Ontogenetic digestive physiology and expression of nutrient transporters in Anguilla japonica larvae

Freshwater eel (Anguilla japonica) is the main aquaculture fish species in Korea, but artificial mass seed production has not been succeeded yet; eel aquaculture is therefore dependent on the seed of wild catches. Major problems in artificial production of eel seed are the low growth and survival rates of early larvae, likely may be caused by the feed used for the first and subsequent feedings. Thus, a physiological understanding of the digestive and absorptive capacities is necessary. This study was conducted for 50 days after hatching (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 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, increasing the protein and carbohydrate content and reducing the lipid content.