Ultrastructure of the toxic, chain-forming dinoflagellate Gymnodinium catenatum (Dinophyceae)

The ultrastructure of the toxic, chain-forming dinoflagellate Gymnodinium catenatum Graham (Dinophyceae)is described from wild and cultured cells from estuaries in south-east Tasmania, Australia. The cell covering (amphiesma) comprises numerous (> 1500) small polygonal vesicles, which lack plate-like contents, and are arranged in irregular rows except in the area of the cingulum, where five or six roughly parallel rows occur. This pattern is also seen in the arrangement of microreticulate fields on the resistant wall of the resting cyst (hypnozygote). The vegetative cell contents include a large central nucleus with about 120 chromosomes, numerous elongated chloroplasts, typical dinoflagellate trichocysts, and a small but distinctive pusule with a central collecting chamber surrounded by a system of radiating, flattened accessory vesicles. The pyrenoids are multiple-stalked with starch caps, and are not penetrated by thylakoids. Cells in a chain are linked by cytoplasmic connections. No intracellular or endonuclear bacteria were observed. Despite previous speculation that G. catenatum may have arisen from a thecate species which produces similar paralytic shellfish poisons, such as Alexandrium catenella (Whedon et Kofoid) Balech, the structure of its amphiesma, pyrenoids, pusule and resting cyst provide evidence against any close relationship with species of that genus or with any other known armoured toxic dinoflagellate. On present evidence, G. catenatum appears to occupy a somewhat isolated position within the Gymnodiniales.