This genus includes 14 marine species and 5 freshwater species (2). There are at least two marine species found in the Pacific Northwest. Most abundant and easiest to find is H. rubra (Sommerfelt) Meneghini, which is now considered synonymous with H. prototypus Nardo (the first species of Hildenbrandia to be described, back in 1834). The other common species is H. occidentalis Setchell. It is difficult to distinguish between these two species in the field, but H. rubra has irregularly divided tetrasporangia, while H. occidnetalis has zonately divided tetrasporangia (3). H. rubra tends to be thinner and more red, while H. occidentalis tends to be thicker and darker (and can be confused with the 'Petrocelis' phase of Mastocarpus papillatus).
Sherwood & Sheath (2003) report that H. rubra is not monophyletic. The crustose morphology of Hildenbrandia is so simple that variation is hard to document. As such, this taxonomic confusion does not come as a complete surprise. Perhaps the reported distribution of H. rubra is so wide (see Habitat) because the name includes a handful of different species.
There is a rumor that the diverse group of fungi, Phylum Ascomycota, traces its origins to florideophyceaen algae (5). Indeed, roughly half of all ascomycetes (approx. 125,00 species) are lichenized (i.e., associated with an alga) (6). This evolutionary rumor stems from early microscopic investigations of the conceptacles of Hildenbrandia spp., where ambiguous structures were identified as fungal filaments (see Life History for more details about this confusion) (7).