Southern Catfish Research Articles

Properties of Skin Collagen from Southern Catfish (Silurus meridionalis) Fed with Raw and Cooked Food.

The southern catfish (Silurus meridionalis) is an economically important carnivorous freshwater fish in China. In this study, we compared the properties of skin collagen from southern catfish fed with raw food (RF) and cooked food (CF). The skin collagen yield in the RF group (8.66 ± 0.11%) was significantly higher than that of the CF group (8.00 ± 0.27%). SDS-PAGE, circular dichroism spectroscopy, and FTIR analyses revealed that the collagen extracted from southern catfish skin in both groups was type I collagen, with a unique triple helix structure and high purity. The thermal denaturation temperature of collagen in the RF group (35.20 ± 0.11 °C) was significantly higher than that of the CF group (34.51 ± 0.25 °C). The DPPH free radical scavenging rates were 68.30 ± 2.41% in the RF collagen and 61.78 ± 3.91% in the CF collagen, which was higher than that found in most fish collagen. Both the RF and CF groups had high ability to form fibrils in vitro. Under the same conditions, the CF group exhibited faster fibril formation and a thicker fibril diameter (p < 0.05). In addition, the RF group exhibited significantly higher expression of col1a1 compared to the CF group. These results indicated that feeding southern catfish raw food contributed to collagen production, and the collagen from these fish may have potential in biomaterial applications.

Characterization of the male-specific region containing the candidate sex-determining gene in Amur catfish (Silurus asotus) using third-generation- and pool-sequencing data

Amur catfish (Silurus asotus) is an ecologically and economically important fish species in Asia. Here, we assembled the female and male Amur catfish genomes, with genome sizes of 757.15 and 755.44 Mb, respectively, at the chromosome level using nanopore and Hi-C technologies. Consistent with the known diploid chromosome count, both genomes contained 29 chromosome-size scaffolds covering 98.80 and 98.73 % of the complete haplotypic assembly with scaffold N50 of 28.87 and 27.29 Mb, respectively. The female (n = 40) and male (n = 40) pools were re-sequenced. Comparative analysis of sequencing and re-sequencing data from both sexes confirmed the presence of an XX/XY sex determination system in Amur catfish and revealed Chr5 as the sex chromosome containing an approximately 400 kb Y-specific region (MSY). Gene annotation revealed a male-specific duplicate of amhr2, namely amhr2y, in MSY, which is male-specific in different wild populations and expressed only in the testes. Amur catfish shared partially syntenic MSY and amhr2y genes with the southern catfish (S. meridionalis, Chr24), which were located on different chromosomes. High sequence divergence between amhr2y and amhr2 and high sequence similarity with amhr2y were observed in both species. These results indicate the common origin of the sex-determining (SD) gene and transition of amhr2y in the two Silurus species. Accumulation of repetitive elements in the MSY of both species may be the main driver of the transition of amhr2y. Overall, our study provides valuable catfish genomic resources. Moreover, determination of amhr2y as the candidate SD gene in Amur catfish provides another example of amhr2 as the SD gene in fish.

Dietary xanthophyll improved growth, antioxidant, pigmentation and meat quality in the southern catfish (Silurus soldatovi meridionalis Chen).

Xanthophyll has multiple physiological functions to improve the quality of farmed animals. The present study aimed to investigate the effects of xanthophyll on the growth performance, antioxidation, immunity, pigmentation and meat quality of southern catfish (Silurus soldatovi meridionalis Chen). Juvenile southern catfish (18.35±0.04g) were randomly allocated into 24 cages (30 juveniles per cage), and fed diets with different dietary xanthophyll levels (at 14, 42, 80, 108, 126 and 152mg/kg, dry matter of diet) twice daily for 8 weeks. Results indicated that the diet with 80mg/kg xanthophyll induced a higher specific growth rate (SGR), feed efficiency ratio and protein productive value. Moreover, the 80mg/kg xanthophyll dietalso increased complement 3, immunoglobulin M (IgM) and lysozyme content more than the other groups. The mRNA expression level of inflammation-related genes and antioxidant enzyme activities further confirmed the effects of 80mg/kg dietary xanthophyll on improving immune response. The present study also found that the 126mg/kg xanthophyll diet significantly enhanced the content of total carotenoids and xanthophyll, hydroxyproline, collagen and amino acid in muscle. The diet with 126mg/kg xanthophylls also induced lower drip loss, thawing loss, centrifugal loss, cooking loss and higher muscle adhesiveness, cohesiveness, springiness, gumminess and chewiness than the other treatments. In conclusion, quadratic regression model analysis based on SGR and IgM revealed that the optimum xanthophyll level in the diet was 86.78 and 84.63mg/kg, respectively. Moreover, broken line regression analysis based on xanthophyll content in dorsal skin and chewiness in muscle demonstrated that the optimal xanthophyll level was between 89.73 and 108.66mg/kg in the diet of southern catfish (S.soldatovi meridionalis Chen).

