FARLEY ET AL.: MICROCELL DISEASE IN OYSTERS 



parasites (Perkins 1979). Larger "membrane" 

 bound structures (500 nm) consistent with the ap- 

 pearance of mitochondria were also present in the 

 cytoplasm; however, cristi could not be distin- 

 guished. 



Ultrastructural studies were also performed on 

 positively diagnosed oysters (C. gigas) from Denman 

 Island, British Columbia (sample WWC-18, collected 

 9 June 1980). Microcells were always associated with 

 focal abscesses, but their occurrence was restricted 

 to the periphery. Parasites were found as cytozoic 

 organisms primarily in VCT cells (Fig. 18). Micro- 

 cells were 3-4 p<m in diameter, had nuclei 1 ^m in 

 diameter, and nucleoli 250-300 nm in diameter. 

 Nucleoli were spherical, eccentrically located within 

 the nucleus, but never peripheral (Fig. 19). One to 

 many parasites occurred within the cytoplasm of 

 vesicular cells. None were ever found within hemo- 

 cytes. Figure 20 shows an organism possibly under- 

 going division. The cytoplasm was densely packed 

 with free ribosomes and contained a variety of 

 organelles as follows: double membrane bound dense 

 bodies 50-185 nm in diameter (Fig. 19A); double 

 membrane bound, dumbbell-shaped structures (Figs. 

 20, 21h), approximately 37 x 18 nm to 85 x 260 

 nm; and dense bodies 40-45 nm in diameter that 

 appeared to be membrane bound, and a suggestion 

 of six- and five-side angularity (Figs. 19, 20, 21v). 

 Endoplasmic reticulum was extremely sparse if 

 present at all. The plasma membrane complex con- 

 sisted of possibly two membranes with the external 

 membrane containing dense material. An electron- 

 lucent zone was present around the cell, suggestive 

 of a glycocalyx. 



Taxonomic Descriptions 



The information acquired on these diseases and 

 the organisms associated with them and outlined 

 previously allows us to propose taxonomic descrip- 

 tions of them. 



Mikrocytos g. n. (Protista incerta sedis) Gittle cell)— 

 Definitive life cycle stages that would permit 

 higher classification of this protistan parasite have 

 not been observed. Named after the term micro- 

 cell as coined by the late John G. Mackin. Small 

 (1-4 ^m), unicellular, protistan, cytozoic parasite 

 normally infecting VCT cells of oysters. Always 

 associated with abscess-type focal inflammatory 

 lesions. 



Type species - Mikrocytos mackini sp. n. 

 Bonamia, the other closely similar genus, infects 

 hemocytes of ostreid oysters only and is associated 



with systemic non-abscess type disease manifes- 

 tations. 



Mikrocytos mackini sp. n. — Named in honor of the 

 late John G. Mackin who discovered this parasite 

 in the early 1960s. 



Type specimen - A 6 /im thick hematoxyhn-and- 

 eosin-stained histologic section of an infected 

 oyster, C. gigas (WWC-2-7), was deposited in the 

 Registry of Marine Pathology, Northeast Fish- 

 eries Center, Oxford, MD 21654. 



Host - Crassostrea gigas 



Type locale - Henry Bay, Denman Island, British 

 Columbia, Canada. 



Range - Occurrence confined to above site. (A closely 

 related candidate for inclusion within this species 

 was found in C. gigas from Hawaii.) 



Morphologic characteristics - Small, 1-4 nm intra- 

 cellular parasites of VCT cells; infections always 

 associated with focal inflammatory tissue ab- 

 scesses. Parasites are unicellular and contain a 

 small 1 p/m nucleus that has an eccentric nucleolus 

 250-300 nm in diameter. The cytoplasm contains 

 dense, double membrane bound, dumbbell-shaped 

 haplosporosome-type organelles 50-180 nm in 

 diameter and 40-45 nm membrane bound five- 

 and/or six-sided dense bodies. 



Mikrocytos roughleyi sp. n. — Named in honor of 

 T. C. Roughley who published the initial study of 

 the Australian winter disease in the 1920s. 



Type specimen - A 6 fim thick hematoxylin-and- 

 eosin-stained section of an infected oyster, S. com- 

 mercialis (S-41-7), was deposited in the Registry 

 of Marine Pathology, Northeast Fisheries Center, 

 Oxford, MD 21654. 



Host - Saccostrea commercialis 



Type locale - Georges River, Woolooware Bay, New 

 South Wales, Australia. 



Range - Known only from the above location and 

 other high salinity estuaries in this region of New 

 South Wales. 



Morphologic characteristics - Infections occur in 

 hemocytes and are associated with focal abscess- 

 type lesions in the gill, connective, and gonadal 

 tissues. Organisms are small 1-3 ptm cells that con- 

 tain an eccentric nucleus and a cytoplasmic 

 vacuole. Ultrastructural characteristics are not 

 known. 



Comparisons — Mikrocytos g. n. is always associated 

 with focal abscesses and occurs in crassostreid 

 oysters. Bonamia is always associated with general- 

 ized infections and only occurs in ostreid oysters. 



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