Factors affecting the size of spores of Myxidium zealandicum Hine, 1975 (Protozoa : Myxosporida)

Variations in the size and shape of Myxidium zealandicum spores were not related to cyst size or host size, but variation was apparent between host species and individual fish. Spores from Anguilla australis were larger (7.9–15.3 μm long) and more uniform in shape than the small (6.1–11.7 μm long) inegularly shaped spores from A. dieffenbachii. Spore size was also consistent between cysts in individual A. australis, but variable between cysts in A. dieffenbachii. Abnormal spores were rare in A. australis, but more common and associated with small arcuate spores in A. dieffenbachii.

It is proposed that size and shape are genetically determined but may be highly modified by the biochemistry/physiology of the host at the site of sporogony. A. australis is a more suitable host for M. zealandicum than A. dieffenbachii, and constancy in spore size between cysts in A. australis was attributed to plasmotomy prior to sporogony. Spore valve striation numbers are genetically determined and variable, but no trends in variation were discernable. Cyst shape may be of taxonomic use within a host species, but not between host species.