The Effect of Shell Infestation by Polydora sp. and Infection by Haplosporidium nelsoni (MSX) on the Tissue Condition of Oysters, Crassostrea virginica

Abstract

A number of stressors, including the endoparasiteHaplosporodium nelsoni (MSX), reduce metabolic condition of oysters; however, previous studies of the effect of the shell-boring polychaetePolydora sp. on condition have produced conflicting results. To assess the effects of both symbionts, singly and in combination, we examined 214 oysters for shell infestation byPolydora sp. mud blisters (by image analysis), intensity of infection byH. nelsoni (by histology), and nutritional state (estimated by soft tissue gravimetric condition index). Among all oysters tested, condition index was negatively (p <0.05) correlated withPolydora sp. infestation, although the explained variance (r2) was only about 5%. When oysters withoutH. nelsoni were considered alone, the variance explained byPolydora sp. rose to approximately 30%, indicating that the presence of one deleterious symbiont can confound the effect of another. In oysters withoutH. nelsoni, a 36% loss in tissue condition was associated withPolydora sp. blister infestation covering 45% of the shell. After a single season's exposure to both symbionts, oysters withH. nelsoni had significantly (p<0.05) greaterPolydora sp. infestation than did animals withoutH. nelsoni, but the difference disappeared after a second year, when nearly all shells were infested. The mean condition index for oysters with bothPolydora sp. andH. nelsoni was 13% lower than that for individuals with neither; oysters with only one symbiont had intermediate values. The interaction between percent blister coverage andH. nelsoni intensity on tissue condition was significant (p<0.002), verifying that the effect of the two symbionts was not additive. We conclude thatPolydora sp. reduces the ability of oysters to accumulate nutritional reserves, but that this effect may be obscured by other deleterious influences.