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Neurokinin B (NKB) is a hypothalamic neuropeptide containing 10 amino acids and is an essential member of the tachykinin family. Based on studies in mammals, Kiss, NKB, and dynorphin A are located in the common neurons, named KNDy neurons, and NKB and Kiss can stimulate the GnRH pulses. In humans, the mutations of NKB and its receptor NK3R can lead to hypogonadotropic hypogonadism and infertility. NKB is also involved in many other physiological activities in mammals and has received increasing attention. NKB was first purified from porcine spinal cord extracts and named neurokinin B or neuromedin K. Subsequently, NKB has been identified in various species, and its encoding gene was named tac2 in ruminants and rodents, and tac3 in other mammals, birds, reptiles, amphibians, and fish. In teleosts, the NKB system was first identified in zebrafish by three laboratories in 2012; it was also found in grass carp, goldfish, Nile tilapia, European eel, orange-spotted grouper, and half-smooth tongue.As teleosts have experienced the third round of genome duplication (3R), the bony fish have two forms of tac3 genes, namely tac3a and tac3b. However, only tac3a is present in highly evolved fish species, such as Nile tilapia and orange-spotted grouper, whereas the tac3b gene has been lost. tac3 in fish and amphibians can encode two mature peptides, NKB and an NKB-related peptide (NKBRP), whereas there is only NKB in mammals, birds, and reptiles. NKB and NKBRP sequences are highly conserved, and sequence analysis showed that NKB and NKBRP share an identical -FXGLM motif at the C-terminus, and X is a hydrophobic or aromatic amino acid residue, which plays an important role in binding to homologous receptors.NKB exerts biological effects by activating the endogenous receptor NK3R. The NK3R receptor belongs to the G protein-coupled receptor family and has a typical seven-transmembrane structure. NK3R is encoded by the tacr3 gene. There are two tacr3 subtypes in teleosts: tacr3a and tacr3b. There was an additional gene, called tacr3a2, in the zebrafish and grass carp, and it was produced by local genome duplication of tacr3a1. The tissue distribution shows that tac3 and tacr3 are widely expressed in the central nervous system and peripheral tissues, indicating that they have essential physiological functions. The specific expression patterns vary depending on the species. They are mainly expressed in the brain, with high expression levels in the pituitary, intestine, and gonads. NKB may participate in regulating reproduction and feeding in teleosts.Currently, studies in teleosts mainly focus on reproductive and feeding regulation. Due to the existence of multiple forms of tac3 and tacr3 genes in teleosts, the action of the NKB system on reproduction control is more complex. Using different experiment methods, such as intraperitoneal injection, intramuscular injection, and incubation of pituitary cells or pituitaries, NKB affected the expression of gnrh, kiss, lhβ, and fshβ and the secretion of LH, FSH, and E2 in fish. The physiological effects varied depending on the gonadal development stages, species, sex, treatment methods, treatment time, and dose. In addition, NKB can act as an anorectic peptide to inhibit food intake and promote gastrointestinal motility. It could also affect the expression of growth-related genes. In summary, as a neurotransmitter or neuromodulator in the central nervous system and a major member of the brain-gut peptides, the NKB system plays an important role in teleosts.Tachykinins activate receptors by coupling to Gαs and Gαq proteins, transducing its signals via PKA and PKC pathways. NKB and NKBRP could activate the PKA/PKC pathway via cognate receptors in some teleosts. This could be verified because different pathway inhibitors could block the effects of NKB and NKBRP. Upon binding to NK3R, NKB induces the secretion of related neuropeptides and the expression of related genes through the AC/cAMP/PKA, PLC/IP3/PKC, and Ca2+/CaM/CaMK-Ⅱ cascades. The C-terminal motif of NKB is crucial to binding receptors; once it is changed, NKB cannot activate the downstream signaling of NK3R, indicating that the integrity of the NKB/NK3R system is essential for the normal functioning of NKB. There are many deficiencies in the research regarding the function of NKB in teleosts. The specific effects and mechanisms of regulating reproductive endocrinology remain unclear, and the functional and signaling interactions between NKB and other neuroendocrine factors such as Kiss, GnIH, and GnRH require further study. In addition, the physiological functions of NKB in fish are mainly focused on reproductive regulation, with less attention given to feeding regulation and other physiological effects.In conclusion, this review provides a summary of the research progress on the NKB system in teleosts, including the identification, tissue distribution, physiological functions, and signaling mechanisms of NKB and its receptors, to enhance the understanding of the NKB system in fish and provide a reference for future research.

