On the Conspecificity of Aglaothamnion tenuissimum and A. byssoides (Ceramiaceae, Rhodophyta)

Crosses between Mediterranean strains of Aglaothamnion tenuissimum and Atlantic strains of both A. byssoides and A. pseudobyssoides support the conclusion that A. tenuissimum is conspecific with A. byssoides but that A. pseudobyssoides is a distinct species. Ceramium tenuissimum (the basionym of A. tenuissimum) antedates Callithamnion byssoides (the basionym of A. byssoides), so that the correct name of this species is A. tenuissimum. Because diagnostic features are lacking in the type specimen of C. tenuissimum, an epitype for that name is chosen. Moreover, A. tenuissimum var. mazoyerae, var. nov., is proposed for Mediterranean populations of this species that grow in habitats of low light intensity and differ morphologically from the nominal variety in the shape and size of the tetrasporangia, in the size and shape of axial cells in the basal region of the thallus, and in the absence of rhizoidal cortication.