The morphology of Ostiophyllum sonderopeltae gen. et sp. nov. (Gigartinaceae, Rhodophyta) from south-eastern Australia

Abstract

Ostiophyllum sonderopeltae Kraft, gen. et sp. nov. (Gigartinaceae, Gigartinales) is an obligate epiphyte of Sonderopelta coriacea (Peyssonneliaceae, Gigartinales) in dimly illuminated, rough-water habitats at depths of 3–20 m in south-eastern Victoria and Tasmania. Plants consist of ovate to lobed, flattened blades < 3 cm in length and breadth that are shallowly anchored in the dorsal surface of the host. Distinguishing features of the genus are the ostiolate tetrasporangial sori that are invariably scattered across the broad faces of the fronds, and restriction of cystocarps to the blade edges, where they communicate with the surface via submarginal ostioles. Tetrasporangial development is unique in the family and begins when roughly circular patches of cortical filaments fail to keep pace with growth of the surrounding cortex, resulting in the formation of a cylindrical pit lined by the apical cells of cortical filaments. Filaments at the base of the pit, which now face towards the interior of the frond, then commence rapid apical growth and produce branched, inwardly directed tetrasporoblastic chains, each cell of which becomes a sporangial primordium. The chains aggregate densely within the lax medulla, forming a compact sorus of cruciate and decussate-cruciate tetrasporangia. Female gametophytes differ from tetrasporophytes in being less ovoid in outline, often broader than long, and usually more lobed. Procarps are typical for the family, and diploidized auxiliary cells issue filiform lateral and inwardly growing gonimoblasts that penetrate a lax weft of primary and secondary medullary filaments. Gonimoblasts then establish secondary pit-connections to the gametophytic cells and form consolidated pockets of spherical carposporangia between narrow islands of sterile cells and within an initially narrow hull of encircling filaments, there being no consolidated ‘Faserhülle’ around the mature cystocarp. Published studies of rbcL gene sequences by Hommersand and his colleagues [M.H. Hommersand, S. Fredericq, D.W. Freshwater & J. Hughey (l999) Phycological Research 47: 139–151] suggest that Ostiophyllum occupies an isolated and basal position within the Gigartinaceae, a conclusion reinforced by the anatomical features now reported.