Ontogeny of the digestive enzymes, thyroid hormones and cortisol in developing embryos and yolk-sac larvae of turbot (Scophthalmus maximus L.)

Abstract Activities of the digestive enzymes and concentrations of thyroid hormones and cortisol in developing embryos and yolk-sac larvae of turbot ( Scophthalmus maximus L.) were measured in this study. Results showed that the embryonic mass presented “decrease-increase-decrease-increase-decrease” trend, appearing the minimal value of 0.34 mg at 7.5 h after fertilization (HAF) before the onset of blastula stage. As for egg diameter, it increased on the whole from 1.027 mm at 0.5 HAF to 1.079 mm at 95.67 HAF. In general, digestive enzymes presented four patterns in the present work. The patterns of alkaline phosphatase (AKP), pepsin (PG), chymotrypsin (CP), trypsin (TRY), leucine aminopeptidase N (LAP), lactate dehydrogenase (LDH) and glutamic oxalo acetic transaminase (GOT) were similar, showing low level before segmentation stage followed by increases until hatching day. Besides AKP, the other enzymes reached the peaks at 2 days after hatching (DAH). Enzyme activities of acid phosphatase (ACP), lipase (LIP) and pyruvate kinase (PK) presented similar “decrease-increase” changes on the whole. Leucine-alanine peptidase (LAPP) was detected from fertilized eggs onwards, increased till 86 HAF at segmentation stage and then decreased thereafter. Amylase (AMY) activity was undetectable before gastrula stage and presented upward tendency thereafter. T 3 concentration declined to the lowest value of 61.82 ng/g sample at 3.17 HAF and exhibited “increase-decrease-increase” patterns during cleavage stage, then it elevated markedly until 29.5 HAF, and then maintained high stable levels before the onset of yolk-sac larval days, reaching the highest value of 220.62 ng/g sample at 72.5 HAF, followed by obvious decline until the end. T 4 concentration showed opposite tendencies to that of T 3 before segmentation stage with the maximal value of 160.95 ng/g sample at 39.17 HAF and the minimal value of 85.94 ng/g sample at 24 HAF, and remained high levels before 0 DAH, followed by a slight decrease thereafter. Cortisol concentration presented three obvious declines during 2.67–4.67 HAF, 5.67–9.67 HAF and 10.67–24 HAF. The maximal value was 147.86 ng/g sample at 5.67 HAF and the minimal value was 73.60 ng/g sample at 24 HAF, respectively. The above results are very valuable to illuminate the evolvement of enzymes and hormones to better understand their functions during endogenous nutritional periods and serve as a basis for future related studies in this fish.