth. Only one percent of the light, which is completely absorbed to the border to the bathypelagial, penetrates to its upper limit. Graphic: divemagazine.co.uk

Goulet, Christophe Guinet of the Centre d’Etudes Biologiques de Chizé, France, and Johnson were curious about how these cat and mouse games play out in depth and therefore decided to document hunting scenes of the elephant seals in detail to answer the question.

First, Goulet and Johnson assembled a data logger that combines a sensitive, fast-response light sensor and a high-resolution position and motion sensor to simultaneously record the bioluminescence of the prey and the seal’s attack movement at a resolution of 20 milliseconds. “Because the bioluminescence flashes are so short, usually less than a second, the tags needed a very fast light sensor,” Goulet explains.
The researchers equipped seven elephant seal mothers who already had weaned their cubs with the data loggers and GPS transmitters — five animals on the Kerguelen and two animals on the Argentine peninsula of Valdés. “There’s always a person who takes care of the other seals when you equip them because you’re totally focused on what you’re doing and you’re not aware that an aggressive animal is coming to bite you,” Guinet recalls.

A: A female Southern elephant seal equipped with the specially developed data logger. B: Regions where the animals have been equipped with data loggers. C-D: The routes of four animals recorded with a GPS transmitter. Graphic: Goulet et al. 2020

When the seals returned two months later, the team found that most of the animals had set off for a 3,000 km odyssey in fish-rich marine regions when they recovered four data loggers. However, an intrepid Argentine seal circled Cape Horn and eventually covered 2,300 km before spotting fish off the Chilean coast.
After months of evaluation and close study of the seals’ maneuvers, as well as careful analysis of the more than 2,000 bioluminescent flashes of light that the seals recorded at depths between 79 and 719 meters, Goulet and Johnson realized that the flashing prey animals were trying to deter their attackers.
“The prey always sends a flash the second the seal launches an attack, suggesting that the lightning is a defensive response as soon as the prey detects that they are being attacked,” Goulet says. In addition, the seals quickly snapped such fish that could not light up, while it was harder for them to catch the meal when their prey unexpectedly blinded them. One seal, however, appeared to have turned the tables by making her victims betray themselves with a subtle shrug of the head that triggered a telltale flash.

It seems that bioluminescent fish fight back by trying to scare their attackers, but their attackers can also learn to exploit the bioluminescent betrayal of their prey. Goulet and Johnson also hope to use the animals’ characteristic flashes of light to identify which species are on the seals’ menu the next time they return to the Southern Ocean.

The study was published in the journal Journal of Experimental Biology.

Source: SciTechDaily, Goulet et al. 2020

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