Look closely and you may see these tiny jellies boiling at the surface, or photo-bombing well planned underwater shots.  They are indeed gregarious — in a prolific kind of way (especially in Spring and Summer). It is not uncommon to see them caught in the stomachs or tentacles of other jellies and this is well-illustrated in the photo of the Hanging Stomach Jelly with its prey.  Often these appear to be clear and transparent with whitish radial canals, but a close-up will often reveal reddish pigment spots around their bell margin.  For a complete description and more photos see inaturalist