Size up to 3 inches in diameter. Shell crown-shaped or cask-shaped, narrower at the top than at the base and with a hexagonal opening. Alae corrugated. Three triangular openings produced at each corner of the major opening where the surface of the ribs has been eroded. Cavities of the shell invaded by skin of whale as an anchor. Ribs convex, crossed by narrow, minutely beaded rugae. Radii thin, space behind them filled with branches of the ovaries. Body-chamber cup-shaped. The scuta are small and are placed near the rostral end of the orifice. The terga, when present, are represented by a thin plate of shell barely visible to the naked eye. The opercular opening is covered by a strong membrane with a central hood through which the cirri are thrust. Branchiae in two folds almost completely surround the body.
Colour of shell, white; opercular membrane, light yellow, occasionally with a purplish tinge; cirri and mouth parts, pale yellow. Shell and opercular membrane usually discoloured by a brownish growth of the diatom Licmophora onassis. (The species was recently described by Hustedt, from Antarctic specimens. It was identified from barnacles of British Columbia humpbacks by Dr. Ruth Patrick, Curator of Limnology, Acad. Nat. Sci. Philad.)
World-wide on humpback whale, Megaptera nodosa(Bonnaterre). C. diadema occurs mostly on the summits of the tubercles and protuberances of the head and flippers and on the knob at the tip of the lower jaw. It occurs also on the ventral grooves and in the region of the genital and anal apertures. It occurs in scattered numbers over the sides, rarely on the dorsal surface.
Coronula diadema is the most common whale barnacle but is apparently confined to the one species of whale. It is stated by whaling-station operators that nearly a ton of these barnacles may be attached to one large whale. Their presence must be a source of irritation to the whale as these mammals have been seen engaged in rubbing themselves against rocks apparently in an effort to remove the barnacles. In C. diademathe branchiae are the largest of any known barnacle. It is interesting to note that in all whale barnacles, and in parasites of whales generally, these respiratory organs are comparatively very large - a possible adaptation for living in warm water where less oxygen is available while the whale is migrating through the tropics. Most C. diadema probably have a life-cycle of less than one year. They are mostly small in the early summer months, but are considerably larger and many have dropped off or have been rubbed off by the end of the summer.