Diseases causing high mortality occurred in a hatchery for rearing juvenile oysters, C. gigas, at Bicheno, Tasmania, during 1980. Outbreaks of the disease characteristically showed three peaks: in 7-10-day- old larvae, in older larvae several days before setting, and in spat 1-2 weeks after setting. Various marine bacteria were cultivated from inactive larvae and other hatchery specimens, and the predominant strains were identified. When given to normal young larvae at doses of 1 × 105-5 × 105 or 1 × 10-5-5 × 107 bacteria (total dose), seven strains (five strains of Vibrio, two strains of Alteromonas) caused fatal infections in most animals. This is the first report of pathogenic marine bacteria in an Australian shellfish hatchery.

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SUMMARY Three fixation schedules were devised and compared in terms of their influence on the preservation of mucus and surface‐associated microorganisms contained within it. Different mucus‐secreting epithelial tissues from normal and spoiled oysters and normal rats were examined by scanning and transmission electron microscopy and light microscopy. On all tissues, mucus was best preserved in specimens fixed by 10% acrolein vapour for 1 h then immersed in 3% glutaraldehyde—3% formaldehyde fixative containing 0·05% ruthenium red, cacodylate buffer pH 7·4, for at least 3 h. This fixation schedule also greatly increased the preservation of microorganisms in mucus in specimens from spoiled oysters and normal rats. In contrast, the retention of mucus and surface‐associated microorganisms was poor in tissues fixed either by 1% OsO4 vapour for 1 h followed by immersion in combined aldehyde fixative, or by direct immersion. The quality of preservation of the mucus layer, epithelium and sub‐epithelium was also noted by transmission electron microscopy in tissues prepared by the different fixation schedules. Cellular preservation was satisfactory in directly immersed tissues but poor in vapour fixed specimens.

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