IS IMPROVED REPRODUCTIVE CAPACITY A TOLERANCE ADAPTATION TO Perkinsus marinus IN THE OYSTER Crassostrea virginica?

While oyster aquaculture development in the Chesapeake Bay region has received much recent attention, the rebound of natural Crassostrea virginica populations is even more remarkable following decades of devastation by parasites Haplosporidium nelsoni and Perkinsus marinus. It raises questions of how oysters might be interacting more successfully with these pathogens. With regard to H. nelsoni, a compelling case can be made for resistance evolution based on studies of oysters from Chesapeake as well as Delaware Bay. In the case of P. marinus, however, infection pressure and infection prevalences and intensities remain historically high, yet oyster survival is improving. Instead of resistance, tolerance adaptation may be at work, as oysters continue to be infected at relatively high levels but with reduced physiological consequences.

Histological analyses of oysters collected in recent years from the Virginia part of the Chesapeake Bay show oysters undergoing normal gametogenic and spawning cycles, with relatively few oysters (generally under 5%) rendered non-reproductive by infections. The majority of these individuals are infected with the trematode Bucephalus, a parasitic castrator.  Initial review of histology from the early 1990s, on the other hand, suggested that oysters were markedly more impacted by P. marinus infections in that period than in recent years. To gain perspective into whether the reproductive capacity of P. marinus-infected oysters has improved, we began a systematic evaluation of the reproductive condition of oysters in summer samples from lower Chesapeake Bay from 1992-present. If evident, the timing of such an improvement could be investigated as a potential indicator of increased tolerance in contemporary oyster populations. While quantitative analyses of oyster fecundity through the time period, as evaluated using archival histological slides, still lies ahead, preliminary qualitative analyses of female reproductive maturity have already been revealing. While 29% of oysters in the 1992-2002 period presented numbers of mature oocytes at peak reproduction that were rated "insignificant" or "low", just 5% of oysters in the post-2002 period presented such low numbers of mature oocytes. Greater proportions of oysters in the recent period appear to be more fully reproductive.

The very intense P. marinus epizootic of 1999-2002 has had the appearance, in retrospect, of a transformative event. The baseline of P. marinus infection in Virginia oysters shifted upward after that epizootic to produce the historically high levels of infection we generally observe today. Emerging histological evidence suggests that it may have been so intense as to have been transformative also of the oyster response to P. marinus, which in reproductive terms has improved following the 1999-2002 event.