Haplosporidium sp. of Rock Oysters
Category 3 (Host Not in Canada)
Common, generally accepted names of the organism or disease agent
Scientific name or taxonomic affiliation
Various locations in northern areas of Western Australia.
Impact on the host
This parasite was associated with epizootics among S. cuccullata from a few location in north Western Australia in June 1993 and subsequent investigations detected prevalences of infection ranging from 27.1% (n = 332) to 3.2% (n = 31) in near-by locations (Hine and Thorn 2000). In heavily infected oysters, plasmodia and sporulation stages occupied greater than 50% of the connective tissue but there was little if any haemocyte infiltration in response to sporulation but brown cells were abundant at the sites of infection (Hine and Thorne 2002). Jones and Creeper (2006) indicated that the presence of this parasite will threaten the viability of any rock oyster farms located north of Exmouth, Western Australia.
Histology: Multinucleate plasmodia (round or ovoid, 6.3 to 15.4 µm in diameter) and sporulation stages (sporonts were 17.0 to 22.5 µm in diameter, sporont syncytium were 20.0 to 31.0 µm in diameter, and spore clusters were 28.0 to 31.8 µm in diameter) were observed as focal infections in the connective tissues of the gills or as disseminated infections in the connective tissue of the digestive gland and mantle. Haplosporidium sp. occurred between the epithelial cells of the gut but not in the epithelium of the digestive gland tubules and sporulation was confined to the connective tissue. Spores were flask-shaped (5.6-6.7 µm by 3.3-4.0 µm).
Electron Microscopy: Multinucleate plasmodia were irregular in outline, contained haplosporosomes, and were diplokaryotic, with the two nuclei separated by osmophilic material about 50 nm thick. Sporonts had few or no haplosporosomes and contained 2 to 13 spherical nuclei per section that were usually grouped as diplo-, tetre- or poly-karya. Dense material also occurred between closely apposed surfaces of the tightly grouped nuclei. In multinucleate sporonts, the nuclei were separated and distributed throughout the cytoplasm. Sporoblasts (sporocysts) were uninucleate or large binucleate forms that could be distinguished from sporonts by the lack of a thickened plasma membrane. Spores were ovoid with a single nucleus and had a characteristic operculum, a few filamentous wrappings and a rod-like structures in the posterior sporoplasm. The wall of mature spores comprised of an inner (90 nm thick), middle (30 nm thick) and outer (130 nm thick) layers and a surface coat of microtubules giving them a furry appearance (Hine and Thorne 2002).
Methods of control
No known methods of prevention or control.
Hine, P.M. and T. Thorne. 2000. A survey of some parasites and diseases of several species of bivalve mollusc in northern Western Australia. Diseases of Aquatic Organisms 40: 67-78.
Hine, P.M. and T. Thorne. 2002. Haplosporidium sp. (Alveolata: Haplosporidia) associated with mortalities among rock oysters Saccostrea cuccullata in north Western Australia. Diseases of Aquatic Organisms 51: 123-133.
Jones, J.B. and J. Creeper. 2006. Diseases of pearl oysters and other molluscs: a Western Australian perspective. Journal of Shellfish Research 25: 233-238.
Bower, S.M. (2007): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Haplosporidium sp. of Rock Oysters.
Date last revised: June 2007
Comments to Susan Bower
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