Effects of feeding strategies on growth and development, liver and intestinal function of Lithobates catesbeiana tadpoles

Bullfrog (Lithobates catesbeiana) tadpole culture technology is still in a vulnerable stage, and metamorphosis rate is an important index of L. catesbeiana tadpole culture. Traditional empirical feeding strategy may lead to insufficient or excessive feeding of L. catesbeiana tadpoles, affecting the digestion and absorption of nutrients and the development of internal organs. Eventually, it leads to slow growth and development, light weight after metamorphosis, poor immunity and it is easy to corrupt water body and cause other problems. The feeding strategy has a significant effect on the growth and development of tadpoles, but no scientific feeding standard has been established. Therefore, four feeding frequencies (1, 2, 3, 4 times/d) and four feeding levels (4%, 6%, 8% and 10%) were set to feed L. catesbeiana tadpoles to explore the appropriate feeding strategy for tadpoles. The feeding frequency experiment showed that the metamorphosis rate of tadpoles fed 3 times/d or 4 times/d was significantly higher than that of tadpoles fed once or twice/d, and the weight gain rate of 3 times/d was the highest (P>0.05). The metamorphosis to feeding ratio (MFR) increased with the increase of feeding frequency and was significantly higher for 2, 3, and 4 times/d than once/d. Feeding frequency had no significant effects on liver structure and intestinal protease, lipase, and amylase activities of froglets. The feeding level experiment showed that the metamorphosis rate increased with the feeding level increasing, and the average weight of metamorphosed tadpoles in 8% and 10% groups was significantly higher than that in 4% and 6% groups, but the intestinal protease activity of metamorphosed tadpoles decreased with the feeding level increasing. The content of lipid droplets in hepatocytes of metamorphosis tadpoles increased significantly in 8% and 10% groups, and fat deposition was obvious. The feeding level of 8% increased the relative abundance of Caldilinea (Chloroflexi) and Cetobacterium (Fusobacterium), and improved the intestinal microbial structure to a certain extent. In conclusion, it is recommended to feed L. catesbeiana tadpoles 3 times a day at a feeding level of 8%. Research on the scientific feeding strategy of L. catesbeiana tadpoles can improve the L. catesbeiana research system and increase the production efficiency of L. catesbeiana.