The coral-eating barnacle Hoekia monticulariae (Gray, 1831), the only internal parasite among the Thoracica described to this day, is characterized by an irregularly-shaped shell nestled cryptically between the polyps of the hermatypic coral Hydnophora Fischer, 1807, which occurs throughout most of the Indo-West Pacific. Because of its protean form, cirripedologists have failed to appreciate the diversity of taxa related to Hoekia, a presumed monotypic genus. We describe seven new species divided between Hoekia and three new genera, Eohoekia, Parahoekia, and Ahoekia for which the Tribe Hoekiini is proposed. As in other pyrgomatids, calcareous overgrowth by the coral is inhibited around the edge of the wall and aperture. But in Hoekiini a pseudopolyp, upon which the barnacle feeds with modified trophi, covers the wall and aperture. Furthermore, rather than articulating with a calcareous basis, the wall is suspended in coral tissue. Its hypertrophied lateral margin ( = basal margin), in contact with the host’s tissue, is the site where metabolic activities are inferred to take place. In Hoekia and Ahoekia, the wall develops simple or connecting tubes that lead to openings in the margin, which serve as circulatory pathways. A hypertrophied margin and elaborated circulatory system suggests that the Hoekiini may not be wholly dependent on feeding directly on host tissue and/or coelenteronic material, but may also be absorptive parasites. Although other pyrgomatids, in the tribes Pyrgopsellini nov. and Pyrgomatini nov., exercise some control over their hosts by an apertural frill and through discontinuities between the shell and basis, they are still planktotrophic.
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 ofthe 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 avenuefor chemical mediationof 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).