Effects of the bloom-forming alga Trichodesmium erythraeum on the pearl oyster Pinctada maxima

Farmed pearl oysters (Pinctada maxima) suffered high mortality in the Dampier Archipelago of Western Australia in 1996. The mortality event affected all oyster sizes and coincided with extensive blooms of the cyanobacterium Trichodesmium erythraeum. The potent neurotoxin saxitoxin was detected in small amounts in some of the affected adult oysters but was not detectable in T. erythraeum. Vibrio species were isolated from some of the affected oysters but not in patterns consistent with a primary disease and no virus-like particles were observed. Juvenile oysters were exposed to medium and high concentrations of T. erythraeum in experimental aquaria for 7 days. No mortality of juvenile oysters occurred but individuals exposed to T. erythraeum at 105 cells/ml were less healthy than those fed upon the diatom Chaetoceros calcitrans. Histopathology of adult oysters from the affected farm and juvenile oysters exposed to T. erythraeum in the aquarium experiments were similar and included dilation of digestive gland lumens, sloughing of epithelial cells and granulocytes under the epithelial layer. These symptoms suggest that the T. erythraeum blooms in the archipelago were not a suitable food source for the oysters and may have contributed to the observed mortalities.