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15 pages, 14 figures, 1 table.A cell cycle-analysis method based on morphological recognition of cytokinesis and sulcal list regeneration was chosen to estimate in situ division rates (μ) of 4 dinoflagellate species of the genus Dinophysis, associated with diarrhetic shellfish poisoning (DSP), following 2 different models. Sampling over 24 h was conducted on 4 mini-cruises in the Galician rías during spring and autumn proliferations of these species. Frequencies of paired and recently divided cells in integrated water samples (0 to 20 m) were measured at 30, 60, or 120 min intervals. Cellular division was phased in D. acuminata, D. acuta, D. caudata and D. tripos, but the shape of the phase fraction curves and the values of estimated division rates varied considerably between seasons and cruises for the same species. Frequencies of paired plus recently divided cells were maximal at dawn in D. acuminata, and 2 to 3 h later in the other species. The results presented here confirm that the cytokinetic (paired) phase can be very fast in Dinophysis spp. (0.3 to 2.7 h), but sulcal list regeneration was shown to be a more stable process and an unambiguous marker of cellular division. This Œpostmitotic index¹ allowed estimates of μ at low field concentrations (102 to 103 cell l-1) of the target species and required a short time for sample processing (1 to 2 h per sample). Moderate (0.24) to high (0.57) values of μ were found under oceanographic conditions considered unfavourable for growth of Dinophysis spp., and the phase in the population growth season seemed to be a key factor affecting this value. A critical revision of previous results of asynchronous division obtained in cell cycle studies of Dinophysis spp. is presented. It is suggested that monitoring the content of DNA per cell through the cell cycle in Dinophysis spp. is not a reliable method until a reasonable knowledge on the nuclear behaviour during sexual processes and other nonmitotic processes is available for these species, and that even accepting that mitosis is a non-return process, cell division may be arrested in one of its phases, adding further inconsistencies to μ measurements based on quantification of DNA per cell.This research was supported by\udCYCIT project MAR99-0224.Peer reviewe

Cell cycle patterns and estimates of in situ division rates of dinoflagellates of the genus Dinophysis by a postmitotic index