Short-term feeding of dietary florfenicol influences gut microbiome and growth performance of fast-growing Silurus meridionalis

Florfenicol is a broad-spectrum antibiotic commonly used in aquaculture to control bacterial infections in fish. However, it may induce perturbation on the gut microbiota, compromising growth and fitness. This study aimed to evaluate the gut microbial community, histological alterations, and growth performance of fast-growing southern catfish Silurus meridionalis larvae fed florfenicol-medicated feed (florfenicol group) at 20 mg/kg biomass per day for 7 days as well as the recovery effects for another 7 days by comparing those of the counterparts fed non-medicated feed (control group). The results showed that dietary florfenicol strikingly affected gut microbial community development and caused community perturbation at day 7, with Proteobacteria enrichment up to 99.3% in the florfenicol group and progressive reductions in microbial diversity and evenness. The antibiotic also rapidly led to enrichments of potential opportunistic pathogens such as Citrobacter, Plesiomonas, and unclassified Aeromonadaceae, which may carry more abundant antibiotic-resistance genes. However, the perturbation effects disappeared gradually after dietary florfenicol withdrawal. Metagenomic predictions revealed an increase in the number of altered microbial functions during the administration of dietary florfenicol and a successive decrease after the withdrawal. Dietary florfenicol had no effect on the gut histological structures and mortalities of southern catfish, but it significantly decreased feed intake. The changes in temporally observed community and function coincided with the commencement of decreased feed intake and increased feed conversion ratio in the florfenicol group, consequent to retarded growth during the dietary administration. These alterations were gradually restorable after the dietary florfenicol withdrawal. This study suggests that short-term feeding of dietary florfenicol disturbs the gut microbiota, potentially impairing fish feeding and feed utilization and, lastly, fish growth performance. The results highlight the importance of responsible and prudent use of florfenicol in aquaculture and the need for more studies to understand the health consequences associated with gut microbiome disturbance resulting from antibiotic use in aquatic animals.

Transcriptome Analysis Reveals Effect of Dietary Probiotics on Immune Response Mechanism in Southern Catfish (Silurus meridionalis) in Response to Plesiomonas shigelloides.

To explore whether a probiotic complex composed of Lactobacillus rhamnosus, Lactobacillus plantarum, and Lactobacillus casei can prevent or inhibit the inflammatory response caused by the invasion of Plesiomonas shigelloides in the southern catfish, we screened differentially expressed genes and enriched inflammation-related pathways among a control and three experimental groups and conducted analysis by transcriptome sequencing after a 21-day breeding experiment. Compared with those in the PS (Plesiomonas shigelloides) group, southern catfish in the L-PS (Lactobacillus-Plesiomonas shigelloides) group had no obvious haemorrhages or ulcerations. The results also showed that inflammation-related genes, such as mmp9, cxcr4, nfkbia, socs3, il-8, pigr, tlr5, and tnfr1, were significantly upregulated in the PS group compared with those in the L-PS groups. In addition, we verified six DEGs (mmp9, cxcr4, nfkbia, socs3, rbp2, and calr) and three proteins (CXCR4, NFKBIA, and CALR) by qRT-PCR and ELISA, respectively. Our results were consistent with the transcriptome data. Moreover, significantly downregulated genes (p < 0.05) were enriched in inflammation-related GO terms (lymphocyte chemotaxis and positive regulation of inflammatory response) and immune-related pathways (intestinal immune network for IgA production and IL-17 signalling pathway) in the L-PS vs. the PS group. Our results indicate that the infection of P. shigelloides can produce an inflammatory response, and probiotics could inhibit the inflammatory response caused by P. shigelloides to some extent.