Insulin-like growth factor binding proteins (IGFBPs) play crucial roles in regulating biological activities of insulin-like growth factors (IGFs) and growth performance in vertebrates. IGFBP is a six-member protein family (IGFBP1–6) with a high affinity for IGF. It affects the distribution, stability, and biological activity of IGF by regulating the interaction between IGF ligands and receptors. Recently, IGFBP-1 was identified as a regulator of growth, reproduction, and development in bony fish, such as juvenile Atlantic salmon, where IGFBP-1 interacts with cortisol to regulate growth. IGFBP-1 regulates cell metabolism and growth through interaction with insulin in primary hepatocytes incubated in vitro in orange-spotted grouper (Epinephelus coioides). IGFBP-2 has a wide range of tissue expression characteristics in bony fish, and IGF regulation may occur through autocrine or paracrine pathways. For example, long-term fasting induced increased igfbp-2 mRNA expression in the liver of zebrafish. In addition, IGFBP-2 can inhibit the activity of IGF ligand through its high affinity with IGF to play an inhibitory and regulatory role in zebrafish growth. igfbp-2 mRNA expression was significantly up-regulated in Carassius auratus (goldfish) liver after fasting, and quickly recovered to normal levels after feeding. This indicated that igfbp-2 mRNA expression may be related to the anabolism and catabolism of goldfish, and is regulated by metabolic factors.Yellowtail kingfish (Seriola aureovittata) of the Carangidae family has high economic value and nutrition value. It is a warm and temperate oceanic fish with long distance migration characteristics that is distributed in the middle and upper oceans globally. This species is large with a fast growth rate, and is a promising candidate for aquaculture in land-based industrial circulating water system, deep-water cages, net pen systems, and aquaculture craft, etc. Our laboratory studied the regulatory mechanism of the rapid growth of yellowtail kingfish and cloned gh, igf-1, igf-2, ghr, and other growth-related functional genes to reveal their molecular regulation in early growth and development. igfbp genes regulate the growth, development, and nutrient metabolism of fish through their interaction with growth axis. The mechanism of IGFBP and how it influences the regulation of yellowtail kingfish growth is unreported. This paper further studied the growth of yellowtail kingfish by analyzing the key growth axis factors.RNAiso Plus reagent (TaKaRa) was used to extract total RNA from tissues. RNA integrity was detected by agarose gel electrophoresis, and RNA quality was determined by a NanoDrop 2000C spectrophotometer (Thermo Fisher Scientific, USA). First, cDNA strand was synthesized using a PrimeScript™ RT reagent kit with a gDNA Eraser (Perfect Real-Time) reverse transcription kit (TaKaRa). Primers were designed to clone the predicted sequence of yellowtail kingfish igfbp gene according to the NCBI GenBank database. The product was amplified by polymerase chain reaction, purified from agarose gel electrophoresis, linked with T4 Ligases, transformed into Trans1-T1 Phage Resistant Chemically Competent Cell, and positive clones were selected and tested. Quantitative real time polymerase chain reaction (qRT-PCR) was used to analyze the distribution of yellowtail kingfish tissues and the expression patterns of liver tissues under different densities of industrial culture.The lengths of igfbp-1, igfbp-2a, igfbp-2b open reading frame (ORF) domains were 741 bp, 882 bp, and 801 bp, and encoded 246 amino acids, 293 amino acids, and 269 amino acids, respectively. The conserved domain of insulin growth factor-binding protein homologues (IB) was present in the N-terminus of the three igfbps, and the conserved domain of thyroglobulin type-1 repeat (Ty-1) was present in the C-terminus. They had a wide range of tissue expression characteristics and were highly expressed in the liver. There were differences between the expression of the same gene in the same tissues of male and female fish. For example, the expression of igfbp-1 and igfbp-2b genes was significantly higher in the liver of male fish compared with female fish, while igfbp-2a expression was significantly higher in the liver of female fish compared with male fish. The tissue differential expression of genes between sexes indicates that igfbp may have sex dimorphism when it plays a physiological role. However, the specific characteristics of this difference between males and females and the possible signaling pathway are unclear. The fish in the low-density group showed the greatest growth rate and the highest expression levels of igfbp-1, igfbp-2a, igfbp-2b, igf-1, and igf-2 under industrial culture conditions. These expression levels were significantly different from those of the medium and high-density groups (P < 