Abstract:The Chinese longsnout catfish (Leiocassis longirostris) is an economically important freshwater fish in China; males grow faster than the females, suggesting that all-male production is economically desirable in aquaculture. However, their sex determination, sex differentiation genes, and the related regulatory mechanisms remain unknown. In this study, transcriptome sequencing of the ovaries and testes of Chinese longsnout catfish was performed using an Illumina high-throughput sequencing platform. We obtained 358048456 raw reads and 356168472 clean reads from the transcriptome. In total, 24661 genes were annotated, of which 23708 genes were aligned with the reference genome. Gene expression patterns in the ovaries and testes were compared. The results showed that there were 10872 differentially expressed genes (DEGs) between the ovary and testis, including 9375 up-regulated genes and 1497 down-regulated genes. 21 functional DEGs were selected for RT-qPCR analysis and the results confirmed the reliability of the transcriptome analysis. The DEGs were annotated using the Nr, KEGG, and GO databases, where they were enriched in 58 GO functional annotation components and 338 KEGG secondary branching metabolic systems. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed that the differentially expressed genes were significantly enriched in 16 pathways related to sex determination and differentiation and gonadal development, including the Wnt signaling, TGF-β signaling, MAPK signaling, ovarian steroid signaling, and GnRH signaling pathways. Through functional annotation, 71 important sex-related candidate genes were screened, including 50 highly expressed genes (such as dmrt1, cyp17a1, samd7, wnt6, and wt1) and 21 highly expressed ovarian genes (such as foxl2, gdf9, zp3, zp1, figla, and bmp15). Protein–protein interaction (PPI) network analysis revealed that the testis specificity gene dmrt1 interacts with the testis-biased DEGs (sox9, wt1, cyp17a1, rspo1, gata4, and hsd3b) and the ovary-biased DEGs (foxl2, figla, and bmp1), suggesting that dmrt1 as a sex-determining gene can play a role by inducing the testisbiased DEGs and inhibiting the ovary-biased DEGs during testicular development. In this study, the differentially expressed genes related to sex determination and differentiation of Chinese longsnout catfish were identified and the signaling pathways involved in gonadal development in both males and females were revealed. These findings provide important data for future research on the mechanisms of sex determination and differentiation of Chinese longsnout catfish and thus provide theoretical support for all-male seed production in Chinese longsnout catfish.

Abstract:Twist2 (Twist family transcription Factor 2) is a member of the basic Helix-Loop-Helix (bHLH) family that plays an important role in the regulation of animal development. However, the expression patterns and regulatory roles of twist2 genes in fishes are poorly understood. This study aimed to understand the expression pattern of the twist2 gene and its functions in the regulation of muscle development in Siniperca chuatsi. The twist2 gene sequence was obtained from the transcriptome database of S. chuatsi and Twist2 protein characterization and homology were analyzed using bioinformatic methods. The protein encoded by the twist2 gene of S. chuatsi consists of 162 amino acids with a conserved functional bHLH structural domain and the results of multiple sequence comparison based on the amino acid sequences showed that the Twist2 protein has a high similarity in vertebrates. The spatiotemporal expression pattern of the twist2 gene of S. chuatsi was analyzed by RT-qPCR and the expression pattern of the twist2 gene was found to be differentially expressed at different developmental stages. The expression level was low before the mid-gastrula stage, increased significantly from the neurula stage, reached its highest level in the tail-bud stage, and was widely expressed in S. chuatsi tissues; its expression was highest in the spleen, followed by the brain and red muscle. Whole embryo in situ hybridization was used to detect the localization of twist2 at different developmental stages in early embryos and it was found to be specifically expressed mainly in the neuroectoderm and somites. Targetscan and RNAhybrid tools were used to predict miRNAs that might target the mandarin twist2 gene. miR-30a, miR-30b, miR-30e-5p, and miR-204 were found to potentially act on the twist2 3' UTR, suggesting that twist2 is a potential target gene for miR-30a, miR-30b, miR-30e-5p, and miR-204. The short-term starvation stress experiment in S. chuatsi showed that the expression of twist2 after three days of starvation was significantly negatively correlated with the expression of the above miRNAs, suggesting that there is a potential regulatory relationship between twist2 and the above four miRNAs. Therefore, the results of this study will contribute to understanding the sequence characteristics and spatiotemporal expression patterns of the twist2 gene and the functions of potential upstream miRNAs in regulating muscle growth and development at the molecular level, which will provide a theoretical basis for the developmental biology of fish, as well as for healthy aquaculture.

