A many-ribbed hydromedusae, Aequorea aequorea, photographed off the coast of New Jersey, USA. The "many-ribbed" genus of hydromedusae are similar in appearance to clear jellyfishes, but lack obvious tentacles and do not sting when touched. These are among the largest hydromedusae known, up to 12" across, and the identity of their corresponding polyp stage is still a mystery. Hydromedusae are mostly water, and when they dry up they lose half of their diameter and almost all of their thickness, becoming sand dollar-sized thin, brittle discs.