Growth and survival of larvae of Mercenaria mercenaria (L.) and Crassostrea virginica (Gmelin) relative to broodstock conditioning and lipid content of eggs

Abstract

Artificial conditioning of broodstocks of Mercenaria mercenaria L. and Crassostrea virginica (Gmelin) was initiated at various times throughout the year under both laboratory and commercial hatchery conditions. Attempts were made to spawn broodstocks at weekly intervals, and data were collected on the initial total egg lipid content of eggs by analytical procedures and by visual inspection after staining with Oil Red O. The physical parameters of egg diameter and density (estimated by density gradient centrifugation) were correlated with egg lipid content. Significant correlations were evident between the initial egg lipid content and survival to both straight hinge (24 h) and pediveliger stages ( P<0.01); however, egg lipid content did not correlate with subsequent larval growth rate. A characteristic of high survival to the straight hinge stage (i.e., >75%) was catabolism of a minimum of about 4.5 ng total lipid during the embryonic stages (i.e., fertilization through to prodissoconch I shell formation) in both bivalve species. Good survival to the pediveliger stage (i.e., >1%) was always predicated by eggs containing a minimum of 12% lipid of the ash-free dry weight (AFDW); however, relatively high lipid levels did not guarantee good larval survival. Species-specific egg density profiles were clearly evident; eggs of C. virginica banded unimodally at 1.075 g·cm −3 while eggs of M. mercenaria banded trimodally at 1.038, 1.059 and 1.073 g·cm −3. Egg lipid was logarithmically related to egg diameter. The data suggest that there is a minimum, size-related, threshold lipid level in eggs necessary for optimal survival through the non-feeding embryonic stages; but environmental or genetic factors other than egg lipid content are responsible for a considerable fraction of the mortality during this period. Variations in the broodstock conditioning protocol induced large fluctuations in egg lipid levels, suggesting that strict attention should be paid to conditioning if optimal culture potential is desired. Minor variations in egg lipid levels (i.e., 4–8% AFDW) were visually discernible by staining with Oil Red O in both laboratory and hatchery environments. Evaluating eggs at the time of spawning for their potential for survival using the lipid-staining technique can provide valuable real-time information for both research and commercial hatchery applications.