Abstract:Taste serves as the foundation for choosing food and is one of the most vital senses of animals. Numerous taste cells, specialized structures that can sense taste, form taste buds. Taste receptors are present in the membranes of these cells. Taste receptors include type I and type II receptors, which are responsible for identifying distinct flavors in food. The taste receptor type I (T1R) family plays a crucial role in the identification of nutrients in the environment. TIR family genes rely on different heterodimers for flavor recognition and can recognize fresh (T1R1+T1R3) and sweet (T1R2+T1R3) flavors. Animal feeding habits are frequently associated with the number and replication of T1R family genes. Mandarin fish (Siniperca chuatsi) have a unique carnivorous lifestyle and eat live bait throughout their lifetimes. To understand the correlation between the quantity and expression patterns of T1R family members in mandarin fish and their carnivorous nature, gene family analysis was conducted to identify T1R family member genes in mandarin fish genomic data. Changes in the T1R family member gene expression levels were then determined using Quantitative Real-time PCR (qPCR) from to 0–30 days post hatching (0–30 dph) and after the domestication of their diet. This study found that the T1R family in mandarin fish comprises four genes, t1r1, t1r2a1, t1r2a2, and t1r3, each of which possesses complete sequences and structural domains. The t1r2 gene was duplicated as t1r2a1 and t1r2a2, and the exons of both displayed identical structural domains. However, the length of the 3rd, 4th, and 5th introns and 5' UTR varied among them. Selection pressure (dN/dS) analysis indicated positive selection for t1r1 in mandarin fish, whereas t1r2s and t1r3 showed negative selection. The possible cause of evolutionary pressure on the umami receptor gene t1r1 might be related to the uncommon diet of this fish species. The expression of four taste receptor type I genes, t1r1, t1r2a1, t1r2a2, and t1r3, began before the oropharyngeal cavity of mandarin fish was completely formed, and increased after 20 d. The umami receptor gene (t1r1) was found to have a much higher expression among these genes than that of the sweet taste receptor gene (t1r2s). All four t1r genes showed lower expression levels after domestication, with t1r1, the umami receptor gene, showed the most significant downregulation. These findings provide fundamental data for investigating the effects of taste receptor genes on the development and adaptation of mandarin fish to carnivores.

Abstract:As an endemic economic shrimp along the coast of China, the ridge-tailed white shrimp Exopalaemon carinicauda has biological advantages such as strong reproductive ability, environmental adaptability, and a fast growth cycle. To investigate the genetic variation of different geographic populations of E. carinicauda in the China sea, we set up two spatial sequence samples, including 3 small-scale sequence samples (northern Zhejiang, central Zhejiang, and southern Zhejiang) and 5 large-scale sequence samples [Bohai Sea, Yellow Sea, East China Sea (Zhejiang), East China Sea (Fujian), and South China Sea]. These samples underwent PCR product purification and sequencing and a total of 210 samples of E. carinicauda from seven wild populations were sequenced for CO I and 16S rRNA gene sequences. The CO I gene sequence of 515 bps with 58.65% A+T content and the 16S rRNA gene sequence of 520 bps with 62.02% A+T content were obtained and the analysis results of two gene sequences in population genetic diversity were consistent with those of the Yellow Sea population having the highest genetic diversity and the population from the South China Sea having the lowest genetic diversity. The results of AMOVA analysis were consistent. The differences between the two gene sequence analyses were as follows: the CO I gene sequence analysis detected 63 variable sites, with a haplotype diversity (Hd) of 0.353–0.809 and a nucleotide diversity (Pi) of 0.00140–0.00497, while the 16S rRNA gene sequence analysis detected 41 variable sites, with an Hd of 0.265–0.801 and a Pi of 0.00102–0.00403. The CO I gene showed greater variation and was able to discriminate between samples in smaller-scale spatial sequences. The results of the present study provide basic data for the conservation of Exopalaemon carinicauda germplasm resources.

