The morphology of Radiathamnion speleotis gen. et sp. nov., representing a new tribe in the Ceramiaceae (Rhodophyta) from southern Australia

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Abstract
The southern Australian red alga RADIATHAMNION speleotis gen. et sp. nov. is a tiny (< 3 mm in length) epiphyte restricted to the apical depressions of a few Laurencia species. Plants are anchored to the host by a single embedded basal cell and consist of a short arching main axis unilaterally invested with primary branches that arise singly on most of the axial cells, resulting in a somewhat spoke-like appearance for which the genus is named. The main axis always terminates in a friable hair, as do most of the branch orders (particularly in calm water habitats). Thalli exhibit a specialized sympodial growth pattern in which successive orders of lateral branches develop as each preceding higher order ceases apical growth with the transformation of its distal cells into elements of hair filaments. The genus is unique among Ceramiaceae in its host restrictedness, branching pattern and method of attachment. Procarps are borne on the basal cells of short lateral branchlets, and evidence is given that fertilization probably results in the passing of the zygote nucleus through a short tube from the carpogonium directly to the supporting cell of the carpogonial branch. A distinctive fusion cell is initiated by the union of the first postfertilization cell cut off from the supporting cell with the fertile axial cell on which the supporting cell is borne. As this combination of vegetative and reproductive features is unknown in the Ceramiaceae, the tribe RADIATHAMNIEAE is erected for the new species. Relationships of the new tribe appear closest to the Antithamnieae, Heterothamnieae and Dohrnielleae because of reproductive similarities.