Changes in chemical contents and enzyme activities during embryonic development of bluefin tuna [Thunnus thynnus]

Naturally fertilized bluefin tuna Thunnus thynnus eggs which were obtained from 7-year-old broodstock in a net cage were incubated until hatching at 27 degree C. No conspicuous changes were found in egg moisture and amounts of total nitrogen and phospholipid during the embryonic development. Egg protein gradually increased, but free amino acid decreased with the development. Triacylglycerole (TG), a main egg constituent of 420 mg/g dry egg at the stage of early cleavage, rapidly decreased after the stage of late gastrula and reached 140mg/g dry egg just before hatching. Activities of aspartate aminotransferase, alanine aminotransferase, creatine kinase, and lactate dehydrogenase fell between the stages of early cleavage and Kupffer's vesicle and rose thereafter. Activities of alkaline phosphatase remained high until embryo formation but fell rapidly thereafter. These results indicate that TG plays an important role as a main energy fuel, and that the differentiation and formation of organs are prompted immediately before or after the stage of Kupffer's vesicle during the embryonic development of bluefin tuna.