Alternative pathways in the life history of Gracilaria gracilis (Gracilariales, Rhodophyta) from north-eastern Sicily (Italy)

The reproductive phenology of Gracilaria gracilis has been followed closely in a brackish water lake in Sicily. The life history of this red alga deviated from the typical Polysiphonia-type cycle and was characterized by the predominance of the tetrasporophytic phase over gametophytes, in situ development of germinated gametophytes and the presence of sterile pleustophytic thalli. Release of spherical agglomerates of cells produced by young tetrasporophytes was observed as well. Their ability to germinate forming new thalli appears to represent an alternative reproductive strategy of this organism. The population structure appears to reflect a predominance of the diploid phase except during the warmer summer season when only vegetative juveniles were found in the lake. Release and early stages of development of carpospores and tetraspores were critically observed in culture. A Dumontia type pattern of germination was observed for both types of spores, showing an erect thallus arising from a multicellular basal disk. Compared to tetraspores, carpospores are less readily released resulting in a subsequent delay in the development of new gametophytes.