You've no doubt heard the joke: What does a [insert sea creature here] like to have for lunch? Peanut butter and jellyfish sandwiches, of course. While you could easily insert sea turtles, sharks, swordfish and tuna in that joke, up until recently, you would have left penguins out because no one thought they ate jellyfish. Turns out, we've been wrong.

As they mostly consist of water, jellyfish aren't exactly the most nutritious snack in the sea. Yet, they are part of the diet of several ocean animals, most notably the leatherback sea turtle and the ocean sunfish. Now, thanks to research reported by the Ecological Society of America (ESA), it seems that another diner pulls up to the plentiful jellyfish buffet in our seas: penguins.

Jean-Baptiste Thiebot – a postdoctoral fellow at the National Institute of Polar Research in Tokyo, Japan – and colleagues from five countries, placed cameras measuring about the size of a thumb drive onto 106 penguins in southern oceans. Their specimens included Adélie penguins, yellow-eyed penguins, Magellanic penguins, and little penguins. The camera was attached to each penguin for one dive only so as to disrupt the animals as little as possible.

In all, almost 200 penguin attacks on jellyfish and other gelatinous creatures (which the researchers are naming "gelata") were witnessed during more than 350 hours of filming.

While it may have been previously thought that penguins ate jellyfish accidentally while diving for other food, the videos show them actively attacking the blobs. The researchers say that the penguins seemed particularly drawn to the carnivorous species among the gelata including "true" jellyfish belonging to the Cnidaria phylum and "comb jellies," which belong to the phylum Ctenophora. All four species of birds were observed eating 187 species of true jellyfish, while Magellanic and little penguins also devoured 11 species of comb jellies.

Interestingly, although plentiful, none of the penguins chose to eat a vegetarian species of gelata known as salps.

"Vegetarian gelata species known as salps, filter feeders more closely related to humans than jellyfish they resemble, are also common gelatinous denizens of the southern oceans," says the ESA. "These little jet-propelled jelly tubes feed on phytoplankton, the floating, green, single-celled organisms that get energy from light, like plants. In the congregational phase of their lives, they form great chains, tubes, and wheels, sometimes 60 feet long. Although salps appeared in video, penguins did not pursue them."

That's a bit unfortunate as, when salp populations explode, as they do in the southern oceans, they can devour sea plankton. This has an impact on the entire food chain, as plankton serves as the food source for krill, which is turn eaten by whales, seals and other marine creatures.

Carnivorous gelata also have the tendency to bloom in large populations, so the penguins can help a little in taming such occurrences, even though the overall percentage of gelata in penguin diets is quite small: just one percent of the daily caloric needs for Adélie, Magellanic, and yellow-eyed penguins, and up to two percent for little penguins.

Still, the fact that the penguins are indeed consuming gelata may mean that researchers have underrated their role in the carbon cycle, says Thiebot.

The video below, while not exactly Hollywood quality, shows some penguin attacks on gelata.

The research has been published in the ESA journal Frontiers in Ecology and the Environment.

Source: Ecological Society of America