How do you make a jellyfish wear an activity tracker?

Spying on a jellyfish isn’t an easy task. Jellyfish are hard to see, fragile, and, worst of all, they’re gooey. Those traits make it tough to stick a tracking tag on them. But thanks to glue, suction cups, and cable ties, scientists are finally able to watch what jellies are up to beneath the waves.

Jellyfish are an important food source for hundreds of fish and endangered sea turtles. But they can also be a nuisance when their populations balloon and the stinging creatures wash up on beaches. The problem is, scientists know very little about these mysterious blobs of goo, like where they like to live, what they do, and what makes their populations burst into these destructive blooms.

The best way to watch an underwater creature without having to hold your breath is to stick some kind of tracking tag on it, which can be tricky to do. These tags act like souped-up Fitbits that track an animal’s route, record its activity, and monitor water conditions around it. On a shark, a tag can wrap snuggly around a fin. And on a seal, it can sit sturdily on its head. But how do you tag a jellyfish?

It turns out, there’s more to a jellyfish’s body than just, “jelly and mucus,” says Monterey Bay Aquarium Research Institute scientist Kakani Katija in a blog post published yesterday. “It actually has some heft and structure.”

That means it’s possible to stick a tag to the top of a jellyfish’s bell with surgical glue. These can last for up to a month. For shorter deployments, a suction cup can do the trick for a couple hours, according to the definitive guide to jellyfish tagging published last year by scientists from Stanford and the Monterey Bay Aquarium Research Institute.

For long-limbed species like the barrel jellyfish, it’s actually possible to wrap a tag around the stalk between the jellyfish’s bell and its arms like a bracelet. Depending on the species, these tags can last hours to days. The main advantage with this method is that the…

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