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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.

Leptin is a peptide hormone primarily produced by adipose tissue, which plays a crucial role in regulating energy balance by controlling appetite and energy expenditure. To better understand the intricate physiological functions of leptin in hepatocytes in tongue sole (Cynoglossus semilaevis), this study presents an integrated transcriptomic and metabolomic analysis of tongue sole hepatocytes following stimulation with lepA and lepB. Comparative analysis identified 2971 and 2900 upregulated differentially expressed genes (DEGs) and 1895 and 1821 downregulated DEGs in response to lepA and lepB stimulation, respectively. Notably, 5706 genes were commonly regulated by both stimulations, suggesting overlapping functional roles of these two leptin proteins. GO and KEGG enrichment analysis indicated significant enrichment in immune response (JAK-STAT signaling pathway, NF-κB signaling pathway, etc.) and lipid metabolism pathways, such as fatty acid synthesis, with similar findings for lepB. Metabolomic analysis demonstrated clear separation between treatment and control groups, with 34 medium- to long-chain fatty acids and numerous differentially expressed metabolites (DEMs) identified. KEGG analysis of the metabolome paralleled the transcriptomic findings, with shared pathways in lipid metabolism and immune function. Finally, joint analysis highlighted co-enrichment in linoleic acid metabolism and leishmaniasis pathways. Detailed mechanistic insights revealed the activation of JAK-STAT and NF-κB signaling pathways, modulation of arachidonic acid metabolism, and the influence on the MAPK signaling pathway. Additionally, lepA uniquely co-enriched in the fatty acid synthesis pathway, with specific gene regulation affecting the synthesis of various fatty acids. The study concluded that lepA and lepB exerted significant regulatory effects on lipid metabolism and immune responses in tongue sole hepatocytes through complex molecular networks. To our current knowledge, this represents the first direct evidence of leptin's role in immune function within teleost fish and offers valuable insights into the molecular mechanisms of lipid metabolism underlying the actions of lepA and lepB.

Sea-land relay transportation (SRT) is a key part of the “land-based greenhouse + offshore net cage” mariculture system for yellowtail kingfish. In this study, the physiological status of yellowtail kingfish during SRT was assessed by determining serum biochemical indexes (levels of cortisol, glucose, and malondialdehyde (MDA), and activities of superoxide dismutase (SOD) and catalase (CAT)), the composition of intestinal microflora, and the liver transcriptome before (control), during, and after transportation (T1 and T2) and after recovery for 6 hours (6 h), 12 hours (12 h), 1 day (D1), 3 days (D3), and 7 days (D7). The results show that the serum cortisol and glucose levels, MDA level, and activities of SOD and CAT significantly increased during transportation, and decreased to similar levels to those in the control after 1 day of recovery. Bacteria including Acinetobacter, Enterobacter, and Comamonas accounted for more than 91 % of the microbial community in the gut of yellowtail kingfish, and these genera were associated with changes in serum glucose levels, indicating that they may help to alleviate the effects of transportation stress. Analyses of liver transcriptome data showed that significantly differentially expressed genes (DEGs) in the T2, D1, D3 and D7 groups were significantly enriched in the “Protein processing in the endoplasmic reticulum (ER)” pathway. At the end of transportation (T2 group), the ER-associated degradation (ERAD) pathway was activated, with up-regulation of hsp40, hsp70, and hsp90α. The transcript levels of these genes gradually returned to levels similar to those in the control during the recovery period. At the first day of recovery, the unfolded protein response imposed stress upon the ER, accompanied by significant down-regulation of PEPK and Calmin and significant up-regulation of CRT. The DEGs in the D3 group were enriched in the “ER”, “Steroid biosynthesis”, “Fatty acid degradation”, “PPAR signaling”, and “Fat digestion and absorption” pathways, whereas the number of DEGs related to lipid metabolism and stress pathways was significantly decreased in the D7 group. Our results indicate that hsp70 and hsp90α can be used as stress markers, and hsp40 can be used as a physio-metabolic recovery marker for yellowtail kingfish. These results show that, although serum indices recovered 24 hours after transportation, up to 3 days after transportation is an important period for recovery from transportation stress at the transcriptome level. “Protein processing in the ER pathway” is an important pathway regulating stress and recovery in yellowtail kingfish.

