| The American shad (Alosa sapidissima) is an economically and ecologically valuable anadromous species native to the Atlantic coast of North America from the St. Lawrence River in Canada, to the St. Johns River in northern Florida. In the past few decades, owing to Chinese shad (Tenualosa reevesii) endangered, American shad introduced into China as a candidate species for aquaculture. At present, the farming scale of American shad in china increases gradually and the culture technology becomes more and more successful. However, during the early development of Alosa sapidissima larvae, a comprehensive study on the developmental biology is not currently reported. In this study, the histological traits of embryonic and organic development, feeding characteristic and enzymatic activity in American shad were analyzed. The results are as follows:1. Under the temperature of 21.0±0.5,℃, the hatching occurred about 71h15min after fertilization. The embryogenesis of American shad was divided into six main period. The first cleavage furrow showed longitudinal fissure, after that the horizontal cleavage was visible at the sixth cleavage. The blastocoel and yolk syncytial layer were obvious during blastula period. The brain rudiment, optic capsule and somite were formed at 90%-epiboly period. The neurula and Kupffer’s vesicle were visible at 100%-epiboly period. During the organ differentiation period, the heart, digestive tube, eye, nephridium and brain appeared. The hatching larvae had a large and homogeneous yolksac.2. The present study makes a comprehensive description about the organogenesis at histological level in larvae of American shad from 0 until 45 days after hatching (DAH). According to its nutrient sources and the external morphological features, and the structural changes of the organs, larval development was divided into four stages:stage 1 (0-2 DAH), stage 2 (3-5 DAH), stage 3 (6-26 DAH) and stage 4 (27-45 DAH). At early stage 2(3 DAH), American shad larvae acquired the initial digestive and absorptive necessary equipment, such as mouth and anus opening, buccopharyngeal cavity, oesophagus, incipient stomach, anterior and posterior intestine distinguish, differentiated hepatocytes and exocrine pancreas. With the assurance of predation, the digestive and absorptive capacity further developed in stages 2 to 3 and appeared pharyngeal teeth, taste buds, gut mucosa folds, differentiated stomach, gastric glands. Four defined compartments were discernible in the heart at 4 DAH. From 3 DAH to 13 DAH, the excretory systems started to develop accompanied by urinary bladder opening, the appearance and development of primordial pronephros, and the proliferation and convolution of renal tubules. Primordial gills were detected at hatching, pseudobranch was visible at 6 DAH, and then the filaments and lamellae greatly proliferated during stage 3.The primordial swim bladder was seen at 2 DAH and started to inflate at 9 DAH, from then on, it constantly expanded. The spleen was first seen at 8 DAH and thymus was evident at 12 DAH. From stage 4 onwards, most organs essentially manifested an increase in size, number and complexity of tissue structure.3. In this study, the allometric growth pattern of American shad larvae was studied in ecology method. The results were shown that some of important organs associated to feeling, swimming and feeding exhibited allometric growth in the early ontogeny. The growth of head length, head and body height exhibited positively allometric during the entire period of larval development, while growth of trunk in length was negatively allometric:As for organs in head, the growth inflexion point of month width and rostrum were 24 and 23 days after hatching, respectively, both of them exhibited positively allometric, and then became isometric allometric, the increase of the eye diameter was positively allometric throughout development. As for swimming organs, the inflexion point of tail fin was 24days, after that the increase of tail fin was isometric. Pectoral fin exhibited positively allometric growth. By analyzing the inflexion points in allometric growth curves and the morphological development of the larvae, we found that some selected organs associated to feeling, feeding and swimming had developed prior to other ones. Therefore, in rearing American shad larvae, we must optimize the environment of growth and supply enough nutritious food to ensure the priority order of organ development, and then increase the survival rate of larvae during the early life stages.4. In order to better understand the feeding characteristics of Alosa sapidissima at the early development stage, the effects of starvation on growth and morphology, the point of no return (PNR) of Alosa sapidissima larvae, and the feeding rhythm of Alosa sapidissima larvae and juveniles were researched by experimental ecology approach under the water temperature of 20.5±1.0℃. It has been clearly described that newly hatch larvae were (7.45±0.15) mm in total length (TL), (2.01±0.06) mg in body weight (BW) and the yolk sac was (5.99±0.13) mm3 in volume. At 5 days after hatch (DAH), the yolk sac of Alosa sapidissima larvae was absorbed completed. Both TL and BW of control group increased gradually with the consumption of yolk sac and the establishment of exogenous nutrient. In contrast, the TL and BW of the starved larvae started to decrease from 6 DAH,and all the starved larvae died out at 10 DAH; The first feeding time of Alosa sapidissima larvae was at 3 DAH, the feeding rate reached to 46.67%, the higtest feeding rate was 89.29% in 5 DAH, the PNR was 7-8 DAH, according to the yolk sac absorption and PNR, the the optimal time of initial feeding should be 2-3 DAH. The feeding rhythm of Alosa sapidissima larvae and juveniles exhibited that the feeding peak of early stage larvae (4 DAH) appeared 14:00, and the feeding amount reached to 0.14 mg; the feeding peak of late stage larvae (17 DAH) was at 10:00 and 14:00, the feeding amount was 0.92 mg and 1.36 mg, respectively; the juvenile at 28 DAH reached three feeding peaks at10:00,14:00 and 18:00, the feeding amount was 1.77 mg,2.45 mg and 1.55mg, respectively. Therefore, Alosa sapidissima was typically feeding only in daytime,and the feeing peak period was gradually prolonged with fish growth. The result of daily feeding situation of Alosa sapidissima showed the feeding amount in daytime was 20.08% (4 DAH),29.48%(17 DAH) and 12.41%(28 DAH), which could be a reference for Alosa sapidissima daily feeding.5. In order to assess the digestive physiological capacity of this species and establish feeding protocols matching the larval nutritional requirements, the ontogenesis of digestive enzymes (trypsin, amylase, lipase, pepsin, alkaline phosphatase, leucine aminopeptidase) in American shad larvae was studied from hatching (0 DAH) until 45 DAH. The results showed that with the exception of pepsin, all the analysis of digestive enzymes were already present in American shad larvae at hatching, indicating initial digestive ability of food items and precocious development of the digestive system. Trypsin suddenly increased to reach the maximum value at 14 DAH, and then continued to decrease slightly until 45 DAH. Amylase sharply increased to 10 DAH, after that a slight decline until 16 DAH, and increased again at 33 DAH coinciding with microdiets introduction at 30 DAH, indicated that the increase may be associated with the carbohydrate content of the microdiets used for feeding larvae. Lipase increased until 14 DAH, whereas it went down to 27 DAH and then increased until 45 DAH. Pepsin was firstly detected at 27 DAH, and sharply increased until 45 DAH, suggesting the formation of functional stomach. Both alkaline phosphatase and leucine aminopeptidase markly increased to 18 DAH, indicating the maturation of intestine. After that abruptly increased again at 30 DAH, which suggested a transition of mode of digestion. |
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