0.05). However, there were no significant differences in the expression levels of growth and liver genes between the middle and high-density groups.igfbp-1, igfbp-2a, and igfbp-2b participated in the growth regulation of yellowtail kingfish. The expression regulation of igf-1 and igf-2 had a positive synergistic effect with growth regulation of yellowtail kingfish. The ORF regions of igfbp-1, igfbp-2a, and igfbp-2b genes of yellowtail kingfish were cloned, and the structural characteristics, tissue expression characteristics, and relationships with growth performance were analyzed under different culture densities. The content of serum IGF-1, GH, cortisol, and glucose concentration was detected in the early stage. The expression trend of igfbp-1, igfbp-2, igf-1, and igf-2 was the same as that of serum IGF-1 and GH in the liver of yellowtail kingfish rapidly growing at the low-density group, but contrary to the expression trend of cortisol content and glucose concentration. This indicated that the key factors of growth axis, cortisol, and glucose participated in the growth regulation of yellowtail kingfish at different densities. However, the specific regulatory mechanism requires further study. The results provide a theoretical basis for interpreting the molecular mechanism of the growth of yellowtail kingfish and the regulation of suitable culture densities under industrial conditions.

Yellowtail kingfish, Seriola aureovittata, is a long-distance migratory oceanic species belonging to the Carangidae family of Perciformes, which has a global distribution and inhabits temperate and subtropical marine waters. S. aureovittata is large in size, has a fast growth rate, and is highly favored by international consumers owing to its excellent flesh taste, nutritional quality, and economic value. Furthermore, it is a promising candidate for the global farming industry and is particularly suitable for rapidly developed open ocean aquaculture in China.Currently, yellowtail kingfish aquaculture occurs in over 10 countries including Japan, Australia, New Zealand, South Africa, Chile, Greece, Holland, USA, Mexico, and China. In 2017, a great breakthrough in seedling production of S. aureovittata was achieved, and currently juveniles are mass-produced in China by combining the "engineering pond" and "land based indoor tanks" modes, which led to the rapid development of the Seriola fish farming industry in China. Nowadays, Seriola species are farmed in Liaoning Province, Fujian Province, and Shandong Province of China, and the combined annual farming yield is approximately 500 tons. However, we found that during seedling production of S. aureovittata, especially at the early larval growth stage, the hatching rate of eggs was variable among different spawning batches, and the survival of early larvae was low especially when the larvae reached 8~10 d post hatching. Occasionally, the high total death rate was attributed to the sudden "sinking death" of larvae, which may have been caused by stress as a result of changes in environmental factors. Therefore, it is necessary to determine the ecological and physiological effects of environmental factors, especially temperature and salinity fluctuations, on the early life stages of S. aureovittata under artificial breeding conditions.In the present study, the effects of two key environmental factors, temperature and salinity, on embryonic development and early larval growth of S. aureovittata were investigated using experimental ecology, morphological measurements, and molecular methods under laboratory conditions. The indexes including hatching rate of eggs, deformation rate of newly hatched larvae, absorption of yolk sac, IGF-1 gene expression, survival index (SAI), and point of no return (PNR) were determined. Moreover, the vitality of newly hatched larvae was tested and evaluated. The results showed that the highest hatching rates of 75%~81% were obtained under temperatures of 20~22 ℃, and the deformation rates of newly hatched larvae were lower than 6.7%. In addition, according to the Q10 calculation, the most appropriate water temperature range for embryonic development of S. aureovittata was confirmed to be 20~22 ℃. Meanwhile, the total length and yolk sac volume of newly hatched larvae of yellowtail kingfish hatched from the 20℃ and 22 ℃ groups were larger than those in the other temperature groups. Regarding salinity, the fertilized eggs floated on the water surface when salinity was over 30 ‰, were suspended in the water when salinity was between 20~25, and sank to the bottom of the container when salinity was lower than 15. The optimum salinity range for embryonic development of S. aureovittata was therefore determined to be 30~35, when hatching rates were between 79%~80%, and the deformation rate of newly hatched larvae was 6%. The yolk sac absorption by newly hatched larvae was