FISHERIES RESEARCH INSTITUTE,MOA,TAIWAN

Early development of the oyster Cressostrea gigas cultured in Taiwan wasanalyzed with emphasis on the processes of fertilization, sperm-egg interaction,meiotic maturation, and early embryogenesis, using light and epifluorescencemicroscopy and scanning electron microscopy. The oyster sperm is composed of aspherical head region with an anterior cap-like acrosome, and a posteriormitochondrial region, from which extends a 40 (m long flagellum. Theunfertilized eggs are approximately 50-60 (m, with irregular shape fromellipsoid to pear shape due to the extension of polar lobe in vegetal pole. Theegg is surrounded with thin, protruded vitelline envelope. When the sperm bindto the vetelline envelope they undergo acrosome reaction immediately andpenetrate through the vetelline envelope to fuse with the plasma membrane of theegg. At water temperature of 27 oC, meiotic divisions of the eggs resume afterfertilization. The first polar body extrudes about 8-12 minutes and the secondpolar body forms approximate 20-24 minutes after fertilization. The first andsecond division occur 40 and 45 minutes respectively after fertilization. Theoyster embryos rapidly reach the morula and ciliated blastula stages and becomemotile gastula about 8 hours after fertilization. The embryos pass through atrochophore stage and develop into a straight-hinge D-larvae approximately 16hours after fertilization.
The effects of water temperature (24, 27, and 30 oC) on meiotic maturationand cell division were examined. The kinetics of polar body formation and thefirst mitotic division are influenced by rising temperature, i.e., the higherthe temperature, the faster development of fertilized eggs and embryos proceed.The unsyncronized meiotic and mitotic development of the oyster occur usingstripped oocytes from the female, and this phenominum is more evident when usingthe mixture of the oocytes from several females.
The results clearly define the conditions and features for normal meioticmaturation and embryonic development of the oyster from local Taiwan and canprovide a basic guideline for successful breeding, culturing and induction ofpolyploid of the oyster in Taiwan.