Introduction

Selenium (Se) when available in sufficient amount is an essential trace element which is necessary for growth and physiological function in animals including fish (NRC 2011). Selenium plays a vital role in some important types of selenoproteins such as glutathione peroxidase (GPx) and provides protection against oxidative damage (Rotruck et al. 1973). A dietary Se deficiency results in reduced activity of GPx, oxidative stress, growth reduction, and immune response suppression in fish; thus, dietary Se supplementation is the best way for providing fish requirement (NRC 2011). The optimum Se requirements varied from 0.15 to 0.7 mg kg−1 in different cultured fish species (NRC 2011). It has been reported that the organic forms of Se is more effective in terms of bioavailability than the inorganic forms (Schrauzer 2000; Burk et al. 2006). Moreover, Se nanoparticles have attracted attention due to novel features like lower toxicity, higher chemical stability, biocompatibility, and ability to gradually release Se after ingestion (Skalickova et al. 2017). For example, it has been reported that administration of diet with nano-Se and vitamin E significantly improved growth performance, stress resistance, humoral immune responses, and serum biochemical parameters in rainbow trout (Oncorhynchus mykiss) and also altered the expression of liver proteins involved in metabolic status of this species (Naderi et al. 2017a,b,c). A plethora of studies have been conducted for determining optimum dietary Se requirements in various cultured aquatic species (NRC 2011). For example, the optimum dietary Se requirements in freshwater species such as crucian carp (Carassius auratus gibelio) and blunt snout bream (Megalobrama amblycephala) were determined as 0.5 and 0.2 mg kg−1, respectively (Zhou et al. 2009, Liu et al. 2016). For some marine fish species including grouper (Epinephelus malabaricus), cobia (Rachycentron canadum), and yellowtail king fish (Seriola lalandi), the optimum dietary Se levels were reported as 0.7, 0.8, and 2 mg kg−1, respectively (Lin and Shiau 2005; Liu et al. 2010; Le and Fotedar 2014). However, information about effects of different Se sources on growth performance and health status of cultured aquatic species is scarce. Recently, Ashouri et al. (2015) reported that supplementation of 1 mg nano-Se kg−1 diet improves growth performance, antioxidant defense system, and health status of common carp. However, according to the current literature, the effects of different dietary Se sources on physiological responses of common carp have not been elucidated. Thus, the present study was conducted to evaluate dietary supplementation of different Se sources including organic, inorganic, and nanoparticulate Se on growth performance and hemato-immunological and serum biochemical parameters of common carp, Cyprinus carpio juveniles.

Materials and methods

Diet preparation

Ingredients and chemical composition of the basal diet used in this experiment were according to Ashouri et al. (2015) (Table 1). Basal diet without Se supplementation used as control. Inorganic Se (sodium selenite (selenite), Na2SeO3, 99% purity, Sigma Chemical Co., St Louis, MO, USA), organic Se (selenomethionine (SeMet), 98% purity, Sigma Chemical Co., St Louis, MO, USA), and nano-Se (99.95% purity, 30–45 nm particle size, 3.89 g cm3 true density, Iranian Nanomaterials Pioneers, Mashhad, Iran) were added to the basal diet at 0.7 mg kg−1 dry diet according to the findings of previous studies on optimum dietary Se requirement of common carp juveniles (Jovanovic et al. 1997; Elia et al. 2011; Ashouri et al. 2015). Nano-Se was the particles of red elemental Se in the redox state of zero and prepared by adding bovine serum albumin to the redox system of selenite and glutathione according to Zhang et al. (2001). The ingredients and different Se sources were mixed, extruded, and air dried at room temperature, and then were kept in the − 20 °C until used. The final actual concentration of Se in each diet was measured by an atomic absorption spectrophotometer equipped with transversely heated graphite atomizer system (Younglin AAS 8020, Korea) as described previously by Elia et al. (2011) (Table 2).

