Ultrastructure of Mikrocytos mackini, the cause of Denman Island disease in oysters Crassostrea spp. and Ostrea spp. in British Columbia, Canada

An ultrastructural study was carried out on Mikrocytos mackini, the cause o f Denman Island disease in Pacific oysters Crassostrea gigas in western Can ada. Three forms were identified, quiescent cells (QC), vesicular cells (VC ) and endosomal cells (EC), QC occurred in the vesicular connective tissue (VCT), haemocytes (hyalinocytes), adductor and heart myocytes, and extracel lularly, They had a central round to ovoid nucleus, <7 cisternae of inactiv e nuclear membrane-bound Golgi, few vesicles and lysosome-like bodies. VC w ere rarely extracellular and usually occurred in adductor and heart myocyte s, in close association with host cell mitochondria. The contents of the ho st cell mitochondria appeared to pass through a tubular extension into the cytoplasm of the parasite. Cytoplasmic vesicles resembled the tubular struc ture in appearance and size. EC occurred in the VCT, in haemocytes and extr acellularly. They had a dilated nuclear membrane, sometimes containing a lo oped membranous structure that appeared to derive from the nucleus, and pas s into the cytoplasm. A well-developed anastomosing endoplasmic reticulum c onnected the nuclear and plasma membranes, and endosomes were present in th e cytoplasm. QC and EC cells were frequently observed tightly against, or b etween, the nuclear membranes of the host cell.-Few organelles occurred in all forms of M mackini, especially QC. The lack of organelles found in most eukaryotic cells, including mitochondria or their equivalents, may be due to obligate parasitism and the utilization of host cell organelles reducing the need for parasite organelles. Alternatively, perhaps M, mackini is a p rimitive eukaryote. Although phylogenetic affinities could not be determine d, it is not a haplosporidian. A developmental cycle is proposed from these findings.