Detection of phenoloxidase activity in early stages of the Pacific oyster Crassostrea gigas (Thunberg)

The presence of phenoloxidase (PO) activity was detected in different developmental stages of the Pacific oyster, Crassostrea gigas. A significant reduction in PO activity was observed from the 6 h embryo stage to the day 11 larvae by spectrophotometry. A progressive increase was also observed from the day 13 larvae right through to the juvenile stage. The microscopy studies with ‘6 h embryo’ and adult samples confirmed the presence of PO activity. Various modulators of PO activity were used to study the triggering of pro-phenoloxidase (proPO) activating system of C. gigas but also to confirm the exact nature of the monitored activity. The enzyme activation mechanisms appear to differ with the developmental stage: bacterial lipopolysaccharides constitute an early elicitor of the proPO–PO system, whereas a purified trypsin triggers proPO–PO system in C. gigas spat. Phenoloxidase activity was totally suppressed by PO-specific inhibitors such as β-2-mercaptoethanol, sodium diethyldithiocarbonate and tropolone. This study demonstrated the selective response of PO-like activity by different elicitors and suggested that proPO–PO activating system, which is supposed to play an important function in non-self recognition and host immune reactions in oyster, is expressed early in the Pacific oyster, C. gigas.