Studies On Early Development And Related Physiological Traits In Turbot

Turbot Scophthalmus maximus distributes mainly in northeast Atlantic, and is characterized by rapid growth and good meat quality. At present, turbot is an important commercial species in northern China. The foundation of its breeding and aquaculture technology pushes the development of marine fishes aquaculture in China. The early development of fish is important base for explanation of its life history. In this study, the traits of embryonic development, growth, digestive physiology, immunity and skeletal development of turbot in early life stages were analyzed. The results are as follows:1. Embryonic development of turbot was divided into six main stages. Hatching occurs 108 hours post-fertilisation (hpf) in 14°C. Yolk syncytial layer and blastocoel were formed at multiple cell stage and low stage, respectively. Neural rod and somite were formed at 90%-epiboly stage, and notochord primordium was formed at complete epiboly stage. Kupffer's vesicle appeared at 59h35min hpf and degenerated at 89 hpf. The posterior digestive tract of embryo was ciliated. The specific pigment cluster appeared in fin fold of newly hatched larva.2. The development of digestive system had four stages in turbot. The first one was from hatching to mouth opening. Digestive tube developed rapidly at that period. The larvae started to open mouth and feeding on 4 DAH. At that day, the digestive tube was differentiated into buccopharynx, esophagus, stomach, anterior and posterior intestine. The larval digestive system was morphologically ready to digest external food at this time. The second stage was from 4 DAH to 16 DAH. During this period, the acidophilic granules and vascular structure appeared in digestive tube, indicating the start of pinocytosis ingestion. The number of mucosal folding in intestine increased gradually, and liver and panereas developed further. The third stage was from 16 DAH to 35 DAH. Gastric glands appeared at 16 DAH and pyloric caecum appeared at 20 DAH. Mucosal folding in intestine, brush border and goblet cells grew rapidly. At 60 DAH, the digestive system completed development.3. The sequence of immune organ anlages appearance were head kidney, spleen and thymus. At hatching, kidney was a paired of pronephric tubules and primordial stem cells were observed. Spleen anlage was present at 5 DAH and it developed slowly. Progenitor thymus appeared at 13 DAH and grew quickly. An outer zone and an inner zone in the thymus were observed. Cell migration occurred between thymus and head kidney. The first functional lymphoid organ was thymus followed by head kidney and spleen. During the posterior developmental period of the immune organs, the melano-macrophage centers (MMCs) were found in spleen and head kidney, but not in thymus. The abundance in spleen was higher.4. Amylase was not found in turbot eggs and embryo. Activities of alkaline phosphatase (AP), trypsin, pepsin and leucine-alanine peptidase (Leu-ala) were high in early and late embryonic developmental stages, but relatively low during middle stages. However, pepsin activity disappeared at hatching. Leucine aminopeptidase N (LAP) activity showed no obvious changes during different embryonic developmental stages. Trypsin sharply increased to the climax at 17 DAH. Then abrupt decrease was observed until 31 DAH followed by a relatively stable level until end of this experiment. Amylase was determined at 4 DAH and reached the maximum value at 19 DAH and then declined sharply to 39 DAH and remained at a low level. Pepsin was firstly detected at 17 DAH and increased to 34 DAH, and then remained at a stable level, indicating the transition of digestive mechanism. AP and LAP activities markedly increased to 23 DAH, decreased abruptly to 50 DAH and increased gradually to 60 DAH. Leu-ala specific activity was low before 23 DAH. After that, this enzyme reached the plateau until 39 DAH, followed by a decline to 60 DAH. The activities of the BBM bound enzymes increased from 30% of the total activities at 31 DAH to approximately 81% at 38 DAH, indicating the maturation of intestinal tract and the transition of digestion mode. 5. A significant increase of specific growth rate during metamorphosis and retarded growth rate during post-metamorphic phase were observed. Ontogenetic patterns of DNA, RNA and protein parameters all showed developmental stages-specific traits. RNA:DNA ratio decreased up to 12 DAH, then increased rapidly till 19 DAH and fluctuated until 35 DAH followed by a decline to the end. RNA:DNA ratio was positively correlated with growth rate of turbot juveniles during post-metamorphic phase, while this ratio was not a sensitive indicator of growth during pre-metamorphic phase and metamorphosis. Protein:DNA ratio showed a similar tendency to RNA:DNA ratio. Changes of DNA content and protein:DNA ratio revealed that growth of turbot performed mainly by hyperplasia from 4 DAH to 12 DAH and hypertrophy until 21 DAH during pre-metamorphic larval phase. Growth was dominantly hypertrophical from early- to mid-metamorphosing phase and hyperplastic thereafter.6. Skeletal development of turbot showed that vertebral ontogeny started with the formation of anterior haemal arches at 5.1 mm standard length (LS), and was completed by the full ossification of parapophyses at 16.9 mm LS. Vertebral centra started to develop at 6.3 mm LS and ossification in all centra was visible at 11.0 mm LS . The caudal fin appeared at 5.1 mm LS and ossification was visible at 20.6 mm LS. The onset of dorsal and anal fins elements appeared at 5.8 mm LS and 6.3 mm LS respectively. Ossifications of both dorsal fin and anal fin were visible at 20.6 mm LS. The pectorals were present before first feeding, their ossifications were completed at 23.5 mm LS . Pelvic fins began forming at 7.2 mm LS and calcification on the whole structure was visible at 19.8 mm LS. In turbot, 24 types of skeletal abnormalities were observed. As for each developmental stage, the most common were dorsal fin abnormalities during early-metamorphic period, vertebral fusion during climax metamorphosis, and caudal fin abnormality during both late-metamorphic period and post-metamorphic period.