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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]

Melatonin affects physiological processes that have circadian and seasonal cycles and causes behavioural changes in nearly all organisms, ranging from bacteria to mammals. However, no studies have focused on the presence, biosynthetic pathway, and rhythms of melatonin in bivalves, which are considered as the class of molluscs with the highest economic value. Here, the metabolic pathway and tissue distribution of melatonin, tissue transcription profiles of melatonin-related genes [key synthases arylalkylamine N-acetyltransferase type 2 and 5 (aanat2 , aanat5), N-acetylserotonin methyltransferase (asmt), and melatonin receptor type 1 A1 and C3 (MTNR1A1 , MTNR1C3)], daily changes in melatonin, transcripts of genes, and effects of four lights (blue, cyan, orange, and red) during the night on melatonin secretion were studied in a bivalve species, razor clam Sinonovacula constricta. Our results showed the presence of complete metabolic and circadian entrainment pathways of melatonin. The melatonin content in the lymphocytes and labial palp was higher than that in the other studied tissues, and aanat2 , aanat5 , asmt , MTNR1A1 , and MTNR1C3 mRNA levels were the highest in the labial palp. Melatonin fluctuated rhythmically in lymphocytes and labial palps and increased at night. Similarly, aanat2 , aanat5 , asmt , MTNR1A1 , and MTNR1C3 mRNA levels were rhythmic in the labial palp, peaking at night. In addition, blue light effectively inhibited the nocturnal secretion of melatonin in the clam. In summary, our results suggest that melatonin plays a role in light signal transduction and the circadian system in S. constricta. • Melatonin could be biosynthesized by the clam Sinonovacula constricta. • Labial palp probably is an important tissue where melatonin exerted its function. • The biosynthesis and secretion of melatonin in the clam are enhanced under the dark phase of the photoperiod. • Blue light inhibits melatonin production in the clam. [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]

In the present study, we explored the effects of seven light treatments (white, violet, blue, cyan, green, yellow, and red) on juvenile Sinonovacula constricta (length, 693.79 ± 82.91 μm), employing dark treatment as a control. A rearing experiment was conducted for one week in which the feeding rate was examined daily, e.g., one, two and three h after feeding. At the conclusion of the experiment, overall growth was calculated, and enzymatic activity related to digestion, superoxide dismutase, catalase, and glutathione peroxidase pertaining to antioxidant biomarkers, as well as expression levels of genes related to antioxidation and stress response, were examined. The results showed that the highest growth rates were observed in the yellow light group, while the lowest growth rates were found in the violet and blue light groups; on the other hand, no significant differences were found among the other groups. Meanwhile, when exposed to white, yellow, or green light, juveniles exhibited high feeding rates, while those in the violet and blue light groups showed low feeding rates. Regarding enzymatic activity related to digestion, those of α -amylase, lipase, pepsin, and trypsin were relatively higher in the yellow light group than in the other groups. Regarding oxidative stress, the yellow light group showed higher superoxide dismutase, catalase, and glutathione peroxidase activities than the other groups. In contrast, the lowest malonaldehyde level was found in the white light group, followed by yellow light, while there were no significant differences among the other groups. In addition, the expression levels of genes related to antioxidation and stress response (heat shock protein 70 and glucose-regulated protein 78) in juveniles generally exhibited higher levels in the red light group, whereas they showed relatively lower levels in the yellow light group. Collectively, these results suggest that the application of yellow light may facilitate the growth of juvenile S. constricta , while red and violet light have opposite effects. • Light spectrum differently influenced the growth of juvenile S. constricta. • Yellow light facilitated the growth of juvenile S. constricta. • Yellow light induced less stress to S. constricta. • Red and violet lights impaired the growth of juvenile S. constricta. [ABSTRACT FROM AUTHOR]