The neurokinin B (NKB) system acts as a crucial modulator in the regulation of reproduction and feeding in mammals, however limited information is available in teleosts regarding its potential role in reproductive functions. Herein, we have identified its encoding gene, namely tachykinin 3 (tac3), in the yellowtail kingfish. The full-length tac3 cDNA sequence was 372 bp in size and encoded a 123-aa preprohormone, including two mature peptides, NKB and NKF. Tissue distribution analysis showed that tac3 was primarily expressed in the intestine of yellowtail kingfish. During embryonic development, the highest mRNA levels of tac3 were observed at stage 4 (16-cell) and stage 14 (newly-hatched larva). The peak of tac3 transcript levels was also noticed at 30 days post-hatching during early ontogeny. The physiological effects of NKB and NKF on expression of brain and pituitary genes along with plasma hormones levels were further analyzed by intraperitoneal injection. NKB exerted a stimulatory effect on gnrh1, gnrh2, gnih, and kiss2r mRNA levels, but suppressed kiss1r, gnihr, gal, gh, lhβ, and fshβ mRNA levels. An evident increase was observed on transcript levels of gnrh1, gnrh2, kiss1, kiss2, gnih, and gnihr after NKF treatment, whereas kiss1r, kiss2r, gh, lhβ, and fshβ mRNA levels were down-regulated. Moreover, NKF, but not NKB, reduced serum estradiol levels, and both of these two peptides suppressed serum 11-keto testosterone levels. Taken together, our results provide preliminary evidence that the NKB system participates in the control of reproduction in yellowtail kingfish via acting on brain, pituitary and gonadal hormones.

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.

This study investigated the physiological responses to copper toxicity in liver and gill of juvenile yellowtail kingfish by exposing to 0 (control), 0.0672, 0.12, 0.214, 0.384, 0.672, 1.20, or 2.14 mg/L copper sulfate for 7 days and then recovered for 14 days. The histopathology, enzyme activities and differential gene expression were detected. The results indicate the safe concentration value of copper for juvenile yellowtail kingfish is 0.083 mg/L. Copper exposure caused structural damage to gill and liver, and could be disappeared or restored after the recovery period. Copper exposure enhanced the Cu,Zn superoxide dismutase (Cu/Zn-SOD) and glutathione peroxidase (GSH-Px) activities and the malondialdehyde (MDA) level in liver. However, with increasing copper concentrations, liver aspartate transaminase (AST) and gill Na+/K+-ATPase activities were at first enhanced and then inhibited. Serum lysozyme (LZM) and Cu/Zn-SOD activities were enhanced at low copper concentrations but inhibited at high concentrations, and the activities of alanine transaminase (ALT) and AST significantly increased at greater copper concentrations. After the recovery period, only liver Cu/Zn-SOD activity returned to control level. In contrast, liver GSH-Px activity and MDA levels and gill Na+/K+-ATPase activity, serum LZM and AST activities remained significantly higher than control, but liver AST activity, serum ALT, and Cu/Zn-SOD activities were significantly lower than control. We constructed the gene regulatory networks of molecular pathways in liver and gill in response to copper exposure. The DEGs were enriched mainly in the pathway of lipid metabolism in liver and pathway of oxidative phosphorylation in gill, indicating these respective pathways are involved in the response to copper stress in this species. The present results elucidate some histological, physiological, and molecular mechanisms underlying copper-exposure stresses in yellowtail kingfish, which could help guide the safe utilization of copper sulfate in disease control of farmed individuals.