Abstract:To analyze the genotype-environment (G×E) interaction effects on the growth and survival traits of a rapid-growth strain of Crassostrea gigas, 28 full-sib families were constructed, and each family was divided into three groups, which were cultured in the Rushan, Rongcheng, and Huangdao Sea areas. The heritability and G×E effects on the growth and survival traits of 12-month-old C. gigas were estimated using a two-trait animal model and the REML method. The BLUP breeding value estimation method was used to comprehensively weigh the breeding values for shell height, body weight, and survival traits to screen for universal breeding materials. The results showed that the shell phenotypes of the strains found in the Rushan and Huangdao Sea areas were significantly better than those of the Rongcheng Sea area, which were more suitable for aquaculture. The heritability of growth and survival traits in the Rushan, Rongcheng, and Huangdao Sea areas were 0.16–0.62, 0.07–0.80, and 0.07–0.63, respectively. A scale effect was observed; however, in addition to the heritability of survival traits, they were all medium-high heritability and had good breeding potential. Therefore, the breeding effect was improved using the breeding value estimation method. Survival traits are mainly affected by environmental factors and can be directly selected based on the phenotype. Considering the different sea areas as fixed effects, the heritability of growth and survival traits, calculated by combining the three environments was 0.04–0.31. The decrease in comprehensive heritability indicated that environmental differences affected heritability assessment. In addition, the genetic correlation of the selected target traits between the two sea areas was less than 0.8, which indicated a significant G×E effect. Therefore, environmental factors must be considered comprehensively during breeding. By comparing the comprehensive breeding values, it was found that family G25 showed special adaptability to the Rushan Sea area, family G16 had special adaptability to the Rongcheng Sea area, and family G23 showed adaptability to the Rushan and Huangdao Sea areas, all of which showed significant gene rearrangement effects. Family G2 was found to have universal adaptability to the three sea area. The results obtained in this study provide an important reference for breeding a rapid-growth strain of the orange shell C. gigas.

Abstract:This study was conducted to analyze the effects of 5-HMF (5-hydroxymethylfurfural) on the growth performance, intestinal histology, serum biochemical characteristics, and gastrointestinal microecology of juvenile yellowtail kingfish. A total of 360 6-month-old yellowtail kingfish juveniles with an initial mean body mass of (82.33±2.75) g were randomly divided into four groups (M0 group, M1 group, M2 group, and M3 group) with three replicates per group and 30 fish assigned in each replicate. The M0 group was the control group and was fed the basic diet, whereas the M1, M2, and M3 groups were fed the experimental diets (0.25% 5-HMF, 0.5% 5-HMF, and 1% 5-HMF, respectively). The experiment lasted for six weeks. The results showed that the proliferation experiment of Caco-2 cells (human colorectal adenocarcinoma cells) in vitro showed that the concentration of 5-HMF used in this study was within the safe range. The weight gain rate and specific growth rate of the M2 and M3 groups were significantly higher than those of the M0 group (P＜0.05) and the plumpness of the M2 group was significantly higher than that of the M0 group (P＜0.05). The muscle layer thickness and villus height of the M2 group were significantly higher than those of the M0 group (P＜0.05). The activities of glutathione peroxidase (GSH-PX), superoxide dismutase (SOD), and catalase (CAT) in the serum of M2 and M3 groups were significantly higher than those in the serum of M0 group (P＜0.05), while the level of malondialdehyde (MAD) was significantly lower (P＜0.05). The serum levels of immunoglobulin M (IgM) and alkaline phosphatase (AKP) in M2 and M3 groups were significantly higher than those in the M0 group (P＜0.05). The activities of lysozyme (LZM) and acid phosphatase (ACP) in the M2 group were significantly higher than those in the M0 group (P＜0.05). In the M0 group, the Chao1 index of pyloric caecum microbiota was significantly higher than that of the M1 group (P＜0.05), while the Shannon index of intestinal microbiota in the M2 group was significantly higher than that in the M0 and M1 groups (P＜0.05). The results of this study showed that the addition of 0.5% 5-HMF to feed could improve the growth performance and promote serum immune levels and antioxidant capacity of yellowtail kingfish. 5-HMF may have played a role in optimizing and regulating the intestinal morphology and microbial community structure of juvenile yellowtail kingfish. This provides theoretical support for the development of healthy breeding technologies and green feed additives for yellowtail kingfish.

