Genetic variation in Symbiodinium (=Gymnodinium) microadriaticum Freudenthal, and specificity in its symbiosis with marine invertebrates. II. Morphological variation in Symbiodinium microadriaticum

Morphological studies conducted on axenic cultures of the endozoic gymnodinioid dinoflagellate Symbiodinium ( = Gymnodinium) microadriaticum demonstrated that there are intrinsic variations in the dimensions of the recently divided, but as yet unseparated, vegetative cells (the two-cell stage). Characteristic dimensions of a given strain are maintained when such a strain is artificially introduced into a host other than that from which it was originally isolated. Ultrastructural obser­vations illustrated that, in contrast to the algae in their hosts’ tissues, the algae in culture produced a fibrous or granular ‘pellicle’ approximately 0.2 μm thick. The algae in situ in Tridacna gigas also demon­strated the presence of a ‘pellicle’, but this structure was reduced when compared to that produced by the same cells in culture. Many of the stages in the described life cycle of the alga were observed, with the exception of those stages involving gametogenesis and sexual reproduction. Two distinct types of zoospores were observed; those that contained an ‘accumulation body’ in the epicone and those that did not.