DESCRIPTION OF METAMORPHIC PHASES IN THE OYSTER CRASSOSTREA-VIRGINICA AND EFFECTS OF HYPOXIA ON METAMORPHOSIS

Four phases of metamorphosis in the eastern oyster Crassostrea virginica were characterized:'settlers' have attached to the substrate but retain larval characteristics; metamorphosis and degeneration of the velum has begun in 'prodissoconch postlarvae'; in 'dissoconch postlarvae' shell growth beyond the prodissoconch has begun but the foot persists; and 'juveniles' have lost all larval organs and metamorphosis is complete. These phases were used in examining the metamorphic process during and following continuous and short-term exposures to hypoxia (1.5 mg O-2 l(-1), 20 % of air saturation) and microxia (< 0.07 mg O(2)l(-1), < 1 % of air saturation). We observed no abnormal development in the oysters, but development was delayed following 3 d exposures to hypoxia, and 2 and 3 d exposures to microxia. Under continuous exposure to microxia, oysters did not develop to the dissoconch postlarva or juvenile phases. Approximately 50 % of the control oysters died within the 2 wk period following settlement. Morality was virtually confined to the settler and prodissoconch postlarva phases. Short-term exposures to hypoxia (1 to 3 d) and microxia (1 d) had little effect on the median mortality time or final total mortality, compared to controls. Microxic treatments longer than 1 d did affect mortality and oysters continuously exposed to microxia had a median mortality time of 87 h. Short-term exposures to low oxygen did not have permanent effects on post-settlement growth rates. Oysters exposed to microxic treatments, however, appeared to have slower growth rates during the exposure period. We conclude that low oxygen conditions, in particular those that are microxic and last longer than 24 h, have detrimental effects on the development, growth, and mortality of post settlement oysters.