Leptin (Lep) plays a crucial role in controlling food intake and maintaining energy balance in mammals. While several studies have shown the presence of multiple leptin genes in teleosts, limited information is currently available on how sex steroid hormones regulate the expression of these genes in fish. In this study, we used two previously expressed and purified leptin proteins to incubate four tissues in vitro (hypothalamic-pituitary-gonadal axis and liver) and used the RT-qPCR method to detect the expression of genes related to growth and reproduction in tongue sole (Cynoglossus semilaevis). The results showed that both recombinant LepA and LepB proteins of tongue sole almost entirely suppressed the expression of genes related to growth and reproduction in the hypothalamic-pituitary-gonadal axis and liver, while LepB may have had a positive role on steroid synthesis in gonads. Further, a high concentration of LepA facilitated the expression of IGF-Ι in liver. At the same time, we utilized human leptin to incubate four tissues in vitro; although most of them had trends similar to those stimulated with tongue sole leptins, there were still some differences, indicating differences among leptin homologs between humans and fishes. To our knowledge, this is the first study to explore the function of tongue sole LepA and LepB within the hypothalamic-pituitary-gonadal axis and liver in vitro. Our results provide a valuable resource and foundation for future studies.

LPXRFa, also known as gonadotropin-inhibitory hormone (GnIH), and kisspeptin (Kiss) are two major hypothalamic peptides that modulate the reproductive axis of vertebrates, including teleosts. However, little information is available regarding the actions of nutritional status on the regulation of these two neuroendocrine systems in fish. Herein, we assessed the effects of starvation and refeeding on the expression of lpxrfa, kiss2 and their receptors (lpxrfa-r and kiss2r respectively) at the brain-pituitary level of half-smooth tongue sole (Cynoglossus semilaevis). Food deprivation for 4 weeks induced a rise in brain lpxrfa as well as brain and pituitary lpxrfa-r mRNA levels, and refeeding restored brain lpxrfa and lpxrfa-r expression back to normal. However, pituitary lpxrfa-r mRNA levels still remained high after 1 week of refeeding. Neither lpxrfa nor kiss2 transcripts in the pituitary were altered by fasting, but their mRNA levels increased significantly after 1 week of refeeding, and declined back to the control levels after 2 weeks of refeeding. None of brain kiss2 and kiss2r along with pituitary kiss2r transcripts were modified by the nutritional status. In summary, our results revealed an interaction between energy status and the elements of LPXRFa and Kiss systems in the brain-pituitary axis of half-smooth tongue sole. Food deprivation and refeeding differentially regulated the two systems, which provided additional evidence for the involvement of the LPXRFa and Kiss systems in the regulation of reproduction by energy balance in non-mammalian species.

Yellowtail kingfish has emerged as one of the most promising marine fishes for aquaculture in China because it is tasty, fast growing, and has high economic value. To investigate the tolerance and adaptability to hypoxia of farmed yellowtail kingfish, juveniles were exposed to hypoxia (3.0 ± 0.5 mg/L) for 5 days and then returned to normoxia (7.5 ± 0.5 mg/L) for another 5 days. Using tissue sections and high-throughput sequencing technology, we investigated the histological, microecological, transcriptomic, and physiological adaptation mechanisms of yellowtail kingfish. The results showed that hypoxia increased the gill lamellae length and spacing, which were reversible post-reoxygenation. At the genus level, the relative abundances of Prevotella, Bacteroides, Roseburia, and Blautia in the gastrointestinal tract increased under hypoxia and were maintained post-reoxygenation. The liver transcriptome revealed that, compared with normoxia group, the different expression genes (DEGs) were mainly enriched in Steroid biosynthesis and PPAR signaling pathways in hypoxia group. Compared with normoxia group, the DEGs were mainly enriched in Ribosome biogenesis in eukaryotes, Steroid biosynthesis, Fatty acid biosynthesis, and PPAR signaling pathways in reoxygenation group. Furthermore, compared with hypoxia group, the DEGs were mainly enriched in Ribosome biogenesis in eukaryotes and Ribosome pathways in reoxygenation group. In contrast to normoxia, of the key genes of the PPAR signaling pathway, FABP4 was significantly downregulated, and SCD-1 and FATP were significantly upregulated. These findings indicated reduced lipid deposition and increased lipid decomposition in liver under hypoxia. The genes including PPARα, SCD-1, ANGPTL4, and FASN were significantly upregulated in lipid metabolism-related pathways, which indicated that lipid metabolism activity was more vigorous during reoxygenation. In contrast to the hypoxia group, almost all of the genes involved in Ribosome biogenesis in eukaryotes and Ribosome pathways for protein processing were significantly upregulated during reoxygenation; this is probably related to the clearance of misfolded proteins and the folding of the new proteins repairing there is damage to the body. The present results shed light on the possible synergetic function of lipid metabolism, protein repairment and synthesis, and gastrointestinal microbiota in resistance and homeostasis maintenance of yellowtail kingfish coping with hypoxic stress in aquaculture.

