Hydrolithon braganum sp. nov. (Corallinaceae, Rhodophyta), the first known exclusively fossil semi-endophytic coralline red alga

Hydrolithon braganum sp. nov., the first known exclusively fossil semi-endophytic species of Corallinaceae (Corallinales, Rhodophyta), is based on material from Integrated Ocean Drilling Program Expedition 310 cores of last deglacial age [c. 20,000-10,000 years before present] from reef terraces around Tahiti, French Polynesia. A detailed morphological- anatomical account is provided and morphological-anatomical similarities to other species are discussed. The taxonomic position of H. braganum is problematic in relation to recent proposals for changes to the circumscription of Hydrolithon. Evidence is presented to show that the morphological-anatomical criteria proposed in recent molecularbased studies are untenable for circumscribing genera called Hydrolithon and Porolithon. Thus, pending further research, the two genera are best treated as the Hydrolithon-Porolithon complex with Hydrolithon having nomenclatural priority, being polyphyletic, and encompassing one or several cryptic genera distinguishable on presently known molecular criteria but not distinguishable using presently known morphological-anatomical criteria.