Cells of the potentially toxic dinoflagellate Alexandrium catenella
have a complement of features typical of dinoflagellates, including
dinokaryon, pusule and amphiesma. No endosymbiotic bacteria were found
in the Californian strain examined, which is weakly toxic. The
chloroplast consists of a central pyrenoid complex with radiating chloroplast
lobes, an arrangement found also in certain other
gonyaulacoid dinoflagellates. Cells are often arranged in few-celled chains
and a cytoplasmic connection interconnects adjoining cells.
Striated fibres line the attachment canal and also subtend the apical pore,
making up an ‘apical fibrous complex’. The flagellar apparatus
basically resembles that of other gonyaulacoids. However, Alexandrium
catenella, Gonyaulax spinifera and Protoceratium reticulatum
presence of two striated collar connectives and a single-stranded microtubular
root associated with the longitudinal basal body, in addition
to the multi-stranded root characteristic of dinoflagellates in general.
Ultrastructural features indicate that these three species are closely
related while Ceratium furcoides, another gonyaulacoid dinoflagellate,
apparently lacks these features.