Protein metabolism can provide energy for various biological activities of organisms. However, there have been few studies on the succession of gastrointestinal microbiota structure during the metabolism of protein by the host. In this study, physiological and biochemical, qRT-PCR, and 16S rDNA high throughput sequencing methods were used to detect the activity characteristics of proteases and the related gene expression levels in each gastrointestinal tissue, as well as the distribution characteristics and succession of the gastrointestinal microbiota structure of pond-cultured Japanese flounder (Paralichthys olivaceus). The results showed that the activity of pepsin, trypsin, and chymotrypsin in the pyloric caecum and gut, and the relative expression levels of the related genes, increased first and then decreased, but the aspartate aminotransferase (ASP) and alanine aminotransferase (ALA) activity showed the opposite tendency. These reflected that the nutrient composition of gastrointestinal tract contents was dynamic, which caused obvious changes to the alpha diversity and structure of gastrointestinal microbiota. Lactobacillus, Acinetobacter, Bacteroides, Escherichia-Shigella, Prevotella, the genera represented by the Lachnospiraceae_NK4A136_group and MND1, Sphingomonas, Alistipes, Alloprevotella, Enterobacter, Helicobacter, Myroides, Ruminiclostridium, and Romboutsia were the main gastrointestinal microbiota based on the dominance, commonality, and colonization. The relative abundance of Lactobacillus and Acinetobacter in the three gastrointestinal tissues increased first and then decreased. According to the number of shared and dominant genera, the consistency of succession of some genera, and the increase in the similarity of microbiota structure among the three tissues, we inferred that the microbiota was affected by the gastrointestinal tract nutrient composition. Gastrointestinal microbiota genes were mainly enriched in the metabolism pathway according to the KEGG pathway, and the ratio of Firmicutes to Bacteroidetes, the representative of energy accumulation, followed an at-first increasing and then decreasing trend in each tissue. This shows that the main gastrointestinal microbiota played a synergistic role in the energy accumulation of the host. This study will provide a reference for the development of high-efficiency compound feed formulae and improve feed efficiency by regulating the gastrointestinal tract microecology for pond-cultured Japanese flounder.

Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) has shown great potential for near-shore bathymetry. Unavoidably, the photon data acquired has a lot of noise. In order to remove noise more accurately, a photon denoising algorithm based on adaptive elevation difference thresholding (AEDTA) is proposed under the assumption that the terrain is continuously changing. The algorithm uses a Gaussian function obtained by fitting an elevation histogram to ICESat-2 data for the extraction of underwater photons. Repeatedly count the elevation difference histogram of underwater photons and adaptively determine the threshold. And the final denoised underwater signal photons are obtained. Compared with the conventional clustering method of ordering points to identify the clustering structure (OPTICS), our algorithm achieves better photon detection results for underwater topography for different densities of ICESat-2 data and has the advantage of parameter adaption without human intervention. ICESat-2 data from Ganquan Island, Dongdao Island and Oahu Island were selected for testing, and the ICESat-2 bathymetry results calculated by the algorithm were compared with in situ bathymetry data to assess the reliability and accuracy of the proposed denoising algorithm. The results show that the MAE of the bathymetry results calculated using the ICESat-2 signal photon data extracted by the method ranged from 0.54 to 1.57 m and the RMSE ranged from 0.64 to 1.74 m in different seas. The algorithm can provide high quality denoised data for bathymetric mapping around coastal areas and islands. Meanwhile, the AEDTA algorithm also has a good denoising effect on terrestrial photons, and the effect of cloud shading on the Gaussian fit analysis of ICESat-2 data cannot be ignored.