Sinonovacula constricta is an important bivalve with strong photosensitivity and a robust circadian rhythm. To elucidate the light entrainment of the molecular circadian clock in this species, this study identified the clock genes, characterized their tissue distribution, and investigated their transcriptional variations under different light regimes: LD (12 h light/12 h dark cycle), LL (continuous light), and DD (continuous dark). The results showed that the S. constricta clock system consists of seven genes [ Circadian locomotor output cycles kaput (ScClock), Brain and muscle ARNT-like protein (ScBmal), Cryptochrome (ScCry1 and 2), Period (ScPer), and Timeless (ScTimeless1 and 2)] that are closely related to their orthologs in marine mollusks and contained corresponding conserved functional regions, indicating the conserved evolution of the circadian clock among organisms. These genes were expressed in all tissues, with high expression in the labial palps and hepatopancreas, suggesting a complex peripheral biological clock in S. constricta and the critical role of these two tissues in circadian functions. When exposed to different light regimes in labial palps, genes from the positive loop (ScClock and ScBmal) displayed a daily/circadian rhythm under both LD and DD and their acrophases were shifted by 5–7 h, whereas genes from the negative loop displayed daily rhythms (ScCry1 and ScPer) under LD and circadian rhythms (ScCry2 and ScTimeless2) under DD. However, only one gene (ScTimeless2) displayed a circadian rhythm under LL conditions. These results suggest that the S. constricta circadian system may be regulated by different clock genes to adapt to various light environments. Although similar phenomena were observed in the hepatopancreas, markedly different clock gene expression patterns occurred between the two tissues, indicating a tissue-specific regulated circadian system. Collectively, this study reveals how the circadian oscillators of S. constricta respond to light and may promote light environment optimization in the S. constricta farming industry. • The molecular circadian clock was conserved in S. constricta. • A peripheral and tissue-specific clock might exist in S. constricta. • The S. constricta circadian clock was synchronized by photoperiod. • A functional clock exists in S. constricta. [ABSTRACT FROM AUTHOR]

Fatty acid and sterol changes in razor clam Sinonovacula constricta (Lamarck 1818) reared at different salinities were investigated. In one experiment, juvenile S. constricta with shell length of 1.63 ± 0.11 mm were firstly cultured at 13 psu for 10 days, and then cultured at 8, 13, 18, and 23 psu for another 5 days. In another experiment, adult S. constricta with shell length of 15.54 ± 0.80 mm were cultured at 13 and 23 psu for 40 days. On the 25th day, half of the S. constricta cultured at 13 psu were transferred to 23 psu for another 15 days. The results showed that higher salinity significantly decreased growth rate and crude lipid content in S. constricta . The proportions of long chain-polyunsaturated fatty acids (LC-PUFAs, C ≥ 20, double bonds ≥ 3) were significantly increased in S. constricta cultured at higher salinity as compared with those at lower salinity. The proportion of cholesterol was firstly increased and then significantly decreased with increasing salinity. The proportion of brassicasterol was higher in S. constricta cultured at higher salinity, while proportions of other plant sterols showed no obvious change trends. Interestingly, when S. constricta were around 60 days old with shell length of 32.06 ± 1.13 mm, the inhibitory effect of high culture salinity on growth rate was weakened, but the nutritional value was significantly improved. The results of this study suggest an optimal culture mode for S. constricta , i.e. if S. constricta were firstly cultured at lower salinity (10–15 psu) and then exposed to higher salinity (20–25 psu) for a short period of time before harvest, a combination of both optimal production and nutritional value would be achieved. [ABSTRACT FROM AUTHOR]

In the present study, we aimed to explore the optimal light intensity for the breeding of Sinonovacula constricta , and the effect of light intensity on the growth of S. constricta at different developmental stages was studied. Four developmental stages (spawning, fertilized eggs, planktonic larvae, and juveniles) and different light intensities were set up in our current experiments. The results showed that various light intensities had significantly different effects on spawning amount and initial spawning time, while it had little influence on the hatchability of fertilized eggs (P > 0.05). The spawning amount of the razor clam peaked at a light intensity of 44–250 lx (P < 0.05). In the planktonic larvae experiment, the highest survival rate appeared at 487 ± 31 lx (58.61 ± 4.0%).For juvenile S. constricta , two substratum thicknesses (1 and 5 mm) and four light intensities, including L 1 (14 ± 11 lx), L 2 (380 ± 145 lx), L 3 (3792 ± 2105 lx), and L 4 (12,244 ± 5290 lx), were set up in a 10-day experiment. When the substratum thickness of 1 mm was applied, the body length and survival rate of S. constricta juveniles (0.26 ± 0.02 mm) in the L 2 group were found to be the largest, which were significantly different compared with the other groups. When the substratum thickness was changed to 5 mm, the body length and survival rate among those groups showed no significant difference. However, when the shellfish samples were changed to a larger size of razor clam (1.45 ± 0.16 mm), a more obvious effect of light intensity on the growth was observed in a 1-mm substratum group compared with the 5-mm group. These results indicated that light intensity exerted a significant effect on the growth of S. constricta at different stages, and each stage exhibited a distinct sensitivity to light intensity. Further study from the aspects of photoreceptors and relevant genes is currently in progress in our laboratory. • Spawning, hatching and growth of S. constricta were affected by light intensity. • All developmental stages of S. constricta were photosensitive, except zygote. • Optimal light intensity is critical for S. constricta breeding and farming. [ABSTRACT FROM AUTHOR]

