The Secret Social Lives of Jellyfish

Do helmet jellyfish play follow-the-leader?  

Published July 13, 2015

Jellyfish are not very smart. “They have very simple sensory organs, and no brain to process any information,” says marine biologist Stein Kaartvedt. Which is why it’s so interesting that, off the coast of Norway, scientists have observed what they’re calling “apparent social behavior” in the helmet jellyfish.

Though jellyfish are often found in dense, stinging hordes, they’re not typically thought of as social animals. These blooms, or “smacks,” actually have more to do with converging water currents than any sort of intentional schooling behavior.

Yet by carefully tracking helmet jellies’ movements using sonar measurements, Kaartvedt and his colleagues watched as individual jellyfish somehow took note of other jellyfish swimming directly above them, and then abruptly changed their rate of ascent to sync up with their squishy comrades. The research raises many questions about how jellyfish perceive and interact with the world around them. Even without brains, says Kaartvedt, jellyfish “can do surprising things.”

During the day, helmet jellies hide in the deep, avoiding the sunlight. But each night, they propel themselves up from the ocean’s depths to hunt krill near the surface. While most helmet jellies perform this climb alone, Kaartvedt’s data showed that around ten percent seem to team up in groups of two to three.

As the scientists’ sonar data revealed, helmet jellyfish are capable of recognizing potential swim buddies from a distance of two meters. Yet how they do so, says Kaartvedt, is a mystery.

When helmet jellyfish swim, their movements only disturb the water within a few centimeters. This makes it unlikely that one is simply “feeling” the swimmers around it. Likewise, because the tagalongs didn’t follow a direct path (like some plankton do when tracking chemical signals left behind by their kin), the senses of “taste” or “smell” seem equally unlikely causes for the buddy system.

These jellies lack anything close to what we’d call eyes, but they do have receptors that let them detect, and avoid, sunlight. So maybe “sight” has something to do with the behavior?

“In deep water, about 90 percent of all animal types produce their own light, and [the helmet jellyfish] is very bioluminescent,” says Kaartvedt.

In fact, he says, they’ve done experiments where they put a single helmet jellyfish in a bucket in a dark room and then knocked on the container. The animal, thinking it was being attacked, produced so much light that the people in the room were able to see each other in the darkness. (Jellyfish researchers get into some weird stuff.)

Still, Kaartvedt can’t say for sure whether the jellyfish are using this bioluminescence to follow each other. And as to why the typically solitary animals would want to hang out with each other? That, too, is a mystery.

Reproduction could explain the social behavior, he says, but there may be an alternative motivation: cooperative hunting. The researchers developed a “probabilistic model of cooperation” to see if a few jellyfish working together could catch more krill than a “lone wolf,” and the math seems to support the idea.

So does this mean the oceans are teaming with bioluminescent, pack-hunting cnidarians? Hold your helmet jellyfish. As Kaartvedt says, so far, this is mostly speculation.