New life-cycle stages of Gymnodinium catenatum (Dinophyceae): laboratory and field observations

Abstract

The chain-forming dinoflagellate Gymnodinium catenatum is responsible for outbreaks of paralytic shellfish poisoning (PSP); however, the relative importance of benthic-planktonic lifecycle transitions in the appearance of blooms of this species needs to be clarified. By coupling field and laboratory experiments, the present study is the first to analyze the dynamics of vegetative cells and sexual stages during a bloom of G. catenatum. In natural samples, the sexual stages of G. catenatum were associated with several different cellular behaviors and morphologies. This confirmed laboratory evidence for the reversibility of the species' sexual processes and for the ability of zygotes to either bypass or shorten the route to resting-cyst formation. Moreover chains of up to 4 viable cysts with differing morphologies occurred and these have never been reported previously for this species. At least two of the cysts had reticulated surfaces, a feature related to sexual reproduction in previous studies; this observation suggests the involvement of sexual processes in mechanisms that cannot be explained by any known life cycle route depicted for this species. Morphological variability and abundance of the sexual stages during the bloom indicated the complexity of the G. catenatum sexual cycle and the important role of sexual reproduction in the ecological succession of this species. However, the lack of a dormancy period in the sexual resting stage (evidenced by the large number of germinated cysts in sediments sampled 3 mo after the bloom) indicated that the advection of off-shore populations shoreward, rather than 'seed beds', is the main mechanism explaining G. catenatum bloom formation in the Galician rías. © Inter-Research 2008