Effects of continuous and alternate administration of β‐glucan and mannan‐oligosaccharide on the growth,immunity and resistance against Vibrio splendidus of sea cucumber Apostichopus japonicus (Selenka)

A 4‐week growth trial was conducted to compare the effects of different feeding strategies of dietary immunostimulants on the growth, immunity and resistance against Vibrio splendidus of sea cucumbers Apostichopus japonicus (Selenka). Six feeding strategies were set, including feeding immunostimulants‐free diet continuously (control), feeding dietary β‐glucan (1.25 g kg^?1 diet) continuously, feeding dietary mannan‐oligosaccharides (MOS; 2.00 g kg^?1 diet) continuously, feeding β‐glucan 2 days followed by MOS 5 days alternately, feeding β‐glucan 5 days followed by MOS 2 days alternately and feeding β‐glucan 7 days followed by MOS 7 days alternately. The sea cucumbers fed immunostimulants showed higher specific growth rate (SGR) and lower cumulative mortality than control (P < 0.05). When sea cucumbers were fed with β‐glucan continuously, total coelomocytes counts and superoxide anion were significantly higher than control on the 4th day (P < 0.05). However, these two immune parameters were not significantly higher than those in control after the 18th day (P > 0.05). While sea cucumbers continuously fed MOS, these two immune parameters were not significantly higher than control until the 15th day. All immune parameters of the sea cucumbers fed with β‐glucan and MOS alternately were significantly higher than those in control during the experiment (P < 0.05). The sea cucumbers fed with β‐glucan 7 days followed MOS 7 days alternately showed the highest SGR and second lowest cumulative mortality. It was suggested that this feeding strategy is most suitable for sea cucumbers.     

Effects of dietary supplementation with β‐glucan and synbiotics on growth,haemolymph chemistry,and intestinal microbiota and morphology in the Pacific white shrimp

The goal of this study was to investigate the effects of dietary supplementation with β‐glucan and microencapsulated probiotics (Bacillus subtilis or Pediococcus acidilactici) on growth performance, body composition, haemolymph constituents, and intestinal morphology and microbiota of the Pacific white shrimp Litopenaeus vannamei. Four treatment diets [basal diet (C), β‐glucan‐containing diet (β‐glu), β‐glucan plus B. subtilis‐containing diet (β‐glu+Bs), and β‐glucan plus P. acidilactici‐containing diet (β‐glu+Pa)] were fed to L. vannamei for 90 days. Shrimp fed the β‐glu and β‐glu+Pa diets exhibited similar growth performance and body protein content, which were significantly higher than those of shrimp fed the control diet (P < 0.05). No significant differences in haemolymph triglyceride, cholesterol, protein, haemolymph urea nitrogen or chloride were detected among the experimental diets. However, dietary β‐glucan alone increased the haemolymph glucose level and osmolarity (P < 0.05). Synbiotic supplementation had greater effects on intestinal microbiota and morphology than dietary β‐glucan alone. For example, β‐glu+Bs increased the number of intestinal lactic acid bacteria and decreased the number of Vibrio spp. (P < 0.05), and β‐glu+Pa increased the height of intestinal villi.     

Response of the intestinal mucosal barrier of carp (Cyprinus carpio) to a bacterial challenge by Aeromonas hydrophila intubation after feeding with β‐1,3/1,6‐glucan

The effect of dietary β‐glucan on the bacterial community in the gut of common carp (Cyprinus carpio) was examined after oral application of Aeromonas hydrophila. Carp received either feed supplemented with 1% MacroGard^®, a β‐1,3/1,6‐glucan, or a β‐glucan‐free diet. Fourteen days after feeding, half of the carp from each group were intubated with 10⁹ colony‐forming units (CFU) of a pathogenic strain of A. hydrophila. Gut samples were taken 12 hr to 7 days after application and analysed using microbiological and molecular biological techniques (NGS, RT‐PCR‐DGGE). The reaction of the mucosa and the microbiota to an A. hydrophila intubation differed in carp fed with β‐glucan compared to carp from the control group. In β‐glucan fed carp, the total bacterial amount was lower but the number of bacterial species was higher. Bacterial composition was different for carp from both treatment groups. The number of mucin filled goblet cells was reduced in carp fed the β‐glucan diet. Mucus was obviously released from the goblet cells and was probably washed out of the gut together with high numbers of bacteria. This might be protective against pathogenic bacteria and, therefore, feeding with β‐glucan may provide protection against infections of the gut in carp.     

