Pyrgoma kuri Hoek, 1913: a case study in morphology and systematics of a symbiotic coral barnacle (Cirripedia: Balanomorpha)

The systematics of pyrgomatids, stemming from the early 1800’s, has traditionally been based on the number of plates making up the wall (six, four or one) and specializations in the opercular plates. A recent study of the related bryozobiines focused attention on detailed structural modifications of the basis, which we now find also applies to some highly derived pyrgomatids and an archaeobalanine. Reexamination of the Indonesian coral barnacle Pyrgoma kuri Hoek, 1913 has revealed previously unknown morphological features, including separable opercular plates, a true tergal spur, and a basis lined with ladder to arch-like calcareous structures covering “atrial passageways”. Thus, the present study expands our knowledge of such specializations and our understanding of the evolution and relationships of the derived pyrgomatids as well as the archaeobalanines and bryozobiines. The complex modifications of the basis found in these three groups evidently function as an avenue for chemical mediation of growth between the barnacle and its host. Although monophyly of the archaeobalanines and bryozobiines based on such structures is possible, there is no obvious connection between them and the few derived pyrgomatids in which these features occur. There is apparently a propensity toward such modifications in archaeobalanids resulting in parallel evolution in association with distinctly different hosts (e.g., sponges, corals and bryozoans).