What in the name of Neptune's beard is that thing? A ghost? An alien? The ghost of an alien?

Such were the questions that vexed a team of deep-sea scientists aboard the Nautilus research vessel earlier this month, when their underwater recon robot encountered a limp, limbless creature hovering like a ghostly lantern over the Pacific seafloor. As the team watched, the bell-shaped blob suddenly transformed, ballooning into a long, translucent windsock with a mysterious red splotch stuck to its innards.

The blob, the researchers revealed in a recent video of the encounter, was no alien (it's never aliens), but one of the rarest-seen and least-studied jellyfish in the sea.

It's called Deepstaria (named for the research vessel that first discovered the genus in the 1960s), and has been seen only a dozen or so times in the last half-century. Researchers don't know much about the armless, shapeshifting sack, but they do know it has a habit of expanding its body to engulf any prey trusting enough to swim nearby.

This armless, shapeshifting sack is a jelly called Deepstaria. (Image credit: Nautilus (nautiluslive.org))That could explain the red splotch inside the jelly's belly. When the researchers zoomed in on the shapeshifting jelly, they saw that the red hanger-on was a tiny, still-living isopod — a type of bottom-feeding crustacean — that may have willingly swum into the jelly's open body for protection from fiercer, less-blobby predators. Such "resident isopods," as the researchers called them, have been observed clinging onto other Deepstaria specimens too, though it’s not clear whether they share a symbiotic relationship.

Little, in general, is known about Deepstaria jellies or their isopod consorts, as so few specimens have been studied. The Nautilus team found this deep-sea duo some 2,500 feet (750 meters) underwater in the Central Pacific, about halfway between the continental United States and Australia. Perhaps they’ll find more Deepstaria — or something even weirder — as their adventures through the deep darkness continue through October.

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Originally published on Live Science.