Power to the oyster: Do spionid-induced shell blisters affect condition insubtidal oysters?

Two static allometric condition indices and two static histological indices were assessed in determining whether oyster shell blistering induced by Bo ccardia knoxi infestations affected the health of infested oysters. Analysi s of variance, Spearman's correlation analysis, and a-posteriori power anal ysis were used to assess the condition indices. The use of the dry weight c ondition index CIflesh:shell and the shell volume condition index CIflesh:C V were first validated after checking for confounding due to increased shel l weight and loss of shell volume resulting from blistering. No significant differences in condition were detected between the sexes within the three subjective shell grades chosen to represent varying levels of shell blister ing. Loss of condition was detected by three out of the four indices; the C Iflesh:shell and CIflesh:CV both detected significant differences, but did not have the required level of power to reject the null hypothesis of no ef fect. However, for heavily blistered oysters the highly significant reducti on in oocyte size (CIoocyte) supported rejection of the null hypothesis. Th e loss of condition was considered insignificant in terms of subtidal oyste rs production, but demonstrates the parasitic effect spionids can have even under ideal growing conditions. The only index not detecting an effect, CI gonad area had the lowest power (<3%), illustrating the importance of perfo rming a-posteriori power analysis. The results of the analysis of variance (ANOVA), Spearman's rank nonparametric correlation coefficients (SCC), and power analysis all indicated the CIflesh:shell was the most sensitive of th e four indices compared. The negative effects of shell blistering induced a s a result of B. knoxi infestations rendered this species a parasite of Pac ific oysters in Admiralty Bay.