Jellyfish are doing weird stuff under the Arctic ice

For several years, a team of researchers from the United States and Poland has been in the habit of peeking beneath the thick ice that covers the waters of the Chukchi Sea between Alaska and Siberia. And here's one of the creatures that grabbed their attention:

That's Chrysaora melanaster – a species of jellyfish also known as the northern sea nettle – gliding lazily across the sea floor and dragging its tentacles behind it. At first glance, it's just a jelly, but for the researchers, the sighting was a surprise. In fact, Andy Juhl of Columbia University's Lamont-Doherty Earth Observatory described the experience very succinctly: "[W]e saw them, and it was kind of weird."

For one thing, adult jellyfish are usually in the habit of floating along in the water column. The researchers who studied these floor-crawling jellyfish suspect the animals may have been grabbing food off the sand with their long tentacles, a fairly rare behaviour for these jellies.

Even more surprising? The scientists' collection of videos reveals that the jellyfish are surviving under the ice a lot longer than expected.

Jellyfish start life as little swimming larvae who find a place to settle on the ocean floor and grow into a polyp sticking out of the ground. When the polyp is ready, it buds off new swimming larvae, which grow into that characteristic floating adult jellyfish, called a medusa. Classically, the adults are understood to emerge in the spring and live through autumn, before giving birth to new larvae and dying.

The jellyfish under the Chukchi Sea ice, however, seem to be surviving for more than a few months: they're spending the whole winter hunkering down below. This isn't just an interesting observation of jellyfish longevity – it may be an important bit of information regarding mysterious jellyfish swarms.

"Jellyfish and ctenophore [comb jelly] blooms are of increasing concern for human enterprise in marine waters," the researchers say in their study, "although bloom development remains poorly understood."

For fisheries in the nearby Bering Sea, those jelly swarms can occasionally cause a bit of a jam: in some years, local jellyfish become so numerous that they clog fishing nets. Why this happens – and how to predict these blooms – is something marine biologists have been trying to work out.

The Chukchi Sea researchers suggest the jellyfish can survive through the winter when prey is plentiful and the sea ice thick enough to protect them from storm winds, while keeping the water temperature suitably chilly. The cold means jellyfish metabolisms slow down, allowing the animals to get by with less food.

A couple of such ice-heavy winters in a row might result in an overabundance of adult jellyfish, old and new. And this phenomenon might even prove to be more widespread than just this particular species.

In a world of shifting climates, understanding the connection between winter weather and jellyfish jamborees will be essential for maintaining fisheries. And if that means there's a need to collect more videos of jellyfish bobbing beautifully along the ocean bottom, well, that's a win-win.

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Top header image: Sebastian Niedlich/Flickr