인공생산된 뱀장어(Anguilla japonica) 자어의 성장에 따른 소화기관 구조 및 기능에 관한 연구

Abstract

Freshwater eel (Anguilla japonica) is rapidly decreasing in number and has not yet been successfully mass produced. This may be at least partially attributable to the unique and long early life history of the eel. So, it is necessary to develop feed and breeding technology tailored to unique eel larval stages. For this, understanding of the ecophysiology of eel larvae based on various analysis on ontogenesis should be preceded. Therefore, this study was conducted to understand the early developmental characteristics of artificially-spawned eel larvae through morphological, histological, molecular biological, and biochemical analysis. Finally, it was intended to contribute to the establishment of the artificial mass seed production of eel by providing basic data necessary to improve diet and rearing protocols. Exp. 1: This study was conducted for 200 days after hatching (DAH) and analyzed morphometry and allometry for eel larvae. The following artificially-spawned eel larval stages were identified: the yolk sac larvae stage (0–6 DAH, 3.23–6.85 mm total length (TL)), the pre-leptocephalus stage (7–30 DAH, 6.85–15.31 mm TL), and the leptocephalus stage (50–200 DAH, 15.31–60.06 mm TL). Artificially-spawned and wild eel larvae could be divided into characteristic larval stages at similar sizes. However, compared to wild eels, artificially-spawned eels had a slower growth rate and fewer preanal myomeres. Meanwhile, artificially-spawned eel larvae rarely had a mixed feeding period as the absorption of endogenous reserves was completed by 7 DAH. The lower jaw of eel larvae was significantly longer than the upper jaw at 50 DAH. In the pre-leptocephalus and leptocephalus stages, eel larvae showed continuous positive allometric growth at trunk height and tail muscle height with change to the willow leaf-like form. These growth characteristics may be the result of adaptation to the migration over long distances and to a diel vertical migration. The inflection point in the body parts growth patterns showed only before 30 DAH, and mass mortality appeared at this period. Therefore, to improve the growth and survival rates of artificially-spawned eel seed, it is necessary to focus on improving the feeding and rearing protocol until 30 DAH. Exp. 2: Histological changes in the digestive organs of the larvae as they grew from 0 DAH to 50 DAH were studied. The endogenous reserves were completely absorbed at 7 DAH, and the first exogenous feeding started thereafter. The teeth appeared at 4 DAH and dramatically changed from needle-like to broad-based types by 50 DAH. The oesophagus was a thin tube composed of simple cuboidal epithelium at hatching, but 5 DAH, it began to be divided into of mucosa, submucosa, muscularis externa, and adventitia. The intestine remained as straight gut until 50 DAH, and the intestinal cilia appeared first at 6 DAH and their density continuously increased thereafter. Interestingly, goblet cells appeared in the intestine, but not in the esophagus. In addition, high amounts of eosinophil substances, assumed to be zymogen granules, accumulated in the pancreas, suggesting a specific digestive strategy of eel larvae. Exp. 3: This study was conducted for 50 DAH; we investigated the activities of digestive enzymes (trypsin, chymotrypsin, amylase, lipase), expression levels of digestive enzymes (trypsin (try), amylase (amy), pancreatic lipase (pnlip)), and expression levels of nutrient transporters (large neutral amino acids transporter small subunit 2 (slc7a8), sodium/glucose co-transporter member 1 (sglt1), Niemann-Pick C1-like 1 (npc1l1)). The try and slc7a8 expression increased with fluctuations until 50 DAH and showed a similar trend. The amy and sglt1 expression increased until 50 DAH and appeared to a similar trend. The pnlip and npc1l1 expression were relatively high until 15 DAH and decreased thereafter. And then, pnlip expression increased again from 30 DAH and npc1l1 expression maintained low until 50 DAH. Meanwhile, expression levels of digestive enzymes were highest in try, followed by amy and pnlip. And, the expression levels of nutrient transporters showed the highest values in the order of sglt1, slc7a8, and npc1l1. These findings suggested that protein and carbohydrates are more important than lipids in eel larvae. All digestive enzymes activities were low until 15 DAH and then rapidly increased to 30 DAH; these levels were maintained until 50 DAH. Eel larvae are considered to have insufficient and unstable digestive ability before 30 DAH. Therefore, in order to improve the growth and survival rate of early eel larvae, this study is suggested that the composition of the current slurry-type diet could consist of easily digestible low-molecular substances before 30 DAH, and increase the protein and carbohydrate content and reduce the lipid content.