The genus Metagoniolithon Weber-van Bosse (Corallinaceae, Rhodophyta)

Abstract

The characters of the genus Metagoniolithon Weber-van Bosse, the single genus within the subfamily Metagoniolithoideae Johansen, are defined. The thalli are differentiated into basal crusts, genicula and intergenicula, with all the tissues displaying cell fusions but lacking secondary pit connections. Initial apical branching is dichotomous, but later the meristematic genicula produce false whorls of branches. The branches lack apical cover cells but have mucilaginous caps. The conceptacles, with their overlying mucilaginous caps, are initiated in the cortical tissue of the intergenicula. The plants are dioecious. In the cystocarpic conceptacle the fusion cell is continuous and the carpospore formation is restricted to the periphery of the fusion cell. The spermatangia develop only on the floor of the conceptacle. The uniporate roof of the tetrasporangial conceptacle is formed by the growth of filaments within the ring of developing marginal tetrasporangia. Spore germination is of the Amphiroa type.

The three species are endemic and restricted to western and southern Australia: M. radiatum (Lamarck) comb. nov. is epilithic; M. stelliferum (Lamarck) Weber-van Bosse and M. chara (Lamarck) comb. nov. var, chara and var. dichotomum var, nov. are epiphytic, commonly on the seagrass Amphibolis C. Agardh. The basal crust of the epiphytic species is epiphytic on species of Hetevoderma Foslie growing on Amphibolis. The characters of the genus Metagoniolithon are compared with those of Amphiroa Lamouroux, and a key to the articulated coralline algae of southern Australia is given.