Correction to: 'Identification of sex chromosome and sex-determining gene of southern catfish based on XX, XY and YY genome sequencing' (2022) by Zheng et al.

Correction to: 'Identification of sex chromosome and sex-determining gene of southern catfish based on XX, XY and YY genome sequencing' (2022) by Zheng et al.

Effect of feeding different diets on postprandial metabolic response, digestive capacity and growth performance in juvenile southern catfish (Silurus meridionalis)

To investigate the effect of feeding different diets (non-artificial food and formulated diet and their mix) on specific dynamic action (SDA), digestive capacity and growth performance in carnivorous fish species, we measured the postprandial metabolic response and meal remaining in the stomach, specific growth rate (SGR) and intestinal microbial diversity of juvenile southern catfish (Silurus meridionalis) fed loach meat, formulated diet and their mix at 25 °C. The peak metabolic rate (PMR) and peak metabolic scope (PMS) of both the formulated diet and mixed diet group were higher than those of the loach meat group, and the SDA duration of both the formulated diet and mixed diet groups was shorter than that of the loach meat group. The percentage food remaining in the stomach was the lowest in the formulated diet group and the highest in the loach meat group at 16 and 24 h postfeeding, and the gastric evacuation rate was the highest in the formulated diet group and the lowest in the loach meat group. Both SGR and feeding rate (FR) were the highest in the formulated diet group and the lowest in the loach meat group. The bacterial community richness and relative abundance at the phylum and genus levels in the gut microbiota were higher in most samples of formulated diet group and lower in all samples of the loach meat group. These results indicated that southern catfish fed formulated diet had a higher metabolic response, faster digestion process and better growth performance than those fed loach meat. We suggest that these differences were related to the macronutrient composition (i.e., the relative content of protein, carbohydrate and lipid), type of food, as well as ingredients of formulated diets and gut microbiota.

High resistance to Ichthyophthirius multifiliis in Percocypris pingi, an endemic fish in upper Yangtze River

Ichthyophthirius multifiliis is a ciliated ectoparasite that infects almost all freshwater fishes with low host specificity. However, an endemic fish in the upper Yangtze River, Percocypris pingi, was thought by aqua-culturists to be completely resistant to I. multifiliis. We therefore aimed to determine P. pingi resistance to I. multifiliis. We compared the survival, feed intake, infection intensity, relative number of I. multifiliis tomont and tomont diameter, and the short- (2 months) and long-term (7 months) effects of I. multifiliis on growth between P. pingi, southern catfish (Silurus meridionalis), grass carp (Ctenopharyngdon idella), Schizothorax prenanti and Culter alburnus post I. multifiliis infection at 20,000 theronts/fish under controlled laboratory conditions. We also tested serum and skin mucus immobilization activity to I. multifiliis in uninfected individuals of the five fish species. The results detected mild I. multifiliis infection in P. pingi reflected by significantly lower parasite density and no expression of parasite I-antigen gene IAG52A comparable to the four other species. The low total relative number of tomonts collected from P. pingi during almost the entire parasitic stage (from the third to sixth day post infection) and small tomont size during the later parasitic stage (the fifth and sixth day post infection) suggest that P. pingi was able to limit trophont settlement and/or development. When infected by I. multifiliis, P. pingi displayed almost no mortality compared to high mortality in the other four fish species (14 days post infection) and surviving individuals recovered from the initial appetite loss within seven days post infection and represented normal growth over the following two and seven months. Furthermore, P. pingi mucus and serum have strong immobilization activity to I. multifiliis. It appears that P. pingi is highly resistant to I. multifiliis and may be a good model to understand the underlying immune mechanism of fish to I. multifiliis.

Identification of sex chromosome and sex-determining gene of southern catfish (Silurus meridionalis) based on XX, XY and YY genome sequencing.

