Closeup of the mouth of a many-ribbed hydromedusae, Aequorea aequorea, off the coast of New Jersey, USA. 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.