SMALL-CELLS IN THE LIFE-HISTORY OF DINOFLAGELLATES (DINOPHYCEAE) - A REVIEW

Small cells are observed in dense populations of many dinoflagellate species, both in clonal cultures and in natural bloom populations. They range in size from 0.5 to 0,16 of the normal cell volume and have reduced nucleus and cytoplasmic components. Two possible origins are considered for such small cells: unequal cell division or budding-like division, and successive 'depauperating' cell divisions. Small cells, which are reported here for 34 species, can proliferate actively in the late stages of exponential growth, and may become the dominant form in cultures when nutrients are exhausted. When new nutrients are added to the exhausted growth medium, small cells increase in size and structural components, and may give rise to forms of a more typical size, Small cells may also serve as gametes. In the sexual cycle of the species studied here, gametes were morphologically indistinguishable from vegetative small cells. Fusion was isogamous in Gymnodinium splendens Lebour, Gyrodinium instriatum Freudenthal et Lee, Alexandrium lusitanicum Balech and Coolia monotis Meunier, but anisogamy was also observed in Gyrodinium instriatum. The nuclear structure of several stages of the life history was examined, including small cells. The results agree with earlier observations: the dinoflagellate nucleus is polyploid or aneuploid and the chromosomes are polytenic. The existence of small forms may have implications for dinoflagellate taxonomy.