Nicholas Sorensen looks at a many-ribbed hydromedusae, Aequorea aequorea, washed up on a Rhode Island, USA, beach. In the "many-ribbed" genus, similar in appearance to clear jellyfishes but lacking obvious tentacles and not stinging when picked up, these are among the largest hydromedusae known, and their exact origin is still a mystery. When they dry up they lose half of their diameter and almost all of their thickness, becoming sand dollar-sized thin, brittle discs.