Hydroids are colonial animals, growing in communities to share resources, primarily food. Most hydroids are carnivorous, catching prey in the water column with the aid of stinging nematocysts (nee-mat-o-sist). Nudibranchs (nud-i-brank) prey on most families of hydroids, and in most cases each nudibranch species prefers one or two species of hydroid.

The image above features Obelia spp. a common species of hydroid. They look like leafless branches, in this image, growing attached to a clump of barnacles, sponge, and a mussel shell. Click on the picture to get a closer look at Obelia.