Green-lip mussel Pema canaliculus spat (15 to 30 mm length) in the outer Marlborough Sounds, New Zealand, suffered 50 to 100% mortality during January to April 1994 (summer/autumn) following thinning and reseeding of the mussel lines by farmers. Adult mussel mortalities of 2 to 5 % continued from February to early May 1994. Histological examination showed extensive haemocytosis and multifocal liquefaction necrosis of interstitial cells, basal cells and digestive tubule epithelial cells. Sloughed pyknotic or karyolytic digestive tubule epithelial cells formed characteristic rounded granular bodies 10 to 15 μm diameter both in digestive tubules and free in lesions . No viral inclusion bodies were observed. Ultrastructural examination showed highly modified rough endoplasmic reticulum associated with small, 25 to 45 nm diameter, electron- dense uncoated virus-like particles Identical cell damage and virus-like particles were subsequently found in monbund adult (75 to 110 mm length) P canaliculus and stunted (25 to 4 7 mm length) subt1dal Mvtilus galloprovincialis from the same area. Following purfication of extracts of the moribund spat by isopycnic centrifugation in CsCJ, large numbers of 25 nm diameter, unenveloped, virus particles were seen by electron microscopy. These particles had a density of 1.364 g cm- 3 A broad band at a density expected for enveloped particles (1.21to1.24 g cm 1 ) was also observed but contained few virus-like particles. Cell damage and mussel mortalities are thus likely due to a small unenveloped virus