Induction,purification and partial characterization of vitellogenin in an Indian major carp Catla catla (Ham.)

Vitellogenin was purified from the serum of 17‐β estradiol (E₂)‐induced juvenile Catla catla using a simple two‐step purification procedure i.e. selective chemical precipitation followed by gel filtration chromatography. Purified protein migrated as single band in a native gradient PAGE which indicated the purity of the sample. The molecular weight of the native catla vitellogenin (~440 kDa) was determined using gel filtration chromatography. In SDS‐PAGE under reducing conditions catla vitellogenin dissociated into three major sub units at 115 kDa, 102 kDa and 73 kDa along with a few faint bands. Confirmation of purified protein as catla vitellogenin was supported by multiple physiological evidences, e.g. absence in male as well as juvenile sera and presence in matured female fish, ability to be synthesized upon estradiol injection in immature fish and certain unique biochemical properties like high molecular weight, phospholipoglycoprotein nature of the molecule. Western blot analysis showed that polyclonal antibody raised against purified protein detected vitellogenin in the sera of catla and in a few species selected from Cyprinidae family. These antisera were used to detect vitellogenin in liver tissue of hormone‐induced catla using immunohistochemistry and its applicability in other immunoassays is discussed.     

Effects of dietary β‐glucan and glycyrrhizin on non‐specific immunity and disease resistance of white shrimp,Litopenaeus vannamei (Boone) challenged with Vibrio alginolyticus

The white shrimp Litopenaeus vannamei, fed immunostimulant‐free, 0.2%β‐glucan and 0.06% glycyrrhizin diets for 18 days, respectively, were challenged with Vibrio alginolyticus at 6.4 × 10⁴ CFU shrimp^?1. The total haemocyte count (THC), phenoloxidase (PO) activity, respiratory burst (RB) and superoxide dismutase (SOD) activity changes for a 120‐h period were investigated, and shrimp mortality was also recorded. The results showed that PO activity, RB and SOD activity were significantly higher in shrimp fed the two immunostimulant diets after 18 days than those in shrimp fed immunostimulant‐free diets. The THC and SOD activity decreased significantly from 0 to 24 h post challenge, and then reverted to normal levels at 96 and 72 h respectively. The values for PO activity and RB increased from 0 to 48 h post challenge. Compared with those fed the control diets, shrimp fed immunostimulants had significantly higher PO activity and RB values at 120 h post challenge. Mortalities after challenge with V. alginolyticus were significantly lower in shrimp fed with β‐glucan or glycyrrhizin than in those fed with a diet without immunostimulants. It was concluded that dietary β‐glucan and glycyrrhizin increased the shrimp immunity. Furthermore, β‐glucan caused an increase in some immune parameters 12 h earlier than glycyrrhizin after V. alginolyticus challenge.     

Effect of treatment of chum salmon Oncorhynchus keta (Walbaum) eggs with 1,3;1,6‐β‐D‐glucans on their development and susceptibility to Saprolegnia infection

The effects of six 1,3;1,6‐β‐D‐glucooligo‐ and polysaccharides with different structures (ranging from 1 to 10 kDa in molecular mass and containing 10–25% of β‐1,6‐linked glucose residues) from brown algae, Saccharina cichorioides, on development of the chum salmon, Oncorhynchus keta (Walbaum), were evaluated. Exposure of chum salmon eggs to 1,3;1,6‐β‐D‐glucans with a molecular mass of more than 2 kDa increased the survival of embryos and juveniles and their resistance to Saprolegnia infection by up to 2.5‐fold, leading to a weight gain in juveniles of 40–55% compared with The control chum salmons. The 1,3;1,6‐β‐D‐glucans with molecular mass of 6–8 kDa and used at a at concentration of 0.5 mg mL^?1 rendered the best stimulative effect.     

