A LONG-TERM STUDY OF "MICROCELL" DISEASE IN OYSTERS 



WITH A DESCRIPTION OF A NEW GENUS, MIKROCYTOS (G. N.), AND 



TWO NEW SPECIES, MIKROCYTOS MACKINI (SR N.) AND 



MIKROCYTOS ROUGHLEYI (SR N.) 



C. Austin Farley,' Peter H. Wolf,^ and Ralph A. Elston^ 



ABSTRACT 



Continuing long-term studies of oyster disease problems have been carried out over the past 26 years 

 using field monitoring, gross, histologic, and ultrastructural pathologic methods. 



A microorganism of uncertain taxonomy was discovered in 1963 by J. G. Mackin in association with 

 lesions and mortalities of Japanese oysters, Crassostrea gigas, from Denman Island, British Columbia, 

 Canada. Mackin coined the term "microcell" for this organism and described the parasite as 1-3 fim 

 cells with small nuclei which occurred within vesicular connective tissue cells adjacent to characteristic 

 abscesses. We are describing this organism as Mikrocytos mackini sp. n. in his honor. Similar appearing 

 organisms were seen by the senior author in flat oysters, Ostrea edulis, from Milford, Connecticut, on 

 three different occasions: 1) in oysters transferred from Milford, Connecticut, to Chincoteague Bay, 

 Virginia; 2) in oysters transferred from Milford to Elkhom Slough, California; and 3) in oysters trans- 

 ferred from Milford to Oxford, Maryland, and held in recirculated sea water. The causative organism 

 in these three episodes has been shown by electron microscopy to be Bonamia ostreae, the parasite that 

 was implicated in recent mortalities in flat oysters in Europe. Similar organisms have also been seen 

 in Olympia oysters, Ostrea lurida, from Oregon and in the Sydney rock oyster, Saccostrea commercialis, 

 from Australia. Presence of the organism in the latter species is associated with the winter mortalities 

 originally described by T. C. Roughley, and the pathogen is here described as Mikrocytos roughleyi (sp. 

 n.) in his honor. 



"Microcell" type parasites of oysters are associated 

 with a complex of diseases that occur in Japanese 

 oyster, Crassostrea gigas; Sydney rock oyster, Sac- 

 costrea commercialis; flat oyster, Ostrea edulis; 

 and Olympia oyster, 0. lurida, in North America, 

 Europe, and Australia. Severity of disease varies 

 from an acute, highly lethal form to a chronic, 

 seasonally recurring disease that does not produce 

 massive mortalities. The etiologic agents are small, 

 morphologically simple, and very difficult to com- 

 pare and characterize taxonomically at light micro- 

 scope levels of resolution. Associated lesions vary 

 according to species affected and provide some of 

 the differences that may be used to distinguish the 

 agents involved. The complexity of this group and 

 the difficulties involved in achieving an understand- 

 ing regarding whether we are dealing with one or 

 a group of organisms and how they were transferred 

 to new locations, the long time span involved in 



'Northeast Fisheries Center Oxford Laboratory, National 

 Marine Fisheries Service, NOAA, Oxford, MD 21654. 



^62 MacKenzie Street, Bondi Junction, New South Wales, 

 Australia 2022. 



'Center for Marine Disease Control, Battelle/Marine Research 

 Laboratory, Sequim, WA 93282. 



answering these questions, and the continuing dis- 

 semination of unpublished privileged information 

 shared in informal workshop gatherings of scientists 

 with common interests, make it necessary to use un- 

 published anecdotal information in order to provide 

 as complete a story as possible. 



The first oyster mortality known to be associated 

 with "microcell" disease was reported in C. gigas 

 from Denman Island, British Columbia, Canada by 

 Quayle (1961). Quayle's report documents the epi- 

 zootic aspects from 1956 to 1960 and demonstrates 

 the gross appearance of the disease in the Pacific 

 oyster. A causative agent was not identified until 

 several years later, when the late J. G. Mackin"* 

 (unpubl. data) discovered a small intracellular 

 organism intimately associated histologically with 

 tissue abscesses in diseased oysters (C gigas) from 

 Denman Island and called this organism "micro- 

 cell". He demonstrated this material at the 1963 

 Shellfish Mortality Conference held at Oxford, MD. 



Mackin's demonstration provided us with the in- 

 sight to identify similar organisms in histologic sec- 



^J. G. Mackin, deceased, Texas A&M University, College Station, 

 TX. 



Manuscript accepted May 1988. 



FISHERY BULLETIN: VOL. 86, NO. 3, 1988. 



581 