Sinonovacula constricta is the first marine mollusc demonstrated to possess all fatty acyl desaturase (Fad) and elongase of very long chain fatty acids (Elovl) activities required for long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis via the Sprecher pathway, providing a good opportunity to investigate the essential fatty acids (FA) for its larval development. In the present study, we analyzed the Fad and Elovl expressions, FA compositions, and the feed effects of three representative microalgae (Chlorella sp. , Chaetoceros calcitrans and Isochrysis galbana) in S. constricta at early developmental stages. The results showed that S. constricta Fad and Elovl were all highly expressed in zygotes, which might occur from maternal transfer. Compared to transcripts of Elovl4 activity, Elovl2/5 was only expressed relatively high in zygotes, indicating that C ≥ 22 LC-PUFA were more important than C ≤ 20 LC-PUFA for S. constricta larval development. The zygotes accumulated abundant FA from parents, especially eicosapentaenoic acid (EPA), but with small amount of docosahexaenoic acid (DHA). Meanwhile, the trochophore larva fed with single or mixed microalgae with I. galbana (rich in DHA) exhibited significantly higher growth and survival rate compared to those fed with single Chlorella sp. (rich in 18:2n-6 and 18:3n-3) and C. calcitrans (rich in EPA) or both of them. Those findings suggested that the highly expressed Fad and Elovl might be largely responsible for DHA biosynthesis, but this capability was limited to meet the developmental requirement and the dietary DHA was necessary. Taken together, our results compellingly demonstrated that DHA was essential for S. constricta larval development, which is important to guide its breeding. • Fad and Elovl expressions were examined in developing S. constricta. • Fatty acid compositions were analyzed in developing S. constricta. • Trochophore larval growth and survival were evaluated by first-feeding diet. • DHA still appeared to be essential for S. constricta larval development. • Microalgae rich in EPA and DHA were critical for S. constricta breeding. [ABSTRACT FROM AUTHOR]

Temperature is a crucial environmental factor that affects microalgal growth. However, it remains unclear how microalgae respond to high temperature. In the present study, we investigated the changes in organic acids, total lipids and transcriptomic profiling of Isochrysis galbana , an important diet microalgal species, in response to high temperature. The contents of six organic acids and total lipids, as well as gene expression of I. galbana cultivated at optimal temperature (20 °C) versus high temperature (35 °C) were compared. The results showed that the contents of oxaloacetic acid, citric acid, α-ketoglutaric acid, succinic acid, fumaric acid and malic acid were all decreased in I. galbana cultivated at 35 °C. Besides, high temperature decreased the accumulation of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG), while it increased the content of triacylglycerol (TAG). The transcriptomic results of some essential genes involved in tricarboxylic acid cycle (TCA) cycle, lipid metabolism and photosynthetic pathways were consistent with the data from metabolomic analysis. Collectively, comprehensive comparable study of metabolomic and transcriptomic was first conducted to uncover the underlying molecular mechanisms of I. galbana in response to high temperature stress. It was found that a series of biological processes were significantly affected by high temperature, especially TCA cycle, lipid metabolism and photosynthesis. All these lead us to provisionally conclude that the decline of I. galbana in summer was strongly linked to high temperature. Moreover, the findings of our current study had guiding significance for outdoor cultivation of this algal species, which might promote its application in aquaculture. • High temperature moderated TCA cycle of Isochrysis galbana. • High temperature remodeled lipid metabolism of Isochrysis galbana. • High temperature inhibited photosynthesis of Isochrysis galbana. • High temperature altered transcriptomic profile of Isochrysis galbana. [ABSTRACT FROM AUTHOR]