An assessment of the taxonomic status of the Mediterranean endemic genus Acrodiscus Zanardini (Halymeniales, Rhodophyta)

Acrodiscus Zanardini is a poorly known monotypic endemic Mediterranean genus based on A. vidovichii (Menegh.) Zanardini. Rarely reported, its reproductive structures have remained undocumented, leaving its exact taxonomic position uncertain. Solely on the basis of its vegetative structure, Zanardini provisionally placed it in the family Cryptonemiaceae of the order Cryptonemiales (currently the Halymeniaceae of the Halymeniales), although he was uncertain as to whether the new genus actually belonged to that family or should instead be included in the Gigartinaceae of the Gigartinales (where Meneghini had originally placed it). In the present study we have extensively sampled A. vidovichii and documented its vegetative and tetrasporangial features. As well, we provide molecular-sequence data (COI-5P, rbcL, LSU) that indicate its phylogenetic affinities. We confirm Acrodiscus as a member of the Halymeniaceae and its status as an independent genus. Searches of several institutional herbaria have allowed us to locate and lectotypify Meneghini’s Chondrus? vidovichii by the discovery of his original material now held at the Herbarium Horti Pisani (Pisa, Italy).