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Five hundred meters below the ocean’s surface, a female southern elephant seal lunges at a lanternfish. For the briefest of moments, the darkness blazes with light. Just as fighter planes release chaff to thwart radar detection, the lanternfish has emitted a series of brief yet intense flashes from its skin, and narrowly avoided the jaws of the dazzled elephant seal.
Scenes like this, reconstructed from data collected in the Southern Ocean by a new sensor, illustrate the cat-and-mouse games taking place where the sun barely reaches.
Scientists have long hypothesized that bioluminescent animals—those that produce light with their own bodies—can use their illumination defensively. Actually demonstrating this, however, has been challenging. Emitted by many deep-water species, including frequent prey of elephant seals, such as lanternfish, cuttlefish, and squid, these flashes last only milliseconds. To evade an attack, scientists estimate these animals have to flash the instant a predator attempts to grab them.
Though researchers regularly mount cameras on underwater predators, none have been precise enough to capture such brief moments. That is, until marine biologists Mark Johnson and Pauline Goulet from Denmark’s Aarhus University and Scotland’s University of St Andrews, respectively, worked with their colleagues to design and build specialized biological data loggers that include a homemade sensor capable of detecting momentary flashes of light. The loggers also record sound, motion, and location.
The researchers traveled to Argentina and the southern Indian Ocean’s Kerguelen Islands to capture and tag female elephant seals that were hungry after weeks spent nursing their pups. The team was able to recover four of the seven tags that were deployed. The data loggers gave the scientists an unprecedented look at where the seals traveled to feed, and the potential bioluminescent countermeasures deployed by their prey.
Over two months, the data loggers recorded that each seal was hit with more than 2,000 flashes by probable prey. Prey consistently flashed the moment the seals launched their attacks, suggesting that the flashes are, indeed, an attempt to buy a few precious seconds to escape.
Interestingly, one of the seals from the Kerguelen Islands appears to have learned its prey’s trick. Just as it reached its prey, the seal would jerk its head. The movement would trigger its target to flash; only then would the seal attempt to grab it.
Though it’s tempting to speculate that the bioluminescent flashes and the seals’ hunting strategies have coevolved as a sort of arms race, Johnson says too little is known to make that claim. The new loggers, however, may help illuminate whether the Kerguelen seal’s adopted tactic is an individual skill, or a more widespread phenomenon.
Roxanne Beltran, a marine biologist at the University of California, Santa Cruz, who was not part of the research, says she wouldn’t be surprised to learn that individual elephant seals have developed new hunting strategies. “Elephant seals have shown individual variability in many facets of their lives, from the timing of reproduction, to body size, to diving behavior, to foraging routes, to diet.”
Already, Johnson and Goulet have adapted their instruments to examine the seals’ habits in more detail. New versions of the tags carry miniature sonar sensors, which will let the researchers see how far a seal is from its prey when it flashes. Because seals approach each type of prey differently, this may help the team determine which twilight-zone species the seals are chasing at any given time.
Turning the seals into fine-tuned probes might finally shed some light in the dark.