Effects of three microalgal powders on growth, metamorphosis, and intestinal antioxidant function of bullfrog (Lithobates catesbeianus) tadpoles

In the complete culture cycle of the bullfrog (Lithobates catesbeianus), the tadpole stage represents a critical developmental transition, accounting for approximately half of the entire cycle. Therefore, enhancing the growth and metamorphosis rate of tadpoles is of paramount importance. Microalgae are a group of widely distributed autotrophic or heterotrophic microorganisms, which have garnered attention due to their rich nutritional composition and ecological sustainability. This study investigated the effects of three microalgal powders on the growth, metamorphosis, and intestinal antioxidant function of L. catesbeianus tadpoles to identify the most effective supplement. A total of 1800 tadpoles initial body weight: (0.04±0.00) g were randomly assigned to 15 groups with three replicates each. Over a 90-day trial, groups received diets containing graded levels of Schizochytrium powder (0, 2, 5, 10, 15, and 20 g/kg; SC0-SC5), Spirulina powder (10, 20, 40, and 60 g/kg; SP1-SP4), and Chlorella powder (1, 5, 10, 15, and 20 g/kg; CH1-CH5). After the 90-day feeding trial, growth performance, metamorphosis parameters, serum thyroid hormone levels, intestinal enzyme activities, and intestinal antioxidant function were evaluated. The results show that larval average weight (LAW), post-metamorphic average weight (PAW), specific growth rate (SGR), feed intake (FI), and metamorphosis rate (MMR) were significantly higher in the SC4, SP, and CH4 groups than in other groups, while the feed conversion ratio (FCR) showed an inverse pattern. Moisture and ash content did not differ significantly among supplementation groups, nor did crude lipid content in tadpoles fed Spirulina powder. Crude protein content was highest with 20 g/kg Schizochytrium powder. Crude lipid content peaked at 10 g/kg Schizochytrium powder before declining at higher levels. For Spirulina powder, crude protein content reached a maximum at 60 g/kg. Both crude protein and crude lipid contents were highest at 15 g/kg Chlorella powder and decreased with further supplementation. Among the three optimal growth groups (SC4, SP3, CH4), the SC4 group exhibited significantly higher LAW and intestinal activities of α-amylase (α-AMS), lipase (LPS), Na⁺/K⁺-ATPase, creatine kinase (CK), catalase (CAT), and total antioxidant capacity (T-AOC) than the SP3 and CH4 groups (P < 0.05). The SC4 group also showed higher LAW, SGR, FI, MMR, serum levels of triiodothyronine (T₃) and thyroxine (T₄), and intestinal alkaline phosphatase (AKP) and superoxide dismutase (SOD) activities. In contrast, intestinal malondialdehyde (MDA) content was lower in the SC4 group. Schizochytrium, Spirulina, and Chlorella powders promoted tadpole growth and metamorphosis, likely by enhancing intestinal digestive and absorptive enzyme activities, improving intestinal antioxidant status, and elevating serum T₃ and T₄ levels. Based on LAW and MMR, the appropriate supplementation ranges are 13.39~15.06 g/kg for Schizochytrium powder, 34.05~37.42 g/kg for Spirulina powder, and 13.97~15.41 g/kg for Chlorella powder. Furthermore, 15 g/kg Schizochytrium powder proved most effective among the optimal groups. These findings provide a theoretical and practical basis for optimizing microalgal supplementation in bullfrog tadpole feed.