Histological, histochemical and biochemical characterization of larval digestive system ontogeny in black tetra Gymnocorymbus ternetzi to inform aquaculture weaning protocols

Abstract Identifying an appropriate larval diet is a significant challenge in aquaculture. To overcome this bottleneck, characterization of larval fish digestive system ontogeny is critical for optimizing nutritional protocols. Intensive culture of characids for ornamental trade is highly dependent on live Artemia nauplii through the larval stage. Live feeds have unique challenges relative to prepared microparticulate diets (MDs), specifically pertaining to availability, labor, and cost. In this study, a feeding trial was conducted to compare three commercially available MDs in larval Black Tetra Gymnocorymbus ternetzi. Larvae exhibited poor survival and growth for MDs relative to Artemia. Digestive system ontogeny was characterized using histology, histochemistry, and digestive enzyme activity assays from the onset of exogenous feeding through the larval stage. G. terntzi exhibited an agastric larval stage, as well as low digestive enzyme activity at the onset of exogenous feeding, followed by increases in trypsin, lipase, and pepsin activity. Histological differentiation of the stomach, including gastric gland formation and production of periodic acid Schiff positive neutral mucopolysaccharides, as well as the onset of pepsin activity, occurred at 22 days post hatch (dph). This shift from agastric to gastric digestion indicated elevated digestive capacity and increased ability to consume a diversity of prey. Based on these data, experiments were conducted to evaluate different weaning times from Artemia to a MD. Larvae were fed Artemia exclusively from hatch through the end of the trial, a MD exclusively, or were transitioned from Artemia to a MD at three different time points. For G. ternetzi fed until 33 dph, weaning beginning at 13 dph exhibited higher survival (20.6 ± 1.8%) than live Artemia and weaning beginning at 18 dph and 23 dph (mean: 13.6 ± 1.5%), while the MD resulted in the lowest survival (0.8 ± 0.2%). These results indicate that weaning is possible prior to gastric differentiation, suggesting that it is possible to rely less on Artemia as a larval feed in this characid species.