FISHf:RY BULLETIN: VOL. 86, NO. 3 



tions of Denman Island oysters in our laboratory 

 collection and further led to the discovery of micro- 

 cell disease in 0. edulis, 0. lurida, and 5. commer- 

 cialis from Australia ("Australian winter disease") 

 described by Roughley (1926). In 1979, the char- 

 acteristics of microcell disease were demonstrated 

 at a microscopic diagnosis workshop held at the 

 Ministry of Agriculture and Fisheries Laboratory 

 in Weymouth, England. This workshop was 

 attended by molluscan pathologists from several 

 European countries including France. In the late 

 1970s, serious mortalities of 0. edulis in France, 

 associated with microcell infections, were described 

 in a paper by Pichot et al. (1979), in which the 

 organism was named Bonamia ostreae. Reference 

 to the earlier work on microcell by Mackin and 

 others (Katkansky et al. 1969) was not included in 

 their report. 



Since the taxonomic relationship and status of 

 these similar parasites have not been described, it 

 is the purpose of this paper to present a complete 

 background on microcell related epizootics and mor- 

 phological information which, in North America, 

 preceded the French report (Pichot et al. 1979). Fur- 

 thermore, microscopic and ultrastructural compari- 

 sons of the microorganisms are provided and a new 

 genus, Mikrocytos g. n., and two new species, Mikro- 

 cytos mackini sp. n. and Mikrocytos roughleyi sp. 

 n., are described. 



MATERIALS AND METHODS 



General Procedures 



Oyster tissues were collected from a variety of 

 sources as follows: 



Code WWC were C. gigas collected from Henry Bay, 

 Denman Island, British Columbia, Canada on a 

 periodic basis from April 1968 to June 1969 by D. B. 

 Quayle (spring 1969 samples were collected by 

 N. Bourne). Live oysters were sent by air freight 

 to the Oxford Laboratory where clinical and gross 

 features were recorded and they were processed for 

 histological and, in some cases, ultrastructural 

 studies. 



Code S-124-A were C. gigas from Hawaii collected 

 in September 1972. 



Code S-41 were S. commercialis collected by Peter 

 Wolf from 22 July to 23 July 1965 from the Georges 

 River, Woolooware Bay, New South Wales and ship- 

 ped to the Oxford Laboratory for processing. 



Code FK were progeny of 0. edulis from Boothbay 

 Harbor, ME that were spawned at Milford, CT in 

 April 1961. Seed oysters were transplanted to 

 Chincoteague Bay, VA in May 1961 and processed 

 at subsequent intervals (FK-1-1 to FK-2-5, August 

 1961; FK-3-1, August 1961; FK-4-1 and FK-4-2, 

 February 1962). 



Code WAC were 0. edulis bred in the Milford, CT 

 hatchery from 1963 to 1965, and introduced into 

 California bays as follows: 



WAC-1, Milford 1963 seed oysters planted in 

 Morro Bay, CA in December 1964 and sampled on 

 7 December 1965 during a heavy mortality. 



Sample WAC-21-28 consisted of 38 oysters from 

 a Milford 1963 stock shipped to California in 1964 

 and held at Pigeon Point Laboratory, Pigeon Point, 

 CA until heavy mortality occurred and sampled on 

 11 May 1966. 



WAC-3-1-10 were 1963 Milford oysters placed in 

 Morro Bay in 1964. Heavy mortality was noted and 

 samples were taken. WAC-3-11-15 were 1963 

 Milford stock placed in Morro Bay in 1964. Mortality 

 was low. Oysters were necropsied and fixed on 1 

 May 1966. WAC-3-16-26 were Milford 1962 stock 

 placed in Morro Bay in 1963. They experienced 40% 

 mortality and were examined and fixed on 1 May 

 1966. Oysters WAC-4-1 through 4-5 were from 

 Milford 1963 stock placed in Tomales Bay, CA in 

 1964; low mortality was observed. They were ex- 

 amined and fixed on 1 May 1966. 



Code FMT were 0. edulis used in an experimental 

 holding study at the Oxford Laboratory. Ten 2-yr- 

 old 0. edulis from the Milford hatchery were placed 

 in each of three tanks receiving 0.45 /um membrane 

 filtered 26°/oo seawater on 23 February 1968; pH, 

 temperature, salinity, and mortality were monitored 

 daily until 10 October 1968. Crassostrea virginica 

 from the Mispillian River, Delaware Bay, were 

 placed in each tank on 28 March. Tanks were desig- 

 nated A (control), B (fed tissues of moribund 0. 

 edulis from Pigeon Point, CA), and C (fed tissues 

 from Denman Island C. gigas infected with Denman 

 Island disease). Seawater in each tank was recir- 

 culated through a glass wool, charcoal, calcium flow- 

 through filter via an airlift system moving from C 

 to B to A, respectively. Oyster codes were FMT-A- 

 1-20, FMT-B-1-20, FMT-C-1-20, numbered as they 

 were fixed (species were designated at time of 

 fixation). 



Code WAO-A were 50 specimen samples of native 

 oysters (0. lurida) from Yaquina Bay, OR, sampled 



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