Jellyfish don’t look all that appetizing. They’re also armed to the bell with some of the deadliest chemical weapons on Earth. They don’t even make for a very hearty meal. So it seems unlikely that small, feathery penguins would willingly seek this potentially dangerous prey.
Yet that’s the conclusion of a new paper published this month in Frontiers in Ecology and the Environment. “It is hard to believe that penguins, which are endothermic animals and thus need fair amounts of energy from their food to keep their bodies warm, could find any benefit in eating relatively energy-poor prey like jellyfish, especially in freezing Antarctic waters,” says Jean-Baptiste Thiebot, a postdoctoral fellow with Japan’s National Institute of Polar Research and lead author on the study, which confirms jellyfish might be a more important food source for penguins than previously thought.
“Crustaceans, and more so fish, contain much more energy per gram,” he said, and therefore “seem much more adequate to satisfy the energy demand of penguins,” especially when they are rearing chicks.
But the videos don’t lie. Thiebot teamed up with 16 other scientists from five countries to assess the diets of penguins using video cameras attached to the backs of 106 penguins. They included four species—Adélie penguins (Pygoscelis adeliae), yellow-eyed penguins (Megadyptes antipodes), Magellanic penguins (Spheniscus magellanicus), and little penguins (Eudyptula minor)—and captured more than 350 hours of footage of the feathered predators hunting.
The cameras documented over 200 instances of jellyfish consumption, including painful sea nettles (Chrysaora plocamia) and Cyanea. The birds were seen tearing off chunks of larger jellies and consuming smaller ones whole. (Sea turtles do it, too: Watch one snack on jellyfish tentacles.)
This isn’t the first study to suggest jellyfish make up a substantial portion of penguin diets. Back in 2013, marine biologist Simon Jarman and his colleagues isolated jellyfish DNA from Adélie penguin scat. In their analyses, jellies and other gelatinous organisms comprised an average of 40 percent of the animals’ diets, and were the only food detected in some individuals.
But those data were hard for some to swallow. The presence of jellyfish DNA was suggested to be from accidental ingestion or because the penguins’ prey had recently consumed them. The new results refute those hypotheses, says Thiebot, as the penguins deliberately attacked jellyfish, even in cases when “preferable” species such as lobster krill were available.
“When we saw the footage…we could finally see that the penguins indeed go for the jellies themselves,” Thiebot said, “and that this was happening not only in the Antarctic but apparently all across the southern oceans.”
Jarman, who did not take part in the latest study, says it’s compelling to see new evidence supporting his own hypothesis. He’s also not surprised that jellies are on the birds’ menus.
“It is only unexpected if you subscribe to the naïve dogma that animals will forage for the most energy intense food preferentially,” he says, but as a rule, that doesn’t make sense. “Thinking as a human, if we eat the most energy-dense food exclusively—fats, basically—then we die.”
Others have theorized that jellies are a food of last resort—that the consumption of this low-calorie prey points to an ecosystem disorder. But Thiebot believes their data refute this idea as well.
“This behaviour should not be seen as a mistake when penguins face an unknown prey, or something abnormal suggesting that the penguins could not find better prey to feed on,” he says. “These are deliberate captures by the penguins, observed over successive years and within each of the four species and seven populations studied across the southern hemisphere, from temperate to Antarctic waters.”
“We still do not know exactly why penguins choose to capture jellyfish,” Thiebot says. He suggests the quick, easy-to-capture bites along the way could help fuel the animals’ long hunting trips, or that jellies might simply be more nutritious than currently believed. Jarman notes they may also be easy to digest. Whatever the appeal, jellies' chemical defences don't seem to be having any serious documented effects on their penguin predators.
One thing is certain: Now that several species of penguins can be added to the ever-growing list of confirmed jellyfish predators, it’s time to reconsider the roles that jellies might play in marine ecosystems, said Thiebot. “We need to clearly acknowledge that jellyfishes and other gelatinous organisms may be a regular prey for a variety of marine predators.”