Studies on Halimeda 2. Reproduction, particularly the seasonality of gametangia formation, in a number of species from the Great Barrier Reef province

Seventeen species of Halimeda have been grown in the laboratory in an open-circuit seawater cascade during a 6-year-period. All but one grew vegetatively (generated new segments) to some extent and several species grew luxuriantly. A few species also reproduced vegetatively by developing new plants at the ends of fine runners. However, most significant was the production of gametangia by all the species in the cascade whilst several of those species, plus one not grown in the cascade, were also observed fertile in the field. Different material from a single collection sometimes became fertile in the cascade in successive years and then often at very similar times, suggesting a distinct seasonality and response to environmental stimuli. Some lunar periodicity was also detected in the fertility of three species. Species-specific differences in the structure of gametangial clusters, particularly the size of the gametangia, were observed. In all species examined, gametophore branches ended in unexpanded tips which often had a distinct, thickened cap.