To clarify the population genetic structure, intrapopulation diversity, and interpopulation differentiation of yellowtail kingfish (Seriola aureovittata), we sampled 143 individuals from five collections of yellowtail kingfish: farmed (n = 30) and wild (n = 33) collections in China, a wild collection in Japan (n = 20), and farmed (n = 31) and wild (n = 29) collections in Australia. Using 2b-RAD simplified genome sequencing, we obtained an average of 287,594 unique tags per population, with an average sequencing depth of 27.13×. Our final genotype dataset included 48,710 SNPs (Single Nucleotide Polymorphisms). The five collections were all in Hardy–Weinberg equilibrium, and the interpopulation differentiation varied among the sample collections. The genetic differentiation coefficients (Fst) between the Chinese and Japanese yellowtail kingfish collections were low and the gene flow (Nm) values were high. These results suggest continuous gene flow occurs frequently between the collections, indicating that they belong to the same population. In contrast, genetic differentiation was high between the Australian collections and the Chinese and Japanese populations, suggesting different evolutionary origins and belonging to different populations. The farmed and wild Australian collections fell into distinct clades in a neighbor-joining phylogeny tree, suggesting farmed fish have begun to differentiate from the wild collection. A similar level of genetic diversity between the wild collections in China and Japan suggests that they originated from the same spawning ground. This, therefore, reminds us that in future aquaculture processes attention is needed regarding implementing targeted breeding strategies. In addition, our data will contribute to Chinese yellowtail kingfish genetic breeding and the sustainable use of Chinese yellowtail kingfish germplasms.

Spexin (SPX) is an evolutionarily conserved neuropeptide, which was first identified in human proteome by data mining. Two orthologs (SPX1 and SPX2) are present in some non-mammalian species, including teleosts. It has been demonstrated that SPX1 is involved in reproduction and food intake, whereas the functional role of SPX2 is still absent in any vertebrate. The aim of the current study was to evaluate the actions of intraperitoneal injection of endogenous SPX2 peptide on the expression levels of some key reproductive genes of the brain-pituitary axis in half-smooth tongue sole. Our data showed an inhibitory action of SPX2 on brain gnih, spx1, tac3 and pituitary gthα, lhβ mRNA levels. However, SPX2 had no significant effect on brain gnihr, gnrh2, gnrh3, kiss2, kiss2r, spx2 expression or pituitary gh expression. On the other hand, SPX2 induced an increase in pituitary fshβ expression. Taken together, our results provide initial evidence for the involvement of SPX2 in the regulation of reproduction in vertebrates, which is in accordance with previous studies on SPX1.

