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

Small cells are observed in dense populations of many dinoflagellate s pecies, 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 red uced nucleus and cytoplasmic components. Two possible origins are cons idered for such small cells: unequal cell division or budding-like div ision, and successive 'depauperating' cell divisions. Small cells, whi ch are reported here for 34 species, can proliferate actively in the l ate 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 structu ral components, and may give rise to forms of a more typical size, Sma ll cells may also serve as gametes. In the sexual cycle of the species studied here, gametes were morphologically indistinguishable from veg etative small cells. Fusion was isogamous in Gymnodinium splendens Leb our, 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 w ith earlier observations: the dinoflagellate nucleus is polyploid or a neuploid and the chromosomes are polytenic. The existence of small for ms may have implications for dinoflagellate taxonomy.