There are not many jellyvores in the world, or so scientists have long thought.
Gelatinous sea animals, like jellyfish and ctenophores, have traditionally been regarded as “dead ends” in food webs. Because they are so low in calories (jellyfish are about 95 percent water), it was thought that most predators would not benefit from eating them. But a recent study has identified a new, unexpected jelly-eater: penguins.
Like other warm-blooded animals, penguins have high caloric demands and typically seek energy-dense foods, like fish and krill. In a paper published in Frontiers in Ecology and the Environment, however, an international consortium of scientists has reported that an assortment of penguin species frequently attack jellies as food, a behavior that had not been documented before.
Past studies have found DNA from gelatinous creatures in penguin guano, but researchers thought the seabirds might be accidentally ingesting jellies, said Michael Polito, an assistant professor of oceanography at Louisiana State University who was not involved in the study. That “penguins are specifically targeting gelatinous prey” is a surprising finding, he said.
In the new study, led by Jean-Baptiste Thiebot, a postdoctoral researcher at the National Institute of Polar Research in Japan, teams from five countries monitored four penguin species: Magellanic penguins in Argentina, Adélie penguins in Antarctica, little penguins in Australia and yellow-eyed penguins in New Zealand.
Strapping miniature video cameras to the penguins, the scientists documented nearly 200 strikes on jellies at seven different sites.
“We were amazed to realize that all teams observed the same phenomenon,” Thiebot said.
The researchers estimated that jellies provide only 1 to 2 percent of penguins’ daily energy needs. This begs an interesting question, said Nina Karnovsky, an associate professor of biology at Pomona College who did not participate in the research: Why would penguins expend the energy to catch and digest jellies for such low return? Besides sea turtles and ocean sunfish, there are few animals that bother hunting jellies at all.
It would be an easy story if it seemed that the birds were only resorting to jellies when there was nothing else to eat. But Thiebot and his colleagues saw in their footage that there were plenty of other available prey. More than once, they even observed penguins bypassing a swarm of krill to capture a jelly, suggesting there’s some other, unknown benefit in these gelatinous meals.
One possible clue comes from the fact Thiebot and his collaborators only saw penguins eating carnivorous jellies, not herbivorous ones, like salps. It’s possible that the penguins are gaining nutrients from the food eaten by carnivorous jellies, which include crustaceans that are too small for the penguins to target themselves, Thiebot said.
Another possibility is that the jellies contain particular elements, like collagen fibers or amino acids, that penguins need. The birds might also feed selectively on certain parts of jellies, like their gonads, which are rich in fats and proteins. Or jellies may simply make easier targets than fish.
Whatever the answer turns out to be, this study illustrates how animals often have “more complicated foraging behavior than we expect,” Karnovsky said.
The researchers’ findings also have potential implications for the global carbon cycle. Until now, scientists have thought that most jellies live their lives in the water column, then die and sink to the seafloor, transporting carbon from the surface to the depths of the ocean.
But if jellyfish are eaten “more often than expected,” Thiebot said, it might turn out that “estimates of carbon fluxes need to be reassessed.”