The effect of the ectoparasitic pyramidellid snail, Boonea impressa, on the growth and health of oysters, Crassostrea virginica, under field conditions.
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Boonea (= Odostomia) impressa are contagiously distributed on oyster reefs so that some oysters are parasitized more than others. The parasite's mobility and the ability of oysters to recover from snail parasitism may be important in assessing the impact of parasitism on oyster populations. During a 4-week exposure period in the field, B. impressa reduced American oyster, Crassostrea virginica, growth rate and increased the intensity of infection by the protozoan, Perkinsus (=Dermocystidium) marinus, but produced few changes in the oyster's biochemical composition because, although net productivity was reduced, the oysters retained a net positive energy balance (assimilation > respiration). During a 4-week recovery period, growth rate returned to normal (control) levels, but infection by P. marinus continued to intensify in previously parasitized oysters kept B. impressa-free. Most changes in biochemical composition during recovery, including increased lipid and glycogen contents, could be attributed to the continuing increase in infection intensity of P. marinus. Consequently, the temporal stability and size of snail patches, particularly as they regulate infection by P. marinus, may be the most important factors influencing the impact of B. impressa on oyster reefs.