This story appears in the October 2018 issue of National Geographic magazine. Moon jellies, which are found in shallow bays around the world, look like small, not entirely friendly ghosts.

This jellyfish species has a clear body, can glow green and has an almost unnoticeable sting, Jace Tunnell writes.

The newly characterized “elegant jellyfish,” roughly the size of a human hand, had been on display in two aquariums in Japan for more than a decade. Lisa Winter became social media editor for The ...