This colony of Rosacea may look like a single jellyfish, but it is actually a large group of smaller siphonophores clustered and living together. In fact, the zooids (individual siphonophores living in the colony) cannot survive on their own. Siphonophores are hydrozoans, a group of jellyfish look-alikes that are closely related to jellies.
A Rosacea colony’s long tentacles extend a meter away from the main body and contract when disturbed by potential food items. The bead-like dots are stinging cells (nematocysts) that immobilize and kill their prey.
Photo: L. Madin, Woods Hole Oceanographic Institution (www.cmarz.org)