Teleosts are important models to study sex chromosomes and sex-determining (SD) genes because they present a variety of sex determination systems. Here, we used Nanopore and Hi-C technologies to generate a high-contiguity chromosome-level genome assembly of a YY southern catfish (Silurus meridionalis). The assembly is 750.0 Mb long, with contig N50 of 15.96 Mb and scaffold N50 of 27.22 Mb. We also sequenced and assembled an XY male genome with a size of 727.2 Mb and contig N50 of 13.69 Mb. We identified a candidate SD gene through comparisons to our previous assembly of an XX individual. By resequencing male and female pools, we characterized a 2.38 Mb sex-determining region (SDR) on Chr24. Analysis of read coverage and comparison of the X and Y chromosome sequences showed a Y specific insertion (approx. 500 kb) in the SDR which contained a male-specific duplicate of amhr2 (named amhr2y). amhr2y and amhr2 shared high-nucleotide identity (81.0%) in the coding region but extremely low identity in the promotor and intron regions. The exclusive expression in the male gonadal primordium and loss-of-function inducing male to female sex reversal confirmed the role of amhr2y in male sex determination. Our study provides a new example of amhr2 as the SD gene in fish and sheds light on the convergent evolution of the duplication of AMH/AMHR2 pathway members underlying the evolution of sex determination in different fish lineages.

Acute Cd Toxicity, Metal Accumulation, and Ion Loss in Southern Catfish (Silurus meridionalis Chen)

The amounts of cadmium in multiple organs and the amounts of Na+ and Ca2+ in the carcass were measured in dead and surviving southern catfish exposed to different concentrations of Cd. The 96 h median lethal concentration was 6.85 mg/L. The Cd content and Cd accumulation rate were positively correlated with Cd exposure concentrations, and there were significant differences between dead and surviving individuals, indicating that both Cd content in tissues and Cd accumulation rates were correlated with mortality. Cd levels in the liver of dead fish were saturated. A lethal threshold for Cd concentration in the whole fish was obtained. Bioconcentration factors for Cd did not decrease with increasing exposure. Acute exposure to waterborne Cd caused a significant decrease in the ion content of the fish carcass. There was a significant difference between the Na+ content of the carcass of dead fish (34.54 μmol/g wet weight) and surviving fish (59.34 μmol/g wet weight), which was not the case with the Ca2+ content, indicating that the lethal toxicity of Cd was probably related to the decrease in Na+ content. Collectively, these results suggest that whole-fish Cd concentration and carcass Na+ content can be useful indicators of fish acutely exposed to Cd.

Chromosome-level assembly of southern catfish (silurus meridionalis) provides insights into visual adaptation to nocturnal and benthic lifestyles.

The Southern catfish (Silurus meridionalis) is a nocturnal and benthic freshwater fish endemic to the Yangtze River and its tributaries. In this study, we constructed a chromosome-level draft genome of S. meridionalis using 69.7-Gb Nanopore long reads and 49.5-Gb Illumina short reads. The genome assembly was 741.2Mb in size with a contig N50 of 13.19Mb. An additional 116.4Gb of Bionano and 77.4Gb of Hi-C data were applied to assemble contigs into scaffolds and further into 29 chromosomes, resulting in a 738.9-Mb genome with a scaffold N50 of 28.04Mb. A total of 22,965 protein-coding genes were predicted from the genome with 22,519 (98.06%) genes functionally annotated. Comparative genomic and transcriptomic analyses revealed a rod-dominated visual system which was responsible for scotopic vision. The absence of cone opsins SWS1 and SWS2 resulted in the lack of ultraviolet and blue violet sensitivity. Mutations at key amino acid sites of RH1.1, RH1.2 and RH2 resulted in spectral tuning good for dim light vision and narrow colour vision. A higher expression level of rod phototransduction genes than that of cone genes and higher rod-to-cone ratio led to higher optical sensitivity under dim light conditions. In addition, analysis of the genes involved in eye morphogenesis and development revealed the loss of some conserved noncoding elements, which might be associated with the small eyes in catfish. Together, our study provides important clues for the adaptation of the catfish visual system to the nocturnal and benthic lifestyles. The draft genome of S. meridionalis represents a valuable resource for studies of the molecular mechanisms of ecological adaptation.

