Since the early 1990s, we humans have been doing something both odd and eminently sensible: We've been launching jellyfish into space. And we have been doing so for science. During NASA's first Spacelab Life Sciences (SLS-1) mission in 1991, NASA began conducting an experiment: "The Effects of Microgravity-Induced Weightlessness on Aurelia Ephyra Differentiation and Statolith Synthesis." To carry it out, the space shuttle Columbia launched into space a payload of 2,478 jellyfish polyps—creatures contained within flasks and bags that were filled with artificial seawater. Astronauts injected chemicals into those bags that would induce the polyps to swim freely (and, ultimately, reproduce). Over the course of the mission, the creatures proliferated: By mission's close, there were some 60,000 jellies orbiting Earth.
The point of all this, as the experiment's title (sort of) suggests, was to test microgravity's effects on jellyfish as they develop from polyp to medusa. And the point of that, in turn, was to test how the jellyfish would respond when they were back on Earth. Jellyfish, foreign to us in so many ways, are like humans in one very particular manner: They orient themselves according to gravity.
When a jelly grows, it forms calcium sulfate crystals at the margin of its bell. These crystals are surrounded by a little cell pocket, coated in specialized hairs, and these pockets are equally spaced around the bell. When jellies turn, the crystals roll down with gravity to the bottom of the pocket, moving the cell hairs, which in turn send signals to neurons. In this way, jellies are able to sense up and down. All they need is gravity.
Humans, of course, are similarly sensitive. We sense both gravity and and acceleration using otoliths, calcium crystals in our inner ears that move ultra-sensitive hair cells, thus informing our brains which way gravity is pulling us. So if the space-raised jellyfish didn't fully develop their version of gravity-sensors, the thinking goes, it's likely that humans raised in microgravity would have similar trouble.
And here, according to Deep Sea News, is the result of the studies: The astro-jellies' sense of gravity did, indeed, seem to be impaired by being raised in space. The results of the STS-1 experiment, published in the journal Advances in Space Research, noted that while the space-bred jellies were "morphologically very similar to those which developed on Earth," their motor abilities were different on Earth than they were in microgravity. In a kind of lit review of the jelly experiments, Helm notes that "while development of the sensory pockets appears normal, many more jellies had trouble getting around once on the planet." The difficulties included, alas, "pulsing and movement abnormalities, compared to their Earth-bound counterparts."
Basically, the invertebrates had vertigo. (Or, as PopSci puts it: "As cool as being an astronaut baby sounds, the jellies didn't develop the same gravity-sensing capabilities as their Earthly relatives.") Which may not bode well for the vertebrate organisms that may be born in microgravity—space-faring humans among them.
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