Changes in the growth, metabolism, disease levels and energy budget of the American oyster, Crassostrea virginica induced by an ectoparasitic gastropod, Boonea impressa

Abstract

The ectoparasitic gastropod, Boonea impressa, altered the biochemical composition, energy balance, growth and the incidence and intensity of disease in the American oyster, Crassostrea virginica. B. impressa transferred the protozoan parasite Perkinsus marinus to uninfected oysters. Feeding by B. impressa also resulted in the intensification of P. marinus infections in diseased oysters. Oysters parasitized by B. impressa gained significantly less weight than control oysters. Snail parasitism produced 75% mortality in small oysters parasitized by 25 snails. Laboratory results corroborated those predicted by an energy model developed to examine changes in the scope for growth (assimilation - respiration) of parasitized oysters. The model predicted that scope for growth would be reduced by 75% when large oysters (7 cm) were parasitized by 30 large snails (6 mm). A negative energy balance would exist when small oysters (3 cm) were parasitized by 15 large snails. On a typical oyster reef as much as 10-15% of the available energy for growth and reproduction may be diverted to Boonea impressa. Few metabolic changes were observed in small or large oysters when parasitized by low levels of parasites (10). At high levels of parasitism (25 and 30 parasites), all tissues in both large and small oysters demonstrated considerable metabolic alteration. Carbohydrate content decreased in small oysters and increased in large oysters. Lipid content increased in the mantle tissue of large oysters. The free amino acid (FAA) pool generally increased with parasitism examined because of increasing taurine concentrations. When taurine and hypotaurine were removed, the FAA pool decreased or remained stable. In all tissues examined, the presence of B. impressa altered the structure of the FAA pool. The established impact of B. impressa on the incidence and intensity of disease, energy balance, growth and biochemical composition of oysters, indicate that this ectoparasite may well be a determining factor in the health of oyster populations. Information acquired about this host/parasite relationship can be applied to a more general understanding of the functioning of host/parasite symbioses in the marine environment.