Effects of short-term fasting on spontaneous activity and excess post-exercise oxygen consumption in four juvenile fish species with different foraging strategies.

ABSTRACTTo investigate the effects of short-term fasting on spontaneous activity and excess post-exercise oxygen consumption (EPOC) in sit-and-wait carnivorous southern catfish (Silurus meridionalis), active carnivorous black carp (Mylopharyngodon piceus), active herbivorous grass carp (Ctenopharyngodon idellus) and active filter-feeding silver carp (Hypophthalmichthys molitrix), each species was divided into a control group and a fasting group (deprived of food for 14 days). Both groups were maintained at 25°C and, at the end of the experimental period, the total movement distance (TMD), percent time spent moving (PTM), ventilation frequency (Vf), pre-exercise oxygen consumption (M(•)O2) and EPOC response of the experimental fish were measured. The TMD and PTM obtained for the control group of southern catfish were lower than those found for the control groups of the three active species. Short-term fasting resulted in decreases in the TMD and PTM of the southern catfish and black carp and increases in the TMD of grass carp and silver carp. The Vf of southern catfish was significantly higher than those of grass carp and silver carp, whereas the latter was also significantly higher than that of black carp. Short-term fasting resulted in significant increases in the Vf and decreases in the pre-exercise M(•)O2 of southern catfish and silver carp. Southern catfish and black carp exhibited lower peak post-exercise M(•)O2 and recovery rates, and relatively higher EPOC magnitudes than grass carp and silver carp. Short-term fasting exerted no significant effects on the peak post-exercise M(•)O2, but resulted in relatively higher EPOC magnitudes in the four fish species. These results suggest that (1) different fish species exhibit significantly different levels of spontaneous activity and post-exercise M(•)O2 profiles with different characteristics and that (2) short-term fasting exerts different effects on the level of spontaneous activity in four fish species with different foraging strategies.

Screening and characterization of sex-linked DNA markers and marker-assisted selection in the Southern catfish (Silurus meridionalis)

The Southern catfish (Silurus meridionalis), which exhibits a sexual dimorphism with female growth faster than male, is an economically important fish in China. However, the lack of a rapid and accurate sex specific marker has hampered the application of sex control in the breeding process, as well as the investigations of sex-determination in this species. In this study, by comparative analysis of next generation sequencing (NGS) and re-sequencing data from male and female samples, 38 candidate Y chromosome-specific sequences were screened out, their sex-specificity were validated in Southern catfish through PCR amplification. Finally, 8 sex-linked DNA markers were obtained to distinguish the genetic sex of Southern catfish. Importantly, one of those markers (Marker-8) could serve as a highly specific and robust co-dominant marker for genetic sex identification of XX, XY and YY individuals. Subsequently, a marker-assisted selection (MAS) technique was developed for the mass production of XX all-females for aquaculture and YY supermales for basic research especially in sex determination. In conclusion, our current data suggest an XY sex determination system in the Southern catfish. Sex-linked DNA markers obtained in this study will have potential applications in the isolation of sex determine gene, elucidation of the sex determination mechanism, and accelerate the development of sex control breeding in Southern catfish.

Microbiome of Co-cultured Fish Exhibits Host Selection and Niche Differentiation at the Organ Scale.

Fish are the most widespread aquaculture species and maintain complex associations with microbial consortiums. However, the ecology of these associations present in multiple microhabitats in fish remains elusive, especially on the microbial assembly in fish external (skin and gill) and internal (stomach and intestine) niches, and the relationship with the rearing environment. To understand host dependence and niche differentiation of organ-specific microbiome signatures using a 16S rRNA gene-based sequencing technique, we systematically provided characterizations of a comparative framework relevant to the microbiome of stomach, regional intestine, skin, and gill in two important farmed fish species, herbivorous grass carp (Ctenopharyngodon idella) and carnivorous southern catfish (Silurus meridionalis), and of the rearing water. The different feeding habits of grass carp and southern catfish showed a significant separation of microbial community structure, with great compositional differences across body sites within each species. Site-driven divergences relied on host species: the same types of microhabitats between grass carp and southern catfish harbored differential microbiome. Additionally, body sites had remarkably distinct communities and displayed lower alpha diversity compared to rearing water. Unexpectedly, the stomach of southern catfish had the highest microbial diversity in the digestive tract of the two co-cultured fish species. For external sites within each species, a higher diversity occurred in gill of grass carp and in skin of southern catfish. Our results unveil different topographical microbiome signatures of the co-cultured species, indicating host selection in individual-level microbial assemblages and niche differentiation at the organ scale. This work represents a foundation for understanding the comprehensive microbial ecology of cohabiting farmed fish, suggesting potential applications associated with fish microbiome that urgently needs to be assessed in polycultured operations in aquaculture.