Abstract:This study was conducted to evaluate the breeding effect and growth performance of oversized Eriocheir sinensis parents (maternal≥300 g, paternal≥400 g) using normal parents (maternal≥150 g, paternal≥250 g) as the controls. The experimental period lasted for approximately 20 months. During the experiment, multiple growth-proofing and gonadal developmental anatomies were combined with the final harvest size distribution. The growth performance, gonadal development, and culture effect of the two groups were comprehensively evaluated, and the results showed the following: (1) the average body weight of the oversized group was higher than that of the control group at the bean, juvenile, and adult crab stages, and it was significantly higher than that of the control group from September to November during the juvenile crab culture stage (P＜0.05). For adult crabs, the average body weight was significantly higher than that of the control group in July (P＜0.05), and there was no significant difference in the average body weight of the two groups in other months. (2) In terms of puberty-molting and gonadal development, the puberty-molting rates of males and females in the oversized group were significantly lower from September 5 to September 25 and July 25 to September 5 (P＜0.05). In addition, the gonadosomatic indices (GSI) of males and females in the oversized group were significantly lower on October 10th and September 10th (P＜0.05). The hepatosomatic index (HSI) of males in the oversized group was significantly higher than that of males in the control group (P＜0.05) on October 10th and the HSI of individual crabs in the oversized group was higher, but there was no significant difference at the other time points (P＞0.05). (3) For juvenile crabs, the survival rate and yield of the oversized group were higher than those of the control group, and the yield of male bodies at the juvenile stage was significantly different from that of the control group (P＜0.05). For adult crabs, although the oversized group had a lower survival rate, the average body weight of the oversized group was higher and the final yield was higher. The feed conversion rate (FCR) of the oversized group at the juvenile crab culture stage was significantly lower than that of the control group. (4) In terms of body weight distribution, the proportions of large crabs (＞12 g) and large adult crabs (＞230 g for males and ＞200 g for females) in the oversized group were significantly higher than those in the control group, and the proportions of small crabs and small adult crabs in the oversized group were significantly lower than those in the control group (P＜0.05). In summary, individuals in the oversized group had faster growth rates and higher yields than those in the control group, and the proportion of oversized crabs was significantly higher than that in the control group, which gives the offspring of oversized parents a significant economic advantage. The application prospects and markets for large Chinese mitten crab parents are broad, thus it is worth popularizing and applying on a large scale.

Abstract:To explore the feeding habits and trophic niche of mesopelagic fish in the adjacent sea area of Xisha in the South China Sea, this study focused on four mesopelagic fish: Diaphus splendidus, Diaphus mollis, Diaphus fulgens, and Diaphus luetkeni. Carbon and nitrogen stable isotope technique was used to investigate the trophic levels, trophic niches, and primary food sources of these four species. The results showed that: (1) The δ¹⁵N values of the four species ranged from 8.31‰ to 11.08‰, and the δ¹³C values ranged from –21.88‰ to –18.19‰; (2) There were differences in trophic levels among the four species, with D. luetkeni (3.75) ＞ D. splendidus (3.63) ＞ D. fulgens (3.57) ＞ D. mollis (3.41); (3) D. fulgens had a largest trophic niche width and a stronger ability to utilize bait resources. The trophic niche width of D. splendidus was the smallest, and which had strong selectivity and feeding specificity towards bait organisms; (4) There was a certain competitive relationship in food resource intake among the four species, but there was no competition between D. luetkeni and D. mollis; (5) Bayesian mixture models indicated that zooplankton was the main food source for the four mesopelagic fish. This study revealed the main food sources and trophic niche characteristics of the four mesopelagic fish species in the adjacent sea area of Xisha, providing basic data for the study of the feeding characteristics and nutritional structure relationships of mesopelagic fish, and was significant for further research on the energy flow and material cycling in marine ecosystems.

