Gymnodinium catenatum Graham (Dinophyceae): morphology and affinities with armoured forms

Samples of the unarmoured dinoflagellate Gymnodinium catenatum Graham, collected from the plankton during an outbreak of paralytic shellfish-poisoning, were examined with light and electron microscopy. No thecal plates were seen. Instead, a pellicular layer surrounded the cells. Cell size and shape, nucleus shape and position, cingulum arrangement, the pellicular layer, chain formation, apical markings, toxicity and possibly cyst morphology indicate close affinities with members of the thecate genus Protogonyaulax Taylor.