Convergent Evolution of Mucosal Immune Responses at the Buccal Cavity of Teleost Fish.

SummaryThe buccal mucosa (BM) is a critical first line of defense in terrestrial animals. To gain further insights into the evolutionary origins and primordial roles of BM in teleosts here we show that rainbow trout, a teleost fish, contains a diffuse mucosal associated lymphoid tissue (MALT) within its buccal cavity. Upon parasite infection, a fish immunoglobulin specialized in mucosal immunity (sIgT) was induced to a high degree, and parasite-specific sIgT responses were mainly detected in the buccal mucus. Moreover, we show that the trout buccal microbiota is prevalently coated with sIgT. Overall our findings revealed that the MALT is present in the BM of a non-tetrapod species. As fish IgT and mucus-producing cells are evolutionarily unrelated to mammalian IgA and salivary glands, respectively, our findings indicate that mucosal immune responses in the BM of teleost fish and tetrapods evolved through a process of convergent evolution.

English

Recently, a skin ulcerative disease caused by Vibrio mimicus has led to heavy economic losses in catfish, including yellow catfish, southern catfish, and Zhengchuan catfish in China. Currently, there was no effective method of controlling the outbreak of this disease. In this study, the bacterial isolates were obtained from dying channel catfish and identified as V. mimicus, which consist of formalin-inactivated V. mimicus (antigen). After first immunization, four weeks later, fishes were exposed to V. mimicus and the immune response was analyzed: Fish survival, respiratory burst activity of blood leukocytes, serum agglutination titers, and lysozyme activity, every week (during four weeks). Survival was up 90%. Respiratory burst activity of blood leukocytes, serum agglutination titers, and lysozyme activity were determined at 1, 2, 3, and 4 weeks after primary immunization. Immunization of channel catfish protected hosts against V. mimicus infection with a survival percentage of more than 90%. Respiratory burst activity of blood leukocytes was not affected by vaccination. Serum agglutination titer and lysozyme activity were significantly increased after immunization, in comparison with un-vaccinated control fish. The obtained results indicated that vaccination is an effective method to control the outbreaks of V. mimicus through regulation of the humoral immune response. &nbsp; Key words: Vibrio mimicus, catfish, skin ulcer, vaccine.

Effects of waterborne cadmium exposure on growth performance, specific dynamic action, and energy budget of southern catfish Silurus meridionalis

This study investigated the effect of waterborne cadmium (Cd) exposure on the growth performance, specific dynamic action (SDA), and energy budget of southern catfish (Silurus meridionalis). S. meridionalis was exposed to various Cd concentrations (0–500 μg L−1) for 8 weeks. The specific growth rate (SGR), apparent digestibility, protein apparent digestibility, SDA parameters, and components of energy budgets were then determined. Compared to the control group, SGR was significantly reduced upon Cd exposure at 250 μg L−1 and 500 μg L−1. Compared to the control group, the experimental groups (62.5–500 μg L−1) showed significantly lower total feed intake and feed conversion efficiency. The apparent digestibility and protein apparent digestibility was significantly decreased in the 250 μg L−1 and 500 μg L−1 experimental groups compared to the control group. A significantly lower peak metabolic rate, SDA, and SDA coefficient were observed in the 250 μg L−1 and 500 μg L−1 experiment groups compared to the control group. The ratio of growth energy to assimilation energy in the 62.5–500 μg L−1 experimental groups was significantly lower than that of the control group. There was a significant increase in the ratio of metabolic energy to assimilation energy in Cd exposed groups compared to the control group. These results indicate that waterborne Cd exposure decreased the protein apparent digestibility, resulting in decreased SDA and SDA coefficient. Waterborne Cd exposure resulted in increased proportion of excretion energy and feces energy in S. meridionalis, whereas assimilation energy was reduced. Moreover, for assimilation energy, the energetic proportion for metabolism was increased, whereas the energetic proportion for growth was reduced