Abstract:Yellowfin sea bream (Acanthopagrus latus), an economically important fish and stock enhancement species, is distributed in the southeastern coastal areas of China. Study on diet change of yellowfin sea bream will contribute to the knowledge for the accurate food composition and ecological function of food wed for yellowfin sea bream. However, information on the feeding habits of wild yellowfin sea bream in natural marine environments is scarce. In this study, the feeding characteristics of yellowfin sea bream of different body lengths (1-3 years old) collected from the Wanshan Islands were analyzed based on stomach morphology and high-throughput sequencing of the 18S rRNA. Here, the 17 collected fishes were divided into three groups: the S group [body length: (194±13.1) mm], the M group [body length: (231±15.5) mm], and the L group [body length: (294±33.3) mm]. Morphological identification showed that shellfish, crabs, and chyme were present in the stomach contents of the yellowfin sea bream. Most fish contained a high proportion of chyme. A total of 84 species of eukaryotes in 16 phyla were identified in the stomach contents of the yellowfin sea bream using high-throughput sequencing of the 18S rRNA gene. Therefore, 18S rRNA high-throughput sequencing analysis provided a higher taxonomic resolution of the dietary composition than microscopic examination. Furthermore, high-throughput sequencing analyses showed that the major food sources of yellowfin sea bream were Charybdis feriatus and Centropages abdominalis of Arthropoda, Brachidontes variabilis of Mollusca, Nematalosa nasus of Chordata, and Neopyropia yezoensis of Rhodophyta, accounting for 66.73% of total food sources. Yellowfin sea bream in the Wanshan Islands mainly fed on animal food and feeding on plant food was relatively low. At the phylum level, the S group were dominant with Arthropoda, Mollusca and Rhodophyta, accounting for 93.88% of all sequences. Chordata, Cnidaria, and Arthropoda were the dominant phyla in the M group, accounting for 86.72% of all sequences, whereas L group was dominated by Chordata, Platyhelminthes, and Arthropoda, accounting for 89.25% of all sequences. Diet analysis of 1-3-year-old yellowfin sea bream revealed that with an increase in body length, the major feeding species of yellowfin sea bream gradually shifted from Arthropoda and Mollusca to Chordata, and the number of feeding species gradually decreased. Compared with that of the other Sparidae fishes in the Pearl River Estuary, a similar dietary habit was observed between Sparidae fishes, indicating feed-food competition for Sparidae fishes from the Pearl River Estuary. Sparidae are omnivorous and mostly consume benthos, plankton, and fish. These results provide basic data on the feeding ecology of yellowfin sea bream in the Wanshan Islands.

Abstract:Merluccius hubbsi is one of the most prominent fishing species in the southwestern Atlantic Ocean, and its catch has been increasing continuously in recent years. Owing to its delicate taste and rich nutritional value, it is very popular among nationals and is one of the major chilled fish imported in China. Anisakid larvae are one of the most common parasites of Merluccius hubbsi. Available data show that its infection rate can be as high as 90% or more, and the intensity of the infection is also high. Parasite infection not only draws nutrients from the host fish but also affects the aesthetics of the fish and reduces its economic value. This study aimed to understand the infection of M. hubbsi by anisakid larvae in the waters of the Southwest Atlantic, to perform biological experiments on M. hubbsi, to collect parasites for morphological identification, and to record and analyze the data. Samples for this study were collected from sampling sites in open ocean waters outside the exclusive economic zone of Argentina and the Malvinas Islands during the summer months of October 2022 to February 2023, with a total of 435 individuals. The results showed that the infection rate of M. hubbsi infected by anisakid larvae was 99.08% and the mean intensity of infection was (19.56±24.76) fish/tail. The higher the sexual maturity of M. hubbsi, the higher the mean intensity of infection of anisakid larvae was; the mean intensity of infection of anisakid larvae was as high as (50.61±56.78) fish/tail when sexual maturity was stage IV. The sites of infection of anisakid larvae were the abdominal cavity, liver, and stomach. The infection intensity of anisakid larvae was significantly higher in the abdominal cavity and liver than at other sites (P＜0.05). This study showed that the abundance and intensity of M. hubbsi infection by anisakid larvae in the Southwest Atlantic was high, which was related to the stable ecosystem of its living sea area, large host population size, and suitable environment for survival. As the host matures, the range and number of feeding objects increase, and the parasite accumulates in the host during this process; it mainly parasitizes the abdominal cavity where the environment is suitable and the area of parasitism is large, and the liver and stomach, which have high nutritional value, are suitable for its growth, development, and stable parasitism. In summary, the number of M. hubbsi infected with anisakid larvae was related to sexual maturity and different infection sites in the sample fish, and the number of infections was correlated with fork length and body weight. This study provides a basis for understanding the growth characteristics and factors influencing anisakid larval infection of M. hubbsi.