FEEDING ABILITY DURING SETTLEMENT AND METAMORPHOSIS IN THE OYSTER CRASSOSTREA-VIRGINICA (GMELIN, 1791) AND THE EFFECTS OF HYPOXIA ON POSTSETTLEMENT INGESTION RATES

The feeding ability of oyster (Classostrea virginica Gmelin) larvae during Settlement and metamorphosis was observed. Also examined were the effects of hypoxia (1.9 mg O-2.1(-1), 25% of air saturation) and microxia (<0.4 mg O-2.1(-1), <5% of air saturation) on ingestion rates of post-settlement oysters. Velar feeding occurred during the searching and crawling stages of settlement, but not during cementation. In newly settled larvae, 16% engaged in velar feeding while 84% did not feed. All later metamorphic phases fed, although the mode of particle capture was unclear. Under normoxic conditions mean rates of cell ingestion by post-settlement oysters (268-651 mu m in shell height) ranged from 600 to 3000 cells.h(-1) individual(-1). In response to hypoxic treatments, post-settlement oysters with shells heights of 469 +/- 103 mu m and 651 +/- 131 mu m maintained their normoxic rates of ingestion but oysters with shell heights of 436 +/- 60 mu m reduced their ingestion rates to 54-61% of normoxic rates. These oyster sizes differed in the extent of gill development and proliferation. In response to microxic treatments, ingestion rates were 1% to 14% of normoxic rates and decreased with body size. We conclude that oysters have the ability to feed at nearly all stages of settlement and metamorphosis and that hypoxic conditions will affect the feeding activities of only the youngest post-settlement oysters while microxic conditions will affect all post-settlement oysters.