Table 1 Formulation and proximate composition of the basal diet (mean ± SEM, n = 3)
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Table 2 The final actual concentration of Se (mg kg-1) in experimental diets
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Fish maintenance and feeding

Common carps obtained from a local hatchery (Ahvaz, Iran) were used in the study. Fish were randomly distributed into 12 cylindrical fiberglass tanks with a volume of 300 L and each tank stocked with 14 fish (mean body weight 9.7 ± 0.1, mean ± standard deviation). Each dietary treatment was assayed in triplicate. Fish were acclimated for 2 weeks before the onset of the nutritional trial and fed by basal diet without Se supplementation. Fish were fed three times a day at 8:00, 13:00, and 19:00 with the experimental diets for 8 weeks according to Ashouri et al. (2015). Daily feeding rate was ca. 3% of body weight according to batch weighing of fish every 2 weeks. Uneaten food was siphoned out 1 h after feeding and weighed to determine feed intake values. Once every 3 days, 50% of the rearing water was replaced. The mean water quality features were kept as follows: temperature 28.4 ± 0.2 °C, dissolved oxygen 6.4 ± 0.1 mg mL−1, pH 7.8 ± 0.2, and the photoperiod was 12L:12D (light/darkness).

Sample collection

At the end of 8-week feeding trial, all the fish were anesthetized with clove oil (150 ppm) and individually weighed. The weights of fish, liver, and viscera were measured to the nearest 0.1 g. Standard formulae were used to assess growth performance, feed utilization, and other parameters: weight gain (%): WG = (final weight − initial weight) / initial weight) × 100; feed efficiency ratio (%): FER = (weight gain (g) / feed intake (g)) × 100; protein efficiency ratio (PER) = weight gain (g) / protein intake (g); hepatosomatic index (%): HSI = (liver weight (g) / whole body weight (g)) × 100; viscerosomatic index (%): VSI = (visceral weight (g) / whole body weight (g)) × 100; condition factor (%): K = (body weight (g) / (body length (cm)) 3) × 100.

Blood was collected randomly from the caudal vein of seven fish per tank. For hematological analyses the extracted blood was poured into heparinized vials (n = 9 fish per diet treatment, n = 3 fish per diet replicate). For serum antioxidant enzymes and immunological and biochemical parameters (n = 12 fish per diet treatment, n = 4 fish per diet replicate), blood was allowed to clot at 4 °C for 1 h and then centrifuged (4000×g, 10 min at room temperature). Sera were separated and frozen at − 80 °C until used.

Light microscopy study

At the end of the experiment, the visceral mass of three fish per dietary treatment was dissected and fixed in 4% buffered formaldehyde (pH = 7.4), dehydrated in a graded series of ethanol, cleared with xylene, embedded in paraffin, and cut in serial sections (3–5 μm thick). Hematoxylin and eosin-stained sections were then photographed and studied using a digital microscope (Dino-Eye AM4023X, Taiwan). A computerized microscopic image analyzer (Digimizer 4.1.1) was used to determine histomorphometric parameters including villus perimeter and villus at foregut and midgut sections on ten different villi per fish. The criteria for selection of histological sections for examination were based on the presence of an intact villus that was perpendicularly sectioned through the midline axis.

Lipid peroxidation values and antioxidant enzyme parameters

Glutathione peroxidase activity was assayed using the method described by Noguchi et al. (1973). GPx degrades H2O2 in the presence of reduced glutathione (GSH). The activity of GPx is expressed as 1 μmol L−1 of the substrate (GSH) depleted per minute per mg of protein or per mL serum.

Activity of serum superoxide dismutase (SOD) was measured according to the method of McCord and Fridovich (1969). The assay mixture contained 1.2 ml of sodium pyrophosphate buffer, 100 μl of phenazine methosulfate, 300 μl of nitroblue tetrazolium, and 200 μl of the serum and water in a total volume of 2.8 ml. The reaction will be initiated by the addition of 200 μl of NADH. The mixture was incubated at 30 °C for 90 s and arrested by addition of 1.0 ml of glacial acetic acid. The reaction mixture will be then shaken with 4.0 ml of n-butanol, allowed to stand for 10 min and centrifuged. The intensity of the chromogen in the butanol layer will be measured at 560 nm in a spectrophotometer. One unit of enzyme activity is defined as the amount of enzyme that gave 50% inhibition of NBT reduction in 1 min.

