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The nutritional requirements and metabolism of bivalves remain largely unknown. In this study, we investigated the effects of dietary carbohydrate/lipid ratios on growth performance, body composition, digestive enzyme activities, oxidative status, and transcriptome profile in juvenile razor clam Sinonovacula constricta. Five microcapsule diets with varying levels of carbohydrate (30–45%) and lipid (2–15%) were prepared and designated as C45L2 (45% carbohydrate and 2% lipid), C41L5, C36L8, C33L11, and C30L15 groups. Juvenile S. constricta with an initial shell length of 4.9 mm were fed with these microcapsule diets for 2 weeks. The results showed that the clam in the C33L11 group exhibited the fastest growth rate. The body protein content decreased with the decreasing dietary carbohydrate/lipid ratio. The clam in the C33L11 group showed significantly higher glycogen content compared to the C45L2, C41L5, or C30L5 group. The amylase activity of the clam in the C45L2 and C41L5 group was significantly higher than that in the C36L8, C33L11, and C30L15 group. The clam in the C33L11 group showed significantly higher catalase and superoxide dismutase activities compared to other microcapsule groups. The malondialdehyde level of the clam in the C30L15 group was significantly higher than that in the C45L2, C41L5, and C33L11 group. Transcriptome analysis showed that the autophagy pathway in the clam fed C41L5 was upregulated compared with those fed C33L11. Meanwhile, the protein levels of microtubule-associated protein 1 A/1B-light chain 3 membrane type (LC3-II) in the C41L5 group were also significantly higher than those in the C33L11 group. Moreover, the clam in the C41L5 group exhibited increased phosphorylation levels of the eukaryotic translation initiation factor 4E-binding protein (4E-BP) and decreased phosphorylation levels of the ribosomal protein S6 kinase beta-1 (S6K). Our results revealed that the optimal dietary carbohydrate/lipid ratio for juvenile S. constricta was approximately 3.01. The high-carbohydrate/low-lipid diet may repress the growth of clam by inhibiting mammalian target of rapamycin complex 1 (mTORC1) and subsequently activating autophagy, while the poor growth performance in the low-carbohydrate/high-lipid diet group may be associated with high lipid peroxidation levels in juvenile S. constricta. • The optimal carbohydrate/lipid ratio for juvenile S. constricta was about 3.01. • The body protein content was decreased with the decline of carbohydrate/lipid ratio. • Amylase activity increased with the increase of dietary carbohydrate. • Fit carbohydrate/lipid ratio enhanced SOD, CAT activities, and lower carbohydrate to lipid ratio increased MDA level. • Compared to C33L11 diet, C41L5 diet might lead to autophagy through mTORC1 pathway. [ABSTRACT FROM AUTHOR]

A 20 days feeding experiment and 15 days reversed feeding experiment were conducted to study the growth of Pampus argenteus juveniles under different feeding strategies. A total of 900 juveniles (an average ± SD, 5.88 ± 0.98 cm and 5.69 ± 0.64 g) were divided into three feeding groups and the experiments run in triplicate. Each experimental tank contained 1000 L of water and each group was fed to satiation four times a day. Three groups were constructed: group A (fed with 100% jellyfish), group B (fed with 100% Yubao 5# feed), and group C (fed with 50% jellyfish and 50% Yubao 5# mixed feed). The remaining 200 healthy individuals respectively in groups A and B were removed following the 20 days feeding experiment and subjected to 15 days of reverse feeding, resulting in group A' (fed with 100% Yubao 5# feed) and group B′ (fed with 100% jellyfish). Yubao 5# feed is 42% protein while live jellyfish larvae with a 1–2 cm umbrella diameter contain approximately 10% protein content. After 20 days, the results showed similar growth performance in terms of weight gain and specific growth rate (P > 0.05) for group A and B, indicating that the P. argenteus juveniles were able to survive on a jellyfish diet. However, the juveniles in group C showed significant (P < 0.05) weight gain and specific growth rate (8.96 ± 0.77 and 4.739 ± 0.572, respectively) over the same culture period. Serine protease (SP) was selected to study the detoxification mechanism of P. argenteus that were fed on jellyfish, and the SP enzyme activities, mRNA, and protein expression under different culture conditions were analyzed to illustrate the detoxification effect of SP protease on jellyfish toxin. The results showed that the highest SP enzyme activities occurred in the kidneys, liver, stomach, lateral capsules, caecum, and intestine (P < 0.05), with enzyme activity in the order group A > group C > group B, group A > A', and group B′ > B (P < 0.05). The highest enzyme activity of 188.91 UL /L was observed in group A when P. argenteus were fed jellyfish exclusively for 20 days (P < 0.05). The results indicate that jellyfish were the main contributors to the changes observed in the SP mRNA, protein, and enzyme activities. Our results indicate that P. argenteus juveniles can survive when feeding only on jellyfish and grow more rapidly when 50% jellyfish is added to the feed than when fed with jellyfish or Yubao 5# feed alone. SP enzymes, which are sensitive to jellyfish, play an important regulatory role in the digestion and decomposition of jellyfish by P. argenteus , and the kidneys are the most important organ for the detoxification and regulation of jellyfish toxin. • Pampus argenteus juveniles are able to maintain growth by feeding only on jellyfish. • Addition of jellyfish to feed significantly improves the growth of Pampus argenteus. • Serine protease in the kidney of Pampus argenteus plays a detoxifying role against jellyfish toxins. [ABSTRACT FROM AUTHOR]