Dietary β‐glucan modulate haematological parameters,cytokines and gene expression in TLR and ERK pathways of rainbow trout (Oncorhynchus mykiss) during infection by Aeromonas salmonicida

To explore the role of β‐glucan (0, 0.05%, 0.1% and 0.2%) in resisting bacteria, rainbow trout (Oncorhynchus mykiss) was challenged with Aeromonas salmonicida after 42‐day β‐glucan administration, and then sampled at 0, 2, 4 and 6 days post infection (dpi). The data showed that 0.2% β‐glucan reduced the accumulated mortality rates compared with the ICG (infected control group) (p < .05). The white blood cells, red blood cells and haemoglobin were higher in the 0.2% β‐glucan group (BG) than the ICG (p < .05). 0.1% and 0.2% β‐glucan elevated serum total antioxidative capability and glutathione activity but alleviated the increase of serum alkaline phosphatase activity and glucose concentration (p < .05) during infection. Serum TNF‐α, IL‐1β, IL‐8 and IgM in three BGs elevated remarkably on 6 dpi compared with the ICG (p < .05). Expression of tnf, il1b and cxcl8 of the head kidney in the 0.1% and 0.2% BGs was higher than the ICG on 4 dpi while ighm expression in the 0.2% BG was higher than in the ICG on 2 and 6 dpi (p < .05). 0.1% and 0.2% β‐glucan increased the expression of tlr5m, tlr5s, tmek and myd88 in the spleen after infection (p < .05). Similarly, 0.2% β‐glucan up‐regulated the expression of tmek, myd88, oncmyk‐dab and c3 in head kidney (p < .05). Overall, 0.2% dietary β‐glucan effectively decreased accumulated mortality rate by modulating the biochemistry process, cytokines, and activating TLR and ERK signalling pathways during A. salmonicida infection.     

The effect of β‐1,3‐glucan derived from Euglena gracilis (Algamune™) on the innate immunological responses of Nile tilapia (Oreochromis niloticus L.)

Algamune^? is a commercial additive produced from Euglena gracilis, providing a rich source of the β‐1,3‐glucan paramylon. Isolated kidney phagocytes of Nile tilapia were incubated with graded doses (0, 8, 16, 32, 64 and 128 μg/ml) of Algamune^? and purified paramylon to gauge their ability to elicit the production of reactive oxygen species. A linear response was observed for extracellular superoxide anion for both sources but only Algamune^? for intracellular superoxide anion. After corroborating the immunostimulant properties ex vivo, a feeding trial was conducted to evaluate the dietary supplementation of Algamune^? (0, 100, 200, 400 and 800 mg/kg of diet) for Nile tilapia. Fish were fed for 3 weeks, after which, fish were sampled for blood and head kidney phagocytes. The remaining fish were challenged with Streptococcus iniae. Macrophage extracellular superoxide anion production was significantly elevated in fish fed diets with 200 mg of Algamune^? kg^?1 when compared to fish fed the basal diet. Even though the disease challenge did not show statistical differences, it is worth mentioning that fish fed intermediate doses of Algamune^? had lowest numerical mortality values. Therefore, Algamune^? was demonstrated to enhance some immunological responses of tilapia both in ex vivo and in vivo.     

Immunomodulation of rainbow trout (Oncorhynchus mykiss) fry by bath exposure to a β‐glucan from Euglena gracilis

Early developmental stages of fish mostly depend on innate immune factors for their protection. Augmenting these factors by application of different immunostimulatory substances may be beneficial for rearing and survival of the early life stages of fish. Bath administration of stimulants leads to a uniform exposure of fish independent of feed intake and reduces the individual handling. The present study demonstrates the immunostimulatory effect of β‐glucan (bath exposure) in rainbow trout fry at different dosages and exposure time. Rainbow trout fry (avg. wt. 770 mg; 87 days post hatch) were exposed to three different concentrations of β‐glucan (10, 100 and 1000 μg mL^?1) by bath exposure for 1 and 24 h. Expression of immune related genes from pooled internal organ samples of individual fish were analysed using a real time qPCR assay. Expression of complement factors (C3 and factor B) and acute phase proteins (hepcidin, precerebellin and transferrin) was significantly up‐regulated after 24 h bath stimulation with β‐glucan (100 μg mL^?1). These innate immune factors may play a vital role in clearance of pathogens. The expression of most of genes showed both a dose‐ and time‐dependent response. A medium dose (100 μg mL^?1) induced a significant increase in expression of complement factors and acute phase proteins mainly at 24 h exposure, whereas the highest dose of β‐glucan (1000 μg mL^?1) down‐regulated the expression of most of the studied genes. The result from the present study indicates that β‐glucan bath exposure could be applied for enhancing the innate immune factors even in fry.     