The log-ratio model (LRM) with three model parameters (${\boldsymbol{n}}$, ${{\boldsymbol{m}}_1},$ and ${{\boldsymbol{m}}_0}$) has been widely used in shallow water bathymetry of multispectral images. At present, obtaining LRM model parameters by optimization algorithms depends on the setting of initial values [${\boldsymbol{n}}(0)$, ${{\boldsymbol{m}}_1}(0),$ and ${{\boldsymbol{m}}_0}(0)$] or initial value ranges. However, how to set the initial value can obtain model parameters more effectively and credibly, and the significance of each initial value in the optimization process is very important for LRM bathymetric retrieval method. In this article, remote sensing images and bathymetric data were used to determine the initial value ranges of credible model parameters, and then the sensitivity of each parameter was analyzed in the algorithm optimization process from the partial differential perspective, and the bathymetry was retrieved in the sea area near Ganquan Island. The results show that according to the partial differential analysis of model parameters, ${\boldsymbol{n}}$, with the smallest change rate in three LRM parameters and the rate of ${{\boldsymbol{m}}_1}$ and ${{\boldsymbol{m}}_0}$ is at least five times of it. So ${\boldsymbol{n}}(0)$ needs more iterations to achieve the optimal. Therefore, the initial value ${\boldsymbol{n}}(0)$ determines whether the optimization method can obtain credible LRM model parameters and the algorithm convergence speed. And when ${\boldsymbol{n}}(0) \in ({1/{{\boldsymbol{R}}_{{{\bf min}}}},\ 1000} ]$ (${{\boldsymbol{R}}_{{{\bf min}}}}$ is the minimum reflectance in blue and green bands), there must be a credible model parameter solution. The bathymetric retrieval results of the sea area with coral and sand substrates show the applicability of our initial value ranges in different substrate area.

Yellowtail kingfish (Seriola lalandi) is a pelagic marine piscivore with a circumglobal distribution. It is particularly suitable for open ocean aquaculture owing to its large body size, fast swimming, rapid growth, and high economic value. A high-precision genome is of great significance for future genetic breeding research and large-scale aquaculture in the open ocean. PacBio, Illumina, and Hi-C data were combined to assemble chromosome-level reference genome with the size of 648.34 Mb (contig N50: 28.52 Mb). 175 contigs was anchored onto 24 chromosomes with lengths ranging from 12.28 to 34.59 Mb, and 99.79% of the whole genome sequence was covered. The BUSCOs of genome and gene were 94.20 and 95.70%, respectively. Gene families associated with adaptive behaviors, such as olfactory receptors and HSP70 gene families, expanded in the genome of S. lalandi. An analysis of selection pressure revealed 652 fast-evolving genes, among which mkxb, popdc2, dlx6, and ifitm5 may be related to rapid growth traits. The data generated in this study provide a valuable resource for understanding the genetic basis of S. lalandi traits.

Sun glint on the sea surface is the unavoidable noise in optical remote sensing images. Water depth retrieval based on optical remote sensing images is vulnerable to sun glint contamination. Different sun glint correction methods and their possible effects on improving the accuracy of optical remote sensing water depth inversion are worth adequately discussing. Considering the problem that traditional sun glint correction methods are not well applied in shallow or turbid water areas, this paper proposes a sun glint correction method based on noise de-correlation (ND-SGC) which is not affected by the essential characteristics of the water body itself and does not require any auxiliary data. In this paper, we analyze the spectral fidelity of remote sensing images by using ND-SGC method and traditional methods for sun glint correction, and compare the accuracy of bathymetry inversion in different water depth cases and between sun glint pixels and sun glint-free pixels. The experimental results indicated that: (1) the ND-SGC method gives different penalty weights to sun glint pixels and sun glint-free pixels, which meaningfully improves the bathymetric inversion accuracy of sun glint pixels and maintains the bathymetric inversion accuracy of sun glint-free pixels, and is applicable to any water depth range; (2) the ND-SGC method improves bathymetric inversion accuracy in the extremely shallow water region (0–2 m) and shallow water region (2–11 m), while the conventional method suppresses bathymetric inversion accuracy in these two water depth ranges; (3) the ND-SGC method maintains the inversion accuracy of the sun glint-free pixels, while the traditional Hedley method and Goodman method increase the mean relative error (MRE) of these pixels by a maximum of 6.7% and 8.8%, respectively; (4) the ND-SGC method preserves the inherent spectral information of the remote sensing image well, while the spectral fidelity index of the images corrected by traditional methods shows a certain degree of distortion of the image’s spectrum.

