Four new genera, Eoatria, Poratria, Microporatria, and Multatria, are described, joining the previously described genus Bryozobia in the subfamily Bryozobiinae, a subfamily now represented by ten species of which four are new to science. Evidence is presented that Bryozobia is intimately associated with sponges, rather than bryozoans as Ross and Newman (1996) inferred from the intimate proximity of barnacle and bryozoan calcareous remnants. The four new genera are also associated with burrowing or encrusting sponges and generally possess secondary as well as primary radial atria between their basis and the substratum. The primary atria are aligned with portals in the suture between the wall plates and the basis whereas secondary atria can be parietal as well as interparietal. Eoatria has weakly developed, solid atria aligned with portals in the suture between the wall plates and the basis, but the atrial "ceilings" remain solid. The other genera have atria with pores or slits that develop from portals in the suture between the basis and the wall during ontogeny. All species of Bryozobiinae maintain contact with the substratum via calcareous outgrowths of the basis and/or wall, and their sponge symbiont grows from beneath and around them during ontogeny, nearly encasing them in sponge tissue. The basal atria enable the sponge to grow out from beneath the barnacle, and the evidence suggests chemical mediation by the barnacles induces the sponges to grow up around them, as it does in coral-inhabiting barnacles (Ross and Newman. 2000). This symbiosis apparently also provides the barnacles with a habitat relatively free of spatial competitors as well as protection from various predators, and allows the barnacles to thrive where other encrusting organisms cannot. While the porous, atriate bases found in some of the coral-inhabiting barnacles (Pyrgomatidae) are likely convergent, monophyly cannot presently be completely ruled out. The diagnosis of the sponge-inhabiting members of the subfamily Acastinae is amended to include only those species lacking atria that are completely embedded in host tissues: that is, having no basal contact with any other substratum. (C) 2010 Elsevier B.V. All rights reserved.
