The reproductive development of the red alga Actinotrichia fragilis (Galaxauraceae, Nemaliales)

The reproductive structures in Actinotrichia fragilis (Forsskal) Borgesen a re described, including the first report of the structure and development o f the carpogonial branch and cystocarp for the genus. Tetrasporangia usuall y arise in a terminal position on assimilatory filaments; occasionally they are formed laterally and in a secund manner. In male thalli, conceptacles are initiated from the apical pit of a thallus branch. Filaments bearing sp ermatangia issue from the inner face of the conceptacle wall. In female tha lli, a carpogonial branch consists of a carpogonium, hypogynous cell and ba sal cell. Carpogonial branches may arise in two ways, either replacing a ve getative branch or rarely as a third branch between vegetative dichotomous branches. Before fertilization, the hypogynous cell divides to produce four cells. Subsequently, the basal cell produces branched filaments that event ually form the pericarp, which envelops the carpogonial branch and developi ng gonimoblast. After fertilization, the carpogonium divides transversely i nto two cells. The upper progeny cell retains its cytoplasmic connection to the trichogyne. The lower cell produces a lateral gonimoblast initial. Pri mary gonimoblast filaments form a compact pseudoparenchymatous mass at the base of the carposporophyte. Carposporangia are produced singly from the ti ps, of the secondary gonimoblast filaments, which develop from the primary gonimoblast mass. Many simple or little-branched sterile paraphyses issue f rom the inner face of the pericarp and grow into the cavity. A comparison o f the reproductive structures of Actinotrichia with those of the other gene ra of the Galaxauraceae suggests that the genus is closely related to Tricl eocarpa Huisman et Borowitzka and Galaxaura Lamouroux.