They are up to 110 mm in diameter and are not easily confused with any other species. A feature unique to the giant barnacle is the lack of longitudinal striation on the scuta. They also contain the largest individual muscle fibres known to science.
The video on the left is of a juvenile barnacle feeding. It was taken by Sylvia Roach through a microscope camera in the marine lab at Pearson College. The size of the barnacle can be estimated by the small skeleton shrimp which is active in the background. That amphipod is less than 1 cm in length.
Research on the associations of hydroids that live on them has been done at Race Rocks by Dr. Anita Brinckmann-Voss. In her publication on the new species Rhysia fletcheri she has included a photograph of hydroids on the valves of this animal. There has also been an extended essay done on this association by a student from the college. They are often encrusted with the holdfasts of kelp. Occasionally a storm tearing at the kelp will uproot a mass of barnacles and they may end up on a beach in the masses of tangled kelp. One of the reasons we have a ban on anchoring in the reserve is that these barnacles are easily broken away from their locations on rock outcrops by a dragging anchor.
In our research on fouling and colonization of objects deployed in the Tidal Energy Project, this image of a single juvenile barnacle "up-rooting" a special anti-fouling plastic coating was good evidence that these animals can be very persistent and in nature this ability can be a very successful adaptation for competing interspecifically with other organisms and intraspecifically with their own species as well.