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Rectal gland is an important organ for osmoregulation in the sharks, and has species-specific features which are dependent on fish size. In this investigation, 36 individual of Brown banded Bamboo sharks (Chiloscyllium punctatum) were caught from Dervish's Creek located at the northern of Persian Gulf in both autumn and spring seasons. After weighting, biometry, sex determination and the numbering of sharks, the rectal gland were removed and weight were measured and then immediately fixed in Bouin’s solution. Histological sections of 5 micrometers were prepared and stained with Hematoxylin and eosin and then studied by using light microscope.The morphological results showed that about the size of the rectal gland, there was no significant difference between males and females (p>0.05) but the smaller sharks have larger rectal glands in proportion to their body size comperated to larger sharks (p<0.05). Also, it seems that the weight of rectal gland is not dependent upon sex and reproductive season. In addition, considering to the results of both seasons, we can conclude that the rectal gland weight of this species was independent of sex and breeding season. Microscopic results indicated that rectal gland had four structural regions: The capsule, outer parenchyma, inner parenchyma and central canal.

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This study was conducted to determine the effects of dietary methionine and lysine supplementation on digestive enzymes activity in juvenile Sobaity, Sparidentex hasta. For this purpose, 180 juvenile fish with an initial average weight of 31.38 ± 1.4 g were distributed randomly in eighteen (300 L) polyethylene tanks. 6 experimental diets were prepared with different levels of methionine and lysine including control diet (without dietary methionine and lysine), Diet 1: 100% methionine; Diet 2: 75% methionine and 25% lysine; Diet 3: 50% methionine and 50% lysine; Diet 4: 25% methionine and 75% lysine; Diet 5: 100% lysine. During the experimental period, fish were fed to satiation thrice daily (8:00, 13:00 and 18:00 hours) for 8 weeks. At the end of the experiment, digestive enzymes (trypsin, chymotrypsin, alkaline phosphatase, amylase, lipase and total protease were effected by different dietary levels of methionine and lysine supplementations (P<0/05). Also, the activity of trypsin and chymotrypsin had showed an increasing trend with increased amount of diet lysine while enzyme activity of alkaline phosphatase was higher in treatments contains maximum amount of methionine supplementation. According to results, the increasing of lysine in diet reduced enzyme activity of lipase. The results, showed no significant differences between experimental treatments in amylase and total protease (P<0/05). Based on the results of evaluation digestive enzymes, amino acids supplements of lysine and methionine were changed the activity of digestive enzymes in juvenile Sobaity, (S. hasta). Finally, Diet 2 with 75% methionine and 25% lysine was suggested for used by Sobaity, as to the balance of dietary methionine and lysine supplementation.

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In this study that lasted to 8 weeks, was conducted to determine  the effects of dietary supplementation of lysine and  methionine on body biochemical composition and amino acid profile of Sobaity sea bream, Sparidentex hasta. Therefore, 180 juvenile fish with an initial weight of 31.38 ±1.4 g were distributed randomly among eighteen tanks. Fish were fed to satiation three time per day (08:00, 13:00 and 18:00 hours) with formulated diets containing six different levels of dietary methionine and lysine; Diet 1: a control diet without dietary amino acid supplementation; Diet 2: 100% methionine supplementation; Diet 3: 75% methionine and 25% lysine supplementation; Diet 4: 50% methionine and 50% lysine supplementation; Diet 5: 25% methionine and 75% lysine supplementation and Diet 6: 100% lysine supplementation.The results of this study showed Carcass protein content was significantly affected by the amino acid supplements and the highest level of carcass protein observed in fish were fed by diet 3(P<0.05). Also Increasing levels of  lysine in diet caused increases the amount of fat in the carcass although the content of carcass fat, moister and ash were not significantly different among thetreatments (P > 0.05). In addition, essential amino acids (ΣEAA) and non-essential amino acids (ΣNEAA) and  ratio ΣEAA / ΣNEAA, were affected by lysine and methionine amino acid supplementation as ΣEAA and ratio (ΣEAA) / (ΣNEAA) significantly increased with increasing levels of amino acid supplementation and the highest amount of this parameters observed in groups were fed by diet with high levels of methionine.The results showed that adding 75%  dietary methionine supplementation and 25% lysine supplemtation to the diet containg 45/95% protein, have positive effects on biochemical composition and amino acid profile in rearing of Sobaity seabream juveniles.

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In this study, biochemical heamolymph parameters indices of the white leg shrimp (Litopenaeus vannamei) with three dietry protein levels(25, 35 and 45%) in the three levels of water salinity(0-3, 12-15 and 32-35 ppt) was studied. For experimental 350 shrimps with the average weight of 5.55±0.18 g and length of 8.81±0.15 cm were distributed randomly in 27 ten tones (length: 600 cm, width: 170 cm and height: 100 cm) concrete tanks. Experimental group were fed to satiation 4 times a day the experimental period. The results showed that at dietary protein levels of 35 and 45% and the water salinity of 32-35 ppt the amount of protein, urea, HDL and LDL increased (P≤ 0/05). Cholesterol and triglycerides levels in treatment with dietry protein level of 45% and the salinity of 32-35 ppt had increased. The glucose level of the plasma changed only by the modification of the protein level and had a revers relation with this variable (P≤ 0/05). However, different levels of protein and salinity had no effect on the amount of hemolymph uric acid. So besed biochemical heamolymph parameters on the results of this study, in dietry protein level 35% and salinity of 32-35 ppt and at least stress in the levels.

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Shemaya (Alburnus chalcoides), Roach (Rutilus rutilus caspicus) and goldfish (Carassius auratus auratus) were belonging to the family Cyprinidae. The aim of this study was to observe the gastrointestinal tract of these fish. After the sampling, gastrointestinal tract and its accessory glands were taken and studied trough histological procedure using H&E staining. The age of the fish was 2 to 3 years old. Three examined species the digestive system was made up of esophagus and intestine and the accessory glands were liver and hepatopancreas around the intestine. Esophagus was short with small foldings compare to the intestine. The intestinal foldings were high and lanceolate in Roach, but round-tip in Shemaya and goldfish. The foldings have high density in anterior part of the intestine with minor differences between examined species. Goblet cells were observed in esophagus and intestine with higher density at posterior part of the intestine. Liver and hepatopancreas were close or apart from each other along the gastrointestinal tract. In conclusion, it has been observed that these three species has some common characteristics in digestive system structure with some species specific properties.

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This study was performed to survey the relationship between water physicochemical factors and determination the effective parameters on Litopenaeus vannaemi  survival in three ponds of three different farms with same stocking (250000 pl) and same storage time during 109 days (Days of culture), from June to September, in  shrimp culture ponds of Abadan-Choebdeh. The results showed that the highest correlation between salinity and temperature, as well as salinity and pH is established. Nitrate and phosphate values ​​recorded during the culture period were outside of the standard range. The results of correlation between chemical feature and survival showed  in all of three farms, the nitrate and ammonia had the maximum effect on shrimp survival which this relationship was expressed inverse. According to the results, the mean of primary weight, final weight and weight gain was observed in T₃ more than T₁ and T₂ , and survival rate in T₃ and T₂ was more than T₁ at the end of the culture period. It seems that the level of protein in diets play an important role in the nitrate and ammonia scale and finally in survival.
 

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 In this study, the effect of  adding different  levels of  Montmorillonite, on growth indices and some blood factors of rainbow trout frys was investigated. Study carried out during 123 day within 3 experimental treatments,consist of adding different levels of Montmorillonite to the commercial diet with 4 replication in each treatment. In each replication 45pcs of fry with average weight of 500±0.35mlg were distributed. For this study, fiberglass rectangular tanks with 300L capacity, filled with 200L water were used. Frys were fed eight times daily. Weight measurement of frys done each 15 days and blood sampeling from caudal vein was carried out at the end of experimental period. The results showed significant improve in final weight, percentage of weight gain, daily weight gain, SGR and FCR in experimental treatments comparing control (p<0.05). The PER and HSI, showed no significant statistical difference between experimental treatments and control (p˃0.05). The highest final weight and highest percentage of final weight and the best FCR was observed in experimental treatment with 2% Montmorillonite in feed and highest daily weight gain and highest SGR was observed in experimental treatment with 4% Montmorillonite in feed. The blood factors, showed no significant difference between experimental treatments and control (p˃0.05). Based on this results, use of 2% Montmorillonite as a feed additive, in rainbow trout fry feeding, is recommended to improve growth indices without negative effect on blood factors.
              

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The aim of this study was to evaluate the performance of plant nano-gels as a new adsorbent and to remove nitrite at low cost. At first, plant nano-gels (nano bagasse, chitosan-functionalized nano-fiber and lignocellulose nano-fiber) were prepared by mechanical and top-down mechanism. Batch system medium was designed to measure the adsorbent optimum. Then isotherm and synthetic were calculated to estimate absorption coefficients. The optimum amount of adsorbent weight for each nano gel was: 0.1 g for nano bagasse and lignocellulose fiber; 0.3 g for modified nano fiber; 4 for nano bagasse and lignocellulose fiber and 5 for modified nano fibril; also initial concentration and the time for all adsorbents was 25 mg /l and 60 min. According to the equilibrium equations, the most consistent were considered with Langmuir, Freundlich and Temkin models. Results showed that the highest nitrite removal efficiency was related to functionalized nano-fiber with 93.1%; for nano-bagasse it was 65.51% and for lignocellulose nano-fiber was 68.96%. Eventually, the plant nano-gels can be used as bio-filters in the effluent system of fish farm.

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This study was carried out to determine the effect of raibow trout cage culture on the abiotic factors of water in the Abbas Abad area, southern basin of the Caspian Sea. For this purpose, water samples were collected at distances of 5, 50, 100 and 1000 m from the cage culture during December 2014 to August 2015. Based on the results, it was shown that the mean value of water temperature for surface layer varied between 12.2 and 30.16 °C from December to August. The highest and lowest mean value of salinity obtained in August (11.26 g/l) and April (10.66 g/l) respectively. The range of pH value changes (8.36-8.83) was as small as the rate of electro conductivity variations (18.11-19.07 µs/cm). The highest concentration of dissolved oxygen was observed in December at 1000 m station (9.23 mg/l) and the lowest was seen in April at 5 m station (7.27 mg/l). Also the mean values of Total Dissolved Solids, Turbidity, Nitrite, Nitrate, ammonium, Total Nitrogen, Phosphate and Total Phosphorus was obtained 9.59 (g/l), 5.55 (NTU), 3.67, 133.45, 92.6, 929.72, 21.63, 37.73 (µg/l) respectively. The results of abiotic factors of water showed that most of the measured parameters had significant differences only among different sampling periods (p<0.05) whilst, for some factors such as dissolved oxygen, ion ammonium and phosphate, there were significant differences between different stations (The effect of distance from the cage) at each sampling period as well as (p<0.05). It seems that the cage culture of rainbow trout in the Abbas Abad area had only a minor impact on measured factors probably due to low stocking density, short-term of fish farm and high current velocity.

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Technology of probiotics encapsulation is one of the newest methods to increase the stability of probiotics. In the current study, the effects of encapsulation of Lactobacillus rhamnosus by alginate/chitosan in two ways, ionic gelation and internal emulsification on nutrition and growth performance and digestive enzymes activity in rainbow trout were evaluated. An experiment was designed with five treatments each with three replicates. 600 juvenile average (±SD) initial weight of 25.53±.82 g were assigned to 4 treatments (each with 3 replications). Fish in T1 and T2 received Lactobacillus rhamnosus encapsulated by Ionic gelation and internal emulsification respectively, T3 were fed with alginate/chitosan enriched free probiotic diet, T4 received bacteria without any encapsulation and control group. Fish were fed with experimental diets for 60 days and the study continued for a further 15 days with basic diet in all treatments. Fish were sampled on days 30, 60 and 75 and food conversion ratio, protein efficiency ratio, specific growth rate, condition factor, weight gain percentage and digestive enzymes activity (lipase, protease, trypsin and chymotrypsin) were compared among the treatments. The lowest food conversion ratio (0.93±0.03) at day 30 was seen in T2 which was significantly different from control treatment (p<0.05). Protein efficiency ratio values at days 30, 60 and condition factor at day 60 were higher in T1 and T2, that this increase was significant in T2 than control group (p<0.05). Also, the measurement of digestive enzymes showed the high level of lipase and protease in microencapsulated treatments compared to other treatments.
It can be concluded that the microencapsulation of Lactobacillus rhamnosus by internal emulsification with alginate/chitosan has significant effects on some of nutrition and growth performance and digestive enzymes activity in rainbow trout and the use of this method to improve the performance of the probiotic in rainbow trout is recommended.

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Introduction
Aquaculture is increasingly vital for meeting the protein needs of a growing global population. Successful production relies on efficient feed, which accounts for 60% of costs, and the development of high-quality products. Disease management in aquaculture is challenging due to factors like poor nutrition and environmental conditions, making prevention more critical than treatment (Ndashe et al., 2023). Current strategies focus on using immune stimulants and additives, such as organic acids, probiotics, and prebiotics, to enhance fish health and resilience against diseases (Dawood et al., 2018). The use of natural growth stimulants like chitin, chitosan, and lactoferrin has surged due to their economic benefits and minimal environmental impact. These substances not only improve immune responses but also enhance the antibacterial activity of phagocytic cells, contributing to sustainable aquaculture practices. Overall, the shift towards these natural alternatives reflects a growing recognition of their value in promoting fish health and reducing reliance on chemical treatments. Chitosan is a biodegradable and biocompatible polysaccharide derived from glucosamine through the deacetylation of chitin, predominantly found in the exoskeletons of crustaceans, particularly shrimp shells. It offers numerous benefits in aquaculture, notably enhancing fish health and growth. Chitosan stimulates the immune system by increasing levels of immunoglobulin M (IgM) and lysozyme activity, which improves disease resistance. It also boosts antioxidant capacity by enhancing the activity of enzymes like superoxide dismutase (SOD) and catalase (CAT), thereby reducing oxidative stress. Additionally, chitosan promotes gut health by increasing intestinal villi length and digestive enzyme activity, facilitating better nutrient absorption (Mukarram et al., 2023; Zhang et al., 2024). Moreover, chitosan can chelate heavy metals such as cadmium, mitigating their toxic effects on fish, and its antibacterial properties help maintain gut microbiota balance by preventing pathogenic bacteria while promoting beneficial ones. Overall, dietary chitosan supplementation has been shown to enhance growth, health, and stress resistance in various fish species, making it a valuable additive in aquaculture. In the context of sturgeon (Huso huso) farming in Iran, reducing juvenile fish mortality and increasing growth rates are critical for economic efficiency, highlighting the importance of suitable diets and micronutrient balance to enhance immune function and production efficiency.
Methodology
The extraction of chitosan from shrimp shells was carried out through four stages: demineralization, deproteinization, decolorization, and deacetylation. 1260 fish (2.87 ± 0.07 g) after passing the acclimatization period, were fed in 18 tanks with six experimental diets, including control diet (without chitosan, T1), 0.15% (T2), 0.3% (T3), 0.45% (T4), 0.6% (T5) and 0.75% (T6) of chitosan (n=3) for 12 weeks. Then all growth indices such as final weight, growth rate, Protein Efficiency Ratio, and FCR, immunological parameters like lysozyme, ACH50, total immunoglobulin, and IgM were measured. The body composition also was measured after 12 weeks.
Results
The final body weight and body weight gain in treatments containing 0.45%, 0.6%, and 0.75% chitosan were significantly higher compared to the control, 0.15% and 0.3% chitosan treatments. The specific growth rate significantly increased with the higher levels of chitosan, and no significant difference was observed in the group fed with 0.15% chitosan compared to the control group. The hepatosomatic index significantly improved in all chitosan-fed treatments compared to the control group (P≤0.05). The feed conversion ratio significantly improved with increasing levels of chitosan compared to the control group. The protein efficiency ratio also increased in the treatments fed with 0.45%, 0.6%, and 0.75% chitosan compared to the control group, but no significant differences were found in the treatments fed with concentrations of 0.15% and 0.3% chitosan (P≥0.05). The lowest carcass protein level was observed in the control treatment, which showed a statistically significant difference compared to the carcass protein levels of other treatments, except for the 0.15% chitosan treatment (P ≤ 0.05). The carcass protein levels in the 0.45%, 0.6%, and 0.75% chitosan treatments were the highest, but no significant differences were observed among them. The carcass fat level in the 0.45%, 0.6%, and 0.75% chitosan treatments was significantly lower compared to the control group (P≤0.05). With the increase in chitosan concentration in the diets, the serum lysozyme levels significantly increased except for the group consuming 0.15% chitosan (P≤0.05). The results obtained from the complement activity (ACH50) measurements showed that with the increase in chitosan levels in the diet, this indicator significantly increased in the serum (P≤0.05). The total serum immunoglobulin and IgM levels, also increased significantly compared to the control group (P≤0.05).
Discussion and conclusion
Chitosan, has gained attention in aquaculture for its potential as a growth promoter and immunostimulant. Research indicates that dietary chitosan enhances growth performance, feed efficiency, and immune responses in various fish species, including Nile tilapia, common carp, and golden pompano. Optimal chitosan levels, typically ranging from 0.5% to 2%, have been shown to improve weight gain, feed conversion ratios, and overall health status by promoting gut health and enhancing digestive enzyme activities. Studies also highlight chitosan's role in modulating serum biochemical parameters, reducing fat accumulation in the liver, and improving intestinal morphology. The balance of chitosan dosage is crucial, as it can enhance beneficial bacteria while inhibiting harmful ones, thereby supporting the fish's immune system. Overall, chitosan represents a promising, eco-friendly alternative to antibiotics in aquaculture, contributing to sustainable fish farming practices while improving the health and productivity of cultured species. Chitosan also enhances nonspecific immunity and reduces mortality in fish, as evidenced by studies on common carp (Maqsood et al., 2010). However, its effects on feed consumption and conversion ratios can vary, with some studies showing no significant impact on these parameters despite improvements in liver health (Stanek et al., 2023). Overall, chitosan's role in aquaculture highlights its potential as a dietary supplement to improve fish health, growth, and resistance to environmental stressors. Serum parameter measurements are crucial for assessing the metabolic and physiological health of fish, particularly in aquaculture where they are susceptible to opportunistic bacterial infections. Lysozyme activity, an indicator of immune health, was found to increase with dietary chitosan levels. Research indicates that chitosan enhances lysozyme activity across various fish species, including loach, cobia, silver carp, vannamei shrimp, and rainbow trout. Additionally, chitosan supplementation in diets for beluga sturgeon and hybrid sturgeon also boosted lysozyme levels. Chitosan's immune-boosting effects are attributed to its ability to scavenge free radicals, enhancing innate immunity. Complement activity, vital for non-specific immune responses, also increased in fish like Nile tilapia and mrigal when fed chitosan. However, responses to chitosan vary by species and concentration, with some studies indicating potential negative effects at higher doses. Furthermore, IgM levels, a key immunoglobulin in fish, significantly increased with chitosan supplementation, reinforcing its role in adaptive immunity. Overall, optimal chitosan levels in fish diets can significantly enhance immune responses and health. The increase in IgM levels with dietary chitosan is not fully understood, necessitating further research to clarify the mechanisms involved (Salam et al., 2024). Studies indicate that chitosan enhances IgM and total protein levels in fish, such as rainbow trout and golden pompano, although excessive chitosan (10 g/kg) can reduce these benefits (Yu et al., 2023). In tilapia, IgM levels increased significantly up to 1.5% chitosan but decreased at 2% (Zhang et al., 2024). Additionally, recombinant chitosan nanoparticles show promise as alternatives to antibiotics in aquaculture, potentially enhancing disease resistance. This research supports using chitosan in beluga fry diets to improve growth and immune performance while reducing antibiotic reliance, particularly at levels of 0.45% to 0.6%, which positively affected growth, body composition, and immune parameters.
Conflict of Interest
The authors declare that they have no conflict of interest.
Acknowledgment
The authors wish to convey their heartfelt appreciation to all colleagues who supported and facilitated the experiment's necessary infrastructure.