Physiological and behavioral stress responses to predators are altered by prior predator experience in juvenile qingbo (Spinibarbus sinensis)

ABSTRACTAll vertebrates exhibit physiological responses to predator stress and these responses are the basis of appropriate behavioral adaptation. We aimed to identify the physiological and behavioral responses of juvenile qingbo (Spinibarbus sinensis) to its natural predator, the southern catfish (Silurus meridionalis) and to test whether these responses could be altered by prior predator experience. We measured the routine metabolic rate (RMR), cortisol levels and spontaneous behavior of both predator-naive and predator-experienced qingbo under predator-absent, predator-present and non-predator-present (Hemibarbus maculatus) conditions. Predator-naive qingbo showed a typical stress response in the form of increased RMR and cortisol when exposed to predators. Spontaneous activity showed no difference between prior-experience groups or among stimulus conditions when tested alone; however, when tested with a companion, predator-naive qingbo showed increased activity and decreased distance to the stimulus arena under the predator-present condition than they did under the predator-absent condition. Both predator-naive and predator-experienced qingbo showed different physiological and behavioral responses between predatory and non-predatory fish, which suggested that predator-naive qingbo can instinctually discriminate between natural predators and non-predators. Predator-naive qingbo increase their inspection behavior when exposed to a predator compared with the predator-absent condition only when tested with a companion, which is possibly due to decreased predation risk and increased boldness.

Molecular characterization, expression analysis, and ontogeny of complement component C9 in southern catfish (Silurus meridionalis)

The complement system plays an important role in host defense against invading microorganisms. Complement component C9 is the last component that is involved in the formation of the membrane attack complex (MAC) on the surface of target cells. In the present study, the full length C9 cDNA sequence of 1984 bp with an open reading frame (ORF) of 1809 bp was cloned from southern catfish (Silurus meridionalis). The deduced amino acid sequence showed similarity with other teleost fish. The mRNA expression of C9 was detected in the liver, spleen, stomach, intestine, and head kidney, with highest levels detected in the liver. The mRNA of C9 was first detected in the yolk syncytial layer at 34 h post fertilization (hpf) with whole mount in situ hybridization, followed by the liver at 36 h post hatching (hph). The mRNA expression of C9 was upregulated significantly in the liver, spleen, and intestine following the injection with Aeromonas hydrophila, suggesting that C9 played an important role in defense against invading pathogens in southern catfish. Therefore, these results provide important information to understand the functions of C9 during fish early development in fish.

Transcriptome sequencing of the gill and barbel of Southern catfish (Silurus meridionalis) revealed immune responses and novel rhamnose-binding lectins (RBLs)

Southern catfish (Silurus meridionalis) is an economically important species widely cultured in China. It is well known for its fast growth, strong resistance to diseases and euryphage. However, little is known about the mechanisms for its powerful immune systems. Our Fish-T1K project has finished its first phase of 200 fish transcriptomes, with sequencing of gills in most examined fishes. In this study, we performed transcriptome sequencing of the gill and the maxillary barbel of Southern catfish, with the latter as a control. High expression of immune-related transcripts were observed in these two tissues. We observed that genes in the T cell receptor signaling pathway had higher transcription values in the gill than in the barbel. In addition, eight new rhamnose-binding lectins (RBLs) were identified and their carbohydrate recognition domains (CRDs) were classified according to the eight conserved cysteine residues and two conserved motifs (-YGR- and -DPC-). This is the first transcriptome report by high-throughput sequencing of the Southern catfish. Our genomic data and discovery of novel RBLs in this project should be able to promote better understandings of the roles of gills in immune responses and disease prevention for further aquaculture.