Serum catalase (CAT) activity was determined following the method of Abei (1984). Briefly, the activity was determined by measuring the decrease in absorbance at 240 nm (e = 40 M/cm) using 13.2 mM H2O2 in 50 mM phosphate buffer (pH 7.0) and 100 μl of serum. A mixture containing 50 mM phosphate buffer (pH 7.0) and 100 μl of serum was used as a control. Malondialdehyde (MDA) concentration, also known as thiobarbituric acid reactive substances, was measured colorimetrically using the method of Buege and Aust (1978). Briefly, 200 μl of serum was reacted with 2 ml of thiobarbituric acid (TBA) reagent containing 0.375% TBA, 15% trichloroacetic acid, and 0.25 N HCl. Samples were then boiled for 15 min, cooled and centrifuged. The absorbance of the supernatants was measured spectrophotometrically at a wavelength of 532 nm. Lipid peroxidation was expressed as TBARS concentration using1,3,3,3 tetra-ethoxypropane as a standard.

Hematological analyses

Hematocrit (%; Hct), hemoglobin concentration (Hb; g dL−1), and the number of red blood cells (RBCs) and white blood cells (WBCs) as well as differential WBC counts (lymphocyte, monocyte, neutrophil and basophil portions as WBC%) were assessed according to methods described by Blaxhall and Daisley (1973). Blood indices including the mean cell hemoglobin (MCH), the mean cell volume (MCV), and the mean cell hemoglobin concentration (MCHC) were calculated according to the following formulae (Lewis et al. 2001):

$$ {\displaystyle \begin{array}{l}\mathrm{Mean}\ \mathrm{cell}\ \mathrm{volume}\ \left(\mathrm{MCV}\right)=\mathrm{Hct}\ \left(\%\right)/\mathrm{RBC}\ \left(\times 106\ \upmu \mathrm{L}\right)\times 10.\\ {}\mathrm{Mean}\ \mathrm{cell}\ \mathrm{hemoglobin}\ \left(\mathrm{MCH}\right)=\mathrm{Hb}\ \left(\mathrm{g}\ \mathrm{dL}-1\right)/\mathrm{RBC}\ \left(\times 106\ \upmu \mathrm{L}\right)\times 10.\\ {}\mathrm{Mean}\ \mathrm{cell}\ \mathrm{hemoglobin}\ \mathrm{concentration}\ \left(\mathrm{MCH}\mathrm{C}\right)=\left(\mathrm{g}\ \mathrm{dL}-1\right)=\mathrm{Hb}\ \left(\mathrm{g}\ \mathrm{dL}-1\right)/\mathrm{Hct}\ \left(\%\right)\end{array}} $$

Humoral immune parameters

The alternative complement pathway hemolytic activity (ACP) was estimated as described by Tort et al. (1996) by 50% lysis of rabbit red blood cell (RARBC) as target cells in the presence of EGTA and Mg+2. Following washing of the RaRBC three times, the absolute lysis value was prepared by adding100 μl of the RaRBC to 3.4 ml distilled water. The lysate was then exposed to cold centrifugation, and the turbidity of the aqueous phase was determined at 414 nm. Subsequently, the serum specimens were diluted in the buffer and 250 μl of adjusted volume serum was added to 100 μl of RaRBC in test tubes. The prepared solution was kept at room temperature for 90 min with repeated mixing. Then, 3.15 ml of NaCl solution (0.85%) was added to all samples and the tubes were centrifuged for 10 min and the absorbance of the supernatant was quantified again. The level of cell lysis was determined, and hemolysis curve was drawn through plotting the hemolysis degree against the volume of serum added on a log/log-scaