| Simultaneous physiological (metabolic and excretion rates) and biochemical evaluations (shell/tissue growth, protein, lipid, carbohydrate content, aerobic enzyme activities) were performed on six oyster (Crassostrea gigas ) batches, in order to: (a) assess the role of endogenous energy substrates on early postlarval mortality, (b) quantify the energy demands of metamorphic and early postlarval stages, (c) investigate the whole-body response to post-starvation recovery, and (d) generate diagnostic indices for larval-postlarval condition. Evaluations were carried out during normal development (0 to 25 days post-settlement); enforced starvation (for 0 to 8 days), and post-starvation recovery (0 to 9 days feeding) of two week-old postlarvae.;All oyster batches exhibited a catabolic period associated with metamorphosis, followed by exponential growth. Protein/lipid levels were reduced by 23 to 28%, whereas weight-specific metabolic rates (VO2), ammonium excretion rates and activity of the electron transport system (ETS) were highest during metamorphosis. High shell growth rates (13 to 23% d-1) continued during this period. Postlarval mortality was accurately predicted by the amount of protein/lipid utilized during metamorphosis (r2 = 0.99), yet proteins appeared to be the limiting substrate for survival. Enforced starvation exerted both immediate and delayed whole-body responses of C. gigas postlarvae, including a linear decrease in tissue weight and ammonium excretion rate, delayed reduction of enzyme activities, and exponential reduction of metabolism. These response mechanisms effectively limited the loss of endogenous reserves after 2 days of starvation and helped to maintain the functional integrity of C. gigas spat during chronic starvation, thus resulting in minimal mortalities. Starved postlarvae were capable of resuming growth, build up energy reserves and increase their VO2 and enzyme activities after feeding resumption, yet starvation incurred a growth cost proportional to the duration of the starvation period. Protein: ash-free dry weight (AFDW) ratios > 37 and O:N ratios > 20 were associated with "healthy" organisms. Lower values were observed in stressed (metamorphic and starved) oysters. The VO2:ETS ratio readily reflected the physiological state of oyster postlarvae, but not of metamorphic oysters. Low VO2:ETS ratios (<0.18) were associated with stressed postlarvae. |