measured under four temperatures (18 ℃, 20 ℃, 22 ℃, and 24 ℃). It was found that the absorption and exhaustion speed of the yolk sac increased with temperature, and the yolk sac was exhausted at 7 d post hatching at 18 ℃, whereas the time decreased to 6 d, 5 d, and 4 d at 20 ℃, 22 ℃, and 24 ℃, respectively. The highest SAI value for newly hatched larvae was observed at a salinity of 30, whereas the lowest was observed at a salinity of 10, which was consistent with the hatching results of embryos under different salinities. The highest first feeding rate of newly hatched larvae was observed at 6 d post hatching, and the PNR appeared between 7 d and 8 d post hatching at culture temperatures ranging from 20~22 ℃. IGF-1 mRNA levels in newly hatched larvae from different temperatures and salinities were detected, and significantly higher expression levels were found at temperatures of 20~24 ℃ and salinities of 30~35. Under continuous starvation conditions, the IGF-1 mRNA in larvae significantly increased at 2 d post mouth open and decreased at 3 d and 4 d, although expression levels remained relatively high, and then continually decreased to a significantly lower level as starvation continued. Results from the present study provide basic knowledge and useful tools for the construction of standardized technological methods for optimal embryonic hatching and seedling production of S. aureovittata.

Yellowtail kingfish (Seriola aureovittata), a pelagic marine finfish species with a worldwide distribution, is regarded as an emerging candidate for the aquaculture industry owing to its fast growth, superior flesh quality, and farming suitability in both sea cages and land-based facilities in China. The species has high economic value and is the second most produced Seriola species in the world following Japanese yellowtail Seriola quinqueradiata. Researchers worldwide have studied the role of regulatory factors neuropeptide Y (NPY) in fish feeding regulation. In recent years, there has been great progress in research on food intake in fish, however, very little attention has been paid to the endocrine regulation mechanism of food intake. Methods on strengthening the production performance of fish through appetite regulation is still a hot research topic.The control of food intake and energy metabolism in vertebrates are complex processes involving several neural pathways. Some hypothalamic signals are released by peripheral tissues that are associated with energy homeostasis or nutrient availability. Among the signaling molecules involved, NPY plays a key role. NPY is recognized as one of the most effective appetite regulators, which primarily function as a signaling factor to regulate a variety of biological processes such as food intake and glucose homeostasis. The orexigenic actions of NPY have been well investigated thoroughly over the past decades. Much evidence supports that NPY´s functional role as a regulator of energy homeostasis and appetite control is conserved across vertebrates, including teleosts. In several species, including rainbow trout, Nile tilapia, and grass carp, NPY injections increase food intake, supporting an orexigenic role. In line with this, food deprivation increased npy mRNA expression in the brain, such as seen for goldfish and Johnny carp. Moreover, refeeding normalized npy mRNA abundance following food deprivation. As S. aureovittata feeds heavily and fiercely, the breeding industries need to understand its feeding control mechanism. To make real-time adjustment to feeding strategy, it is necessary to obtain high-quality and high-yield aquatic products with the least input to maximize economic benefits. As a potent appetite stimulating factor, npy has been proven to promote feeding, but this gene has not been identified in S. aureovittata. Therefore, it is necessary to explore the variable rules of the npy gene in feeding and starvation compensation mechanism, to provide the special compound feed for breeding.To gain insight into the existence of npy in S. aureovittata, we used homologous cloning, RNA extraction and reverse-transcription to obtain the ORF sequence of npy. npy belongs to the pancreatic polypeptide (PP) family, which plays an important role in appetite regulation and energy expenditure in mammals and fish. The ORF of S. aureovittata npy is 300 nucleotides in size and encodes a 99-amino-acid precusor, with a calculated molecular mass and isoelectric point of 11.24 kDa and 5.02, respectively. The precursor protein is composed of a predicted signal peptide of 28-aa in size, 36-aa putative mature peptide, a GKR protein proteolytic site, and a 32-aa C terminus of unknown function. Bioinformatics analysis on the amino acid sequence identities and evolutionary relationships of the npy was performed. Comparison of homology of the precursor peptide sequences of npy analysis revealed that S. aureovittata npy displayed a high degree of identity with the counterparts of Seriola dumerili (99.0%), Morone saxatilis (98.0%), Micropterus salmoides (96.0%), and Scophthalmus maximus (94.9%), followed by Cynoglossus semilaevis (93.9%) and Oryzias latipes (92.9%). Phylogenetic analysis highly supported that the npy of S. aureovittata was closely related to that of S. dumerili. Furthermore, using real-time quantitative PCR, we found that the npy mRNA is widely expressed in 12 tissues, with abundant expression in the brain, followed by the pituitary and stomach. In addition, except for the intestine and gonad, npy was found to have no significant difference in all other detected tissues of both sexes. To establish the functional link between npy and feeding, the expression profiles of npy mRNA during food deprivation and refeeding were examined in S. aureovittata. We detected the 7 d, 14 d, and 21 d starvation and 7 d refeeding effect on npy mRNA levels. Results showed that fasting induced an increase of npy mRNA levels in brain, pituitary, and stomach when compared to the control groups. Interestingly, the pituitary npy transcripts significantly increased after 21 d of starvation compared with the control group. In addition, refeeding normalized npy mRNA abundance following food deprivation in the brain, pituitary, and stomach. These results indicated that npy is involved in the regulation of feeding and energy homeostasis in S. aureovittata. Collectively, we provided initial evidence for the existence of npy in S. aureovittata and suggested its involvement in the regulation of feeding, which plays an important role in the starvation compensation mechanism.In summary, we obtained the ORF sequence of npy and clarified its role as a potent orexigenic peptide in feeding regulation of S. aureovittata, which would be beneficial for specific feed for this species.

Seriola aureovittata is a long-distance migratory oceanic species belonging to the Carangidae family; it is also known as yellowtail kingfish. It inhabits temperate and subtropical marine waters worldwide. We reached a significant breakthrough in S. aureovittata seedlings production in 2017, enabling current massively juvenile rearing in China, which became a promising candidate for the farming industry, especially for the rapidly growing open ocean aquaculture. However, sudden larvae death caused by environmental changes-related stresses during the early life stages of S. aureovittata leads to great losses. Thus, exploring the physiological mechanism of larvae responses to environmental stresses during seedlings' production under artificial breeding conditions is urgently needed. Heat shock proteins (HSPs) play an important function in the physiological regulation of stress and immune responses in vertebrates, including fish. HSP70 is a member of the HSPs family studied in dozens of fish species, with physiological roles in protein homeostasis, DNA protection, and stress tolerance enhancement, among others. To investigate the possible physiological effects of HSPs on early growth and development of S. aureovittata, we cloned and obtained the full-length cDNA sequence of the hsp70 gene. Furthermore, the structure and spatial and temporal expression patterns of hsp70 during embryonic development, larval and juvenile growth and development of S. aureovittata were determined. The results showed that the full-length cDNA sequence of the hsp70 gene contains 2 332 bp, wherein the 5'-UTR is 187 bp, the ORF is 1 920 bp, and the 3'-UTR is 225 bp in length. A 639 amino acids protein with a molecular weight of 70.1 kDa is encoded by hsp70. The hsp70 spatial expression exhibited a sex dimorphism pattern, with significantly high expression levels in the gill, heart, spleen, and ovary in females, whereas in males, the significantly high expression levels were found in the pituitary, gill, head kidney, and testis. Notably, the highest hsp70 expression levels were observed in the ovary of females and the testis of males. During the S. aureovittata embryonic development, the hsp70 mRNA was detected in fertilized eggs before cleavage, indicating that hsp70 is parentally inherited. Additionally, the hsp70 mRNA could be detected at all stages of embryonic development, wherein the lowest expression levels were observed before the low blastula stage, with a significant increase at the early gastrula stage maintained until the hatching stage. The high levels of hsp70 mRNA were detected in one-day-old larva, which decreased in four-day-old larva, followed by an increase in 15-day-old larva. Remarkably, the hsp70 mRNA level once again decreased in 20-day-old larva, maintaining an average level until 60-day-old juveniles. The results from the present study may provide insights into the origin and physiological function of hsp70 during the early life stages of S. aureovittata.

Black rockfish (Sebastes schlegelii) is a member of the family Sebastidae—an ovoviviparous marine fish widely distributed in the coastal waters of China. This species has important commercial value and is also a delicious table fish. It often inhabits areas at the bottom of the sediment in rocky reefs with clear water quality. The species does not exhibit a long-distance migration habit or remarkable clustering. In recent years, sea ranching has developed rapidly in China. Black rockfish is a good candidate to promote sea ranching, because it does not exhibit a long migratory habit and is called a housekeeping fish. In recent decades, owing to the overfishing of wild resources and deterioration of the environment, the natural pools of black rockfish have decreased sharply. Thus, the national black rockfish release and breeding plan was implemented in 2006. Currently, millions of young black rockfish are released into the sea every year in China, which has played a positive role in promoting the conservation and restoration of natural resources. However, the release activity of marine organisms is random and under lax supervision. In recent years, various tagging technologies, including external markers, such as T-bar markers, and internal markers, such as otolith markers, have been developed to support the evaluation of the enhancement effect of released species. At present, otolith fluorescent labeling is the only reported marker technology that can be used for the proliferation and release of black rockfish. However, otolith staining has some problems, such as difficulty in detection and rapid fading, and it is not effective as an internal marker. Currently, there has been no efficient internal marker for black rockfish juveniles; thus, an effective marking technology must be established for evaluating the enhancement effect of black rockfish to support scientific release and natural resource maintenance. The coded wire tag (CWT) is a magnetized stainless-steel wire segment with a diameter of 0.25 mm. Several rows of numbers are engraved on the marking line to represent a specified batch or a single code. CWTs are widely used internal tags with many advantages, including small size, easy management, accuracy, and high retention. CWTs have been effectively used in over 30 fish genera worldwide because of their high retention rates and minimal biological impact on living fish, although serious effects on growth and survival have been reported among different species and tag placements. In the present study, we investigated the marking effects of CWTs on released juveniles of black rockfish. According to different sizes and marking placements, the experimental juveniles of black rockfish were divided into four groups: small fish [average total length = (8.25±0.84) cm, average body weight = (9.99±2.75) g ] with gill levator arcus palatine muscle tagging group (SLM group); small fish with dorsal muscle tagging group (SDM group), large fish [average total length = (11.13±0.67) cm, average body weight = (20.95±2.99) g ] with gill levator arcus palatine muscle tagging group (SDM group); and large fish with dorsal muscle tagging group (LDM group). The survival rate, tag retention, antioxidant enzyme activity, and expression of genes related to growth or stress were measured to evaluate the effects of CWTs on black rockfish juveniles. The survival rate was 100% in all study groups. The tag retention rates in the SLM and LLM groups were 87% and 97%, respectively. The tag retention rate in the SDM and LDM was 100%. The specific growth rate of all tagged fish was lower than that of the control fish. After 30 days of culture post-tagging, the liver catalase enzyme activity in the SLM and LDM groups was significantly higher than that in the control group, and the liver superoxide dismutase enzyme activity in all experimental groups was significantly higher than that in the control group. The liver IGF-I mRNA levels were significantly higher in the SLM, LLM, and LDM groups than in the control group. There were no significant differences in HSP70 mRNA levels between the small fish groups, while in the large fish groups, HSP70 mRNA level in the LDM group was lower than that in the LLM and control groups. Therefore, CWT is suitable for marking black rockfish juveniles based on survival and tag retention data; however, the specific growth rate of tagged fish was lower than that of the control fish, and the antioxidant enzyme activities and expression of genes related to growth and stress showed significantly different expression responses to CWT marking, indicating that tagging caused physiological stress in released black rockfish juveniles. Therefore, indoor temporary breeding or wild domestication time should be relatively long for the seedlings marked with CWTs, which is conducive to improving the survival rate of the marked released seedlings and their growth after release. Overall, CWTs may be useful to evaluate the enhancement effects of released species and support accurate assessments of the proliferation and release effect of black rockfish.