Although recent studies have revealed its involvement in fish reproduction, the physiological role of neurokinin B (NKB), which is encoded by the tachykinin 3 (tac3) gene, is still largely unclear in teleosts, with most researches on the orders Perciformes and Cypriniformes. In the current study we identified the tac3 gene in a representative species of Pleuronectiformes, the half-smooth tongue sole. The full-length cDNA sequence of tongue sole tac3 was 742 base pairs (bp) in length with an open reading frame of 372 bp that encoded a 123-amino acids preprohormone. The TAC3 precursor contained NKB and an additional peptide NKB-related peptide (NKBRP), both of which had an –FVGLM motif at the C-termini. Tissue distribution analysis indicated that tac3 transcripts were highly expressed in the brain, intestine and gonads of both sexes, and to a lesser extent in other tissues. Among various brain regions, tac3 transcripts were most abundant in the hypothalamus. In addition, the expression profiles of tac3 mRNA were also evaluated along the brain-pituitary-gonadal axis during tongue sole ovarian maturation. The brain tac3 mRNA levels remained unchanged, however, in both pituitary and ovary, the mRNA expression of tac3 increased significantly to the maximal levels at stage III and then declined back to basal levels. Taken together, our results encompass the first study reporting the existence of tac3 gene in a flatfish species and suggest its possible involvement in the control of reproduction in the half-smooth tongue sole.

A 90-day study was performed under three different stocking densities, including high density (10,000 fishes/cage), medium density (8000 fishes/cage), and low density (6000 fishes/cage), in a deep-sea net cage for yellowtail kingfish (Seriola lalandi). The physiological characteristics and growth performance were tested, and structural characteristics of the gastrointestinal microbiota were systematically analyzed. The results show that fishes with high density had a lower weight gain rate and a specific growth rate, as well as higher serum cortisol content. The diversity, types and numbers of dominant microbiota with significant differences, and the numbers of shared genera among the different groups all changed. Core genera in the gastrointestinal tract were obtained according to the principles of dominance, commonality, and difference. The changes in the relative abundance of the core genera might be related to the growth and physiological characteristics of the host. The ratio of Firmicutes to Bacteroidetes in the stomach and pyloric caecum, which favors the accumulation of energy by the host from the diet, was higher in the medium-density group than in the other groups. This indicates that the higher density could cause physiological stress and affect growth performance. In order to reduce the resulting growth differences, gastrointestinal microbiota might assist the host in accumulating energy, participating in the energy distribution by adjusting its structure. Based on the growth, physiology, and production practices, the medium density was the appropriate density in this study. This study provides a reference for the improvement of deep-sea culture technology and the promotion of healthy growth through the gastrointestinal microecological regulation of yellowtail kingfish.

Spexin (SPX) is a newly identified peptide hormone of 14 amino acids (aa) with pleiotropic functions in vertebrates, and recently a second paralog of SPX (named SPX2) has been only found in some non-mammalian species (the previous form of SPX is now designated as SPX1). Results of previous studies have revealed the existence of a functional SPX1 in the half-smooth tongue sole. To clarify the divergence of the spexin family, we further isolated the spx2 gene from the same flatfish species in the current study. The open reading frame of tongue sole spx2 was 378 nucleotides in size that encoded a 125-aa preprohormone, and the tongue sole SPX2 precursor included a 17-aa mature peptide that may be amidated at the C-terminus, rather than a tetradecapeptide reported in most other vertebrates. Tissue distribution analysis showed that spx2 transcripts were found in a wide array of tissues of both sexes with the highest expression in the ovary. In addition, the spx2 mRNA abundance was also ubiquitously detected in different brain regions. Food deprivation for 4 weeks stimulated spx2 mRNA levels at the brain-pituitary-ovary axis, and refeeding of starved fish restored the spx2 mRNA levels back to normal. Taken together, this study provides initial evidence for the existence of a novel spexin gene (spx2) in Pleuronectiformes and suggest its possible involvement in the regulation of energy balance in the female tongue sole.