Partial characterization of digestive proteases of the three‐spot cichlid Cichlasoma trimaculatum (Günter 1867)

Partial characterization of digestive proteases in the three‐spot cichlid Cichlasoma trimaculatum juveniles was conducted. It was determined that there is higher alkaline proteases activity (3.95 ± 0.32 IU mg^?1 protein) compared to acidic proteases (2.01 ± 0.57 IU mg^?1 protein). Optimal temperature for alkaline proteases is 60 °C which resulted in more thermostability to temperature changes. On the other hand, optimal temperature for acidic proteases is 50 °C. Optimal pH for acidic proteases was pH 2, while for alkaline proteases, it was pH 10, which resulted in more stability in relation to pH changes than acidic protease. The use of specific inhibitors and the SDS‐PAGE electrophoresis analysis revealed seven types of bands for alkaline proteases, which make evident the main presence of serine proteases. In acidic proteases, more than 98 g kg^?1 of the activity was inhibited with pepstatin A inhibitor. Therefore, it is evident that C. trimaculatum digestion is composed by acidic and alkaline proteases; thus, it should be considered an omnivorous fish.     

Enhancement of the innate immune system and disease‐resistant activity in Cyprinus carpio by oral administration of β‐glucan and whole cell yeast

The effects of dietary β‐ (1,3) glucan and whole cell yeast (Sacharomyces uvarum) on the immune response and disease resistance to Aeromonas hydrophila were investigated in Cyprinus carpio. β‐(1,3) glucan was extracted from the yeast. Both β‐(1,3) glucan and whole yeast were incorporated into the diet at 1% level and fed to common carp C. carpio for a period of 60 days. Control and treated fish were exposed to A. hydrophila on the 30th and the 60th day of the experimental period. Dietary supplementation of glucan significantly increased the white blood cell count in fish on the 60th day (2.91±0.04 × 10⁴), and the highest nuetrophil nitro blue tetrazolium (NBT) activity was also observed in glucan‐fed fish (30th day). A consistent increase in neutrophil (NBT) activity was also observed in whole cell fed fish until the end of the experiment. Similarly, β‐(1,3) glucan and whole cell yeast enhanced the serum lysozyme activity from the 15th day onwards but higher activity was reported on the 30th day in glucan and the 60th day in whole cell yeast‐fed fish. Suplementation of β‐(1,3) glucan protected the fish from A. hydrophila infection. Nearly 75–80% of the fish survived pathogen exposure (relative percentage survival). However, only 54–60% survival was observed in the whole cell‐fed fish. β‐(1,3) glucan and whole cell yeast protect the fish from pathogens by enhancing the cellular and humoral immune response in C. carpio.     

Inhibition of Vibrio spp. by 2‐Hydroxyethyl‐11‐hydroxyhexadec‐9‐enoate of Marine Cyanobacterium Leptolyngbya sp. LT19

Seven marine cyanobacteria were isolated from two regions of the Gulf of Thailand and evaluated by the agar diffusion method for antibacterial activity. Inhibitory compound was purified from the crude methanol extract and its structure was elucidated based on extensive spectroscopic analysis, including IR, 1D and 2D NMR spectra as well as mass spectrometry. A novel antimicrobial compound produced by the marine cyanobacterium Leptolyngbya sp. LT19 was identified to be a 2‐hydroxyethyl‐11‐hydroxyhexadec‐9‐enoate which has so far never been reported in microorganisms. Biological assays revealed that this novel compound exhibited antibacterial activities against the Gram‐negative, persistent shrimp pathogens, Vibrio harveyi and V. parahaemolyticus with minimal inhibitory concentration of 250–1000 and 350–1000 μg mL^?1 respectively.     

Newly identified C‐type lysozyme in Chinese soft‐shelled turtle (Pelodiscus sinensis) exhibits potent antimicrobial activity

Lysozymes play vital roles in humoural immune response against bacterial invasion by its lytic activity. In the present study, a new C‐type lysozyme was identified and characterized from Chinese soft‐shelled turtle Pelodiscus sinensis. The full‐length cDNA of PslysC was of 923 bp, encoding a polypeptide of 148 amino acid residues. The multiple alignments and phylogenetic relationship analysis revealed the highly enzyme‐related conserved residues. The real‐time PCR analysis suggested that PslysC was constitutively expressed in a wide range of tissues with highest level in blood cells and liver. The expression of PslysC could be significantly up‐regulated under Aeromonas jandaei infection and ammonia exposure, while no significant changes were found under Poly I:C infection. The rPslysC protein was expressed in E. coli and purified by Ni‐NTA. The optimal pH and temperature for rPslysC protein lytic activities were determined at pH 7 and 30℃. rPslysC can inhibit the growth of eight kinds of Gram‐negative bacteria, and three kinds of Gram‐positive bacteria. The binding activity of rPslysC to different microbial polysaccharides and microorganism was analysed. The results showed that rPslysC could bind to selected bacteria, and exhibit a strong binding activity to lipopolysaccharide and peptidoglycan, but a weak binding activity to β‐glucan. This suggests that the binding activity might be the major mechanism of action to realize the antibacterial activity. The present study will provide helpful evidence to further understand the innate immunity of P. sinensis, and the interaction mechanisms of C‐type lysozymes with bacterial membranes.     

Effects of Dietary Fibre Concentrates on growth performance and digestive enzyme activities of jundiá (Rhamdia quelen)

This study was conducted to investigate the prebiotic effect of different Dietary Fibre Concentrates (DFC) (Mucilage =MG; Pectin = PN or β‐glucan + mannan = βg + M) on growth and somatic parameters, body composition and digestive enzyme activities of jundiá (Rhamdia quelen). After acclimation, fish (7.16 ± 0.06 g) were allocated into 24 tanks (30 fish per tank) and triplicate groups were fed with Control diet (0 g kg^?1 of DFC); diet supplemented with 5 g kg^?1 commercial prebiotic (CP) or diets supplemented with 5 or 10 g kg^?1 diet of MG; PN or βg + M. At the end of the trial (8 weeks), growth was significantly (P < 0.05) higher in fish fed diets supplemented with DFCs and did not differ from animals supplemented with CP. The animals that were fed Control diet presented a body protein content higher compared to those supplemented with diets containing pectin or β‐glucan + mannan (P < 0.05). However, fish fed diets added with β‐glucan + mannan yielded a higher level of protein deposited in the whole body. The activity of digestive enzymes was lower in the group supplemented with Pectin. Results indicate that supplementation with DFCs in the diet had positive effects on the performance of jundiá and are prebiotic potential candidate.     

Recombinant expression and comparative bioactivity of tongue sole insulin‐like growth factor (IGF)‐1 and IGF‐2 in Pichia pastoris

Insulin‐like growth factor (IGF) plays a key role in the complex system that regulates bony fish growth, differentiation, and reproduction. In the current study, recombinant tongue sole IGF‐1 and IGF‐2 were obtained using the Pichia pastoris expression system and their comparative bioactivities were investigated. Tricine–SDS–PAGE and western blot analysis showed that the recombinant tongue sole IGFs were secreted into the culture medium and had a molecular weight of 8.7 kDa. The optimal incubation time and pH for recombinant expression of IGFs were 36 hr and 5.0 respectively. Functional analysis demonstrated that both recombinant tongue sole IGF‐1 and IGF‐2 significantly promoted cell proliferation of MFC‐7 in vitro. In addition, the recombinant tongue sole IGF‐1 and IGF‐2 proteins could suppress hepatic mRNA levels of igf‐1 and igf‐2 in vitro, which showed that they have similar physiological functions. Taken together, the biologically active recombinant tongue sole IGF‐I and IGF‐II proteins will allow us to further investigate their physiological roles in growth regulation of this species. Furthermore, the present results also hinted at the potential application of these two recombinant IGF‐I and IGF‐II proteins into the tongue sole farming industry.     

Characterization of β‐catenin 1 during the gonad development in the common carp (Cyprinus carpio)

β‐catenin gene is a pivotal gene for gonad development and maintenance of ovarian function in mammals. However, little is known about its expression and function in gonad development of fish. In this study, a complete cDNA (3342 bp) sequence of β‐catenin 1 was cloned from the common carp, Cyprinus carpio, by RACE PCR, which encodes a 780‐amino‐acid protein. Quantitative real‐time PCR demonstrated that β‐catenin 1 mRNA expressions were high in the testis and ovary tissue and the expression increased as the testes developed and the early stage ovaries developed. Western blot results revealed a single immunoreactive band with an estimated molecular weight of 90 kDa in testes. Immunohistochemistry analysis revealed that the β‐catenin 1 protein was concentrated mainly in the cytoplasm of early development stage of oocyte cells and in the cytomembrane of developing and mature sperm cells. 17β‐Ethinylestradiol injecting intraperitoneally into the fish decreased the relative β‐catenin 1 mRNA expression level except 1 μg/g 72 hr and 5 μg/g 48 hr of treatments in the ovary by real‐time PCR. These results suggest, for the first time, that β‐catenin 1 is an essential protein in gonad development and might be involved in ovarian early development of C. carpio.     

Effects of Vibrio harveyi on plasma clotting protein (coagulogen) of white shrimp Litopenaeus vannamei

Blood clotting exhibits various important functions, including the prevention of body fluid loss and invasion of pathogens in shrimp. The effects of pathogenic Vibrio harveyi on plasma of white shrimp (Litopenaeus vannamei) in vitro and in vivo were investigated in this study. The clotting protein (coagulogen) in plasma of white shrimp pre‐incubated with extracellular products (ECP) of V. harveyi was found apparently decreased and fast‐migrated in crossed immunoelectrophoresis (CIE) gels. In addition, the coagulogen had been degraded to many low molecular‐weight protein bands in plasma pre‐incubated with ECP on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE) gels. When pre‐challenged with bacterial cells and ECP of V. harveyi, the white shrimp began to die at about 30 and 16 h respectively. Moreover, plasma coagulogen was decreased more obvious in shrimp challenged with ECP than that with bacterial cells as visualized in CIE gels, and total plasma protein in both group of shrimp were all decreased. Haemolymph withdrawn from moribund shrimp pre‐challenged with V. harveyi or its ECP was observed unclottable. However, the addition of clotting factors (transglutaminase and/or Ca²⁺) to these unclottable plasma could apparently promote their re‐clotting ability as jelly‐like solid observed in microtubes. The recovery of clotting ability of plasma from moribund shrimp was due to the reformation of coagulogen (200 kDa) after adding the two clotting factors as shown on CIE and SDS‐PAGE gels. The present results suggest that the infection of V. harveyi in white shrimp may not only degrade coagulogen but also influence the presence of transglutaminase and Ca²⁺ ion.     

Dietary supplementation of β‐glucan improves growth performance,the innate immune response and stress resistance of red sea bream,Pagrus major

A 56‐day feeding trial was conducted to evaluate the effects of supplemented diets with β‐glucan (BG) at four levels [0 (D1), 250 (D2), 500 (D3) and 1000 (D4) mg BG kg^?1] on red sea bream, Pagrus major. The obtained results revealed a significant increase (P < 0.05) in final body weight, weight gain, specific growth rate, feed intake, body protein content, lysozyme activity and tolerance against low‐salinity stress test in all BG‐supplemented groups when compared with BG‐free group. Furthermore, D4 group resulted in a significant increase in feed efficiency ratio, protein gain, protein and lipid digestibilities, serum bactericidal activity and peroxidase content when compared with D1 group (P < 0.05). Haematocrit and plasma protein content in D3 group were significantly higher than those in D1 group (P < 0.05). Interestingly, BG supplementation decreased glutamic oxaloacetic transaminase (GOT) in D2 group and reactive oxygen metabolites in D2, D3 and D4 groups when compared with D1 group. Following low‐salinity stress test, significantly higher amounts of secreted mucus were observed in fish fed D2 and D4 diets than those from fish fed D1 diet (P < 0.05). In conclusion, the supplementation of BG improves growth, stress resistance and immune response of P. major.     

Effects of dietary immunostimulant combination on the growth performance,non‐specific immunity and disease resistance of cobia,Rachycentron canadum (Linnaeus)

The aim of this study was to examine the effects of the immunostimulant combination (IC) containing β‐glucan, A3α‐peptidoglycan, vitamin C and vitamin E on the growth performance, non‐specific immunity and protection against Vibrio harveyi infection in cobia (Rachycentron canadum). Fish were fed diets containing six graded levels of IC (0, 1, 2, 3, 4 and 5 g kg^?1 diet) for 8 weeks. The results showed that the survival rate ranged from 81.1 to 84.4% with no significant difference among all the groups (P > 0.05) after the feeding experiment. Dietary IC significantly increased the specific growth rate (SGR), serum lysozyme, alternative complement pathway (ACH50) activity, phagocytosis percentage (PP) and respiratory burst activity of head kidney macrophages of cobia. Moreover, feeding of supplemented diets containing 3.0 g kg^?1 IC resulted in significantly lower mortality against the pathogens, V. harveyi compared with the control group. To elevate the growth and immune resistance ability of cobia, the optimal dose of dietary IC administration, determined by second‐order polynomial regression analysis was 3.43 and 2.71 g kg^?1 diet, respectively, on the basis of the SGR and mortality after challenge with V. harveyi.