| Summary: | Two methods for distinguishing triploid Pacific oysters [Crassostrea gigas (Thunberg)] from diploid oysters were assessed. Adductor muscle diameter in relation to valve height was significantly (p < 0.001) greater in samples of triploid oysters than in diploid samples and was influenced by site. However, variation in this measure was too large to allow individual oysters to be distinguished as either triploid or diploid. A second method was assessed that used differences in the nuclear size of hemocytes and the intensity of staining of hemocyte nuclei to distinguish between diploids and triploids. Histological sections, prepared by standard paraffin histology, were stained for nuclear histones with Gill's hematoxylin. Integrated nuclear optical density and nuclear area were recorded with image analysis. This method was effective in distinguishing individual oysters as diploid or triploid. When histological specimens are required, this method is less expensive than other techniques used to determine triploidy.
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