Effects of sublethal infection by the parasite Haplosporidium nelsoni (MSX) on gameto-genesis, spawning, and sex ratios of oysters in Delaware Bay, USA

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Abstract
Haplosporidium nelsoni (MSX) is an ascetosporan parasite that has caused heavy mortalities of oysters Crassostrea virginica (Gmelin) on the mid-Atlantic coast of the United States. Oysters in some areas have developed resistance to mortality, which involves an ability to restrict infections and tolerate parasitism for prolonged periods. Effects of such sublethal infections on gametogenesis and spawning were examined in native oysters in Delaware Bay, an estuary where the parasite is enzootic. A comparison of infection levels with gonad state in histological sections of 2700 oysters demonstrated a clear inhibition of gametogenesis, in proportion to infection intensity, during late spring when parasite levels were high. Subsequently, however, temperature-associated infection remission occurred; many oysters recovered, developed mature gonads, and spawned before new or recurrent infections proliferated in fall. Inhibition of early gametogenesis was more severe in males than in females. There was no evidence that spawning ''stress'' accelerated the development of infections. There was no correlation between year-to-year fluctuation in parasite abundance and oyster setting in Delaware Bay.

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