M.E. Valdez-Ramirez, M. Le Pennec, G. Dorange, N. Devauchelle, G. Nonnotte
Abstract:
The quality of gametes in aquaculture is usually assessed from their capability of producing viable larvae after fertilization. Results obtained in hatchery on bivalve molluscs have highlighted that at the spawning time, once fertilized the mature-like oocytes produced embryos and larvae with variable survival and abnormality rates.
With the aim of understanding the nature of this problem, we attempted to establish possible relations between some oocyte- and embryo-characteristics and larvae viability from the study of various criteria: i) morphological, like oocyte diameter and shape; ii) cytological, such as overall ultrastructure and membrane integrity; iii) biochemical like, contents in lipids, proteins and glucides; iv) physiological, such as respiration. The survival and abnormalities rates were estimated 24 hours after fertilization.
The gonads of adult reproductive Japanese oysters, Crassostrea gigas grown in the natural environment (Bay of Brest and Aber-Benoit, Brittany, France), were stripped to produce sets of oocytes, 3-hour embryos and 24-hour larvae for analysis purpose.
Cytological investigations were carried out on 39 experimental spawnings whereas physiological studies concerned 38 other ones. The first results showed that 80 to 90% oocytes were cytologically viable before fertilization. This evaluation calculated from oocyte staining with Trypan blue highlighted that this parameter was only slightly correlated with larva viability. Two exceptions were noticed: viability did not exceed 20%. Therefore, this led us to consider that the interest of Trypan blue staining stands in that it allows the recognition of oocytes with damaged membranes.
Respiration was determined first from O2-consumption of unfertilized oocytes measured 5 min and 6 h after stripping, and secondly from O2-consumption of 3-hour embryos. These results showed positive correlations between the O2-consumption of embryos and fertilization rate (r=0.74 p=0.001) and between the O2-consumption of embryos and hatching rate of 24-hour larvae, (r=0.56 p=0.001). On the other hand, no correlation was found between hatching parameters and the O2-consumption of unfertilized oocytes. These results suggest that embryos possess quality indicators, which can be quantified from their metabolism more easily than in oocytes. Morphological and biochemical results are being processed.
(Biologie Marine, UMR CNR5 6539, I.U.E.M., 29280 Plouzane, Brest, France)