Development profile of digestive enzymes and intestinal nutrient transporters during the early ontogeny of Himalayan snow trout, Schizothorax richardsonii

This study characterized the ontogenic profile of key digestive enzymes and intestinal nutrient transporters in Schizothorax richardsonii larvae from 0 to 21 days post-hatch (dph; at 21.5 ± 0.2 °C) using biochemical and molecular approaches. The larvae exhibited an exclusive lecithotrophic phase until 8 dph, followed by an endogenous-exogenous transition phase until 15 dph (fed Artemia nauplii and larval diet), and complete exogenous feeding with larval diet during 15–21 dph. Regarding larval growth, weight gain exhibited an exponential trend, while the gain in total length followed a linear trend, increasing by 478% and 114%, respectively. The specific activity of trypsin and leucine aminopeptidase showed an increase at 8 dph and fluctuated thereafter during exogenous feeding, whereas total proteases and chymotrypsin activity was found to be high from 12 to 21 dph. Similarly, the expression of carboxypeptidase and peptide transporter pept1 showed a significant increase during 15–21 dph, as compared to 5–7 dph. Concerning lipid uptake, the mRNA levels of bile-activated lipase, hepatic lipase (lipc), and mttp showed an increasing trend during exogenous feeding, with peak expression at 21 dph. The amylase activity and the mRNA abundance of maltase (mgam), sodium-glucose cotransporter 1 (sglt1), and glucose transporter-2 (glut2) were found to increase significantly at 15–21 dph during exogenous feeding with formulated feed, as compared to the yolk absorption phase. Conversely, the abundance of apolipoprotein-b (apob) transcripts during the lecithotrophic phase indicates their potential role in utilizing yolk nutrients. The alkaline phosphatase activity showed a significant increase from 15 dph and peaked at 21 dph, indicating gut maturation and its role in aiding alkaline digestion. Overall, the development profile of digestive enzymes and intestinal nutrient transporters clearly indicates dietary stimulation and regulation, which is relevant for weaning and hatchery management of this agastric teleost.