Cypris settlement and metamorphosis into dwarf males were studied in the androdioecious barnacle Scalpellum scalpellum using field collected samples from the North Sea, and experiments with laboratory reared larvae, observed with video. In the field sample, dwarf males were always situated on the rim of the mantle aperture and almost invariably confined to two areas (receptacles) located along the scutal plates near their contact to the terga. In the laboratory experiments, cyprids settling on the mantle rim always developed into males. Those settling elsewhere, whether on the external surfaces of the adults, or on their hydroid substratum, always developed into hermaphrodites. The numbers settling as males did not differ significantly from those settling as hermaphrodites, suggesting that genetic sex determination may operate in S. scalpellum. The N. Sea sample comprised 52 adult hermaphrodites. Of these 15 (29%) lacked males altogether, while 37 (71%) carried males with an average of 4.7 per hermaphrodite. On the hermaphrodite, a thin lamella along the mantle rim protects the settling and metamorphosing male from accidental damage or dislocation by the beating cirri. The cyprid gains additional protection by starting almost immediately after settlement to penetrate into the receptacle tissue. After 12–24 h the developing dwarf male can be almost completely buried. It appears that the cyprid does not use any preformed burrow, but actively descends into the hermaphrodite integument.
Journal of Experimental Marine Biology and Ecology, 2012, Vol 422–423, p. 39-47