Juvenile Ontogeny and Metamorphosis in the Most Primitive Living Sessile Barnacle, Neoverruca, from Abyssal Hydrothermal Springs

Neoverruca brachylepadoformis Newman recently described from abyssal hydrothermal springs at 3600 m in the Mariana Trough, has the basic organization of the most primitive sessile barnacles, the extinct Brachylepadomorpha (Jurassic-Miocene). However, a subtle asymmetry diagnostic of the Verrucomorpha (Cretaceous-Recent) is superimposed on this plan, and it is evident that Neoverruca also represents a very primitive verrucomorphan. A median latus, unpredicted in such a form, occurs on one side as part of the operculum, and the outermost whorl of basal imbricating plates is the oldest, rather than the youngest as in the primitive balanomorphans, Catophragmus s.l. and Chionelasmus and as inferred in Brachylepas. Neoverruca is further distinguished from higher sessile barnacles in passing through a number of well developed pedunculate stages before undergoing an abrupt metamorphosis into the sessile mode. Theses unpredicted ontogenetic events in the life history of an early sessile barnacle indicate that the transitory pedunculate stage of higher sessile barnacles, first noted in Semibalanus balanoides by Darwin, reflects the compression of pedunculate juvenile stages into a single stage, rather than simply a vestigial reminiscence of their pedunculate ancestry. From these observations it is evident that the transition from a pedunculate to a sessile way of life was evolutionarily more complicated than previously understood, and this has a significant bearing on our understanding of the paleoecology as well as the evolution of sessile barnacles.