Hydrolithon (Foslie) Foslie

Maneveldt, G. W., Merwe, E. Van Der & Keats, D. W., 2015, Taxonomic review of Hydrolithon samoënse (Corallinaceae, Corallinales, Rhodophyta) and other taxa found to be conspecific, Phytotaxa 192 (4), pp. 230-253 : 231

Hydrolithon (Foslie) Foslie

Foslie (1909: 55) originally established the genus Hydrolithon to encompass those taxa that possessed a dimerous thallus construction in which basal cells were slightly longer than they were wide, and uniporate tetrasporangial conceptacles that were like those of Goniolithon Foslie (i.e. notably domed) in which a central columella was absent. Foslie (1909) ascribed four species to Hydrolithon but did not designate the type specimen. It was not until much later that Mason (1953: 333) designated the lectotype specimen, Hydrolithon reinboldii (Weber van Bosse & Foslie) Foslie.

The concept of Hydrolithon has undergone substantial change since Foslie’s original description (see Penrose & Woelkerling 1988 for a review of these changes). Woelkerling (1985) and Penrose & Woelkerling (1988) concluded that the characters used to delineate the type of Hydrolithon were not reliable, as the types of the genera Porolithon Foslie and Spongites Kützing also possessed these characters. Penrose & Woelkerling (1988) then subsumed both Hydrolithon and Porolithon in Spongites (the oldest available name for the complex), but did note that Spongites (sensu lato) encompassed a broad range of taxa. During their course of study on the southern Australian taxa of Spongites (sensu lato), Penrose & Woelkerling (1992) demonstrated that two distinct patterns of tetrasporangial conceptacle development occurred and they concluded that Hydrolithon and Spongites were distinct genera.

In Hydrolithon , the tetrasporangial conceptacle roof developed both from filaments peripheral to the fertile area and from filaments interspersed among the developing tetrasporangia. Here the pore canals of the tetrasporangial conceptacles were lined by a ring of conspicuous, enlarged cells that arose from filaments interspersed among the tetrasporangial initials. These cells did not protrude into the pore canal and were oriented more-or-less perpendicularly (vertically orientated) to the roof surface. Based on this new evidence, however, Penrose & Woelkerling (1992) still considered Hydrolithon and Porolithon to be congeneric, with Hydrolithon having nomenclatural priority.

Recently, two independent studies ( Bittner et al. 2011, Kato et al. 2011) demonstrated, using molecular analyses, that Hydrolithon and Porolithon are indeed separate taxa. Kato et al. (2011) demonstrated that the former subfamily Mastophoroideae needed to be split and they proposed, among others, the erection of the subfamily Hydrolithoideae A.Kato & M.Baba with Hydrolithon (Foslie) Foslie emend. A.Kato & M.Baba as the type genus and Hydrolithon reinboldii (Weber van Bosse & Foslie) Foslie as the type species to the genus. Based on the Kato et al. (2011: 669) assessment, the genus Hydrolithon can be separated from the genus Porolithon by the trichocyte arrangement. In Hydrolithon the trichocytes do not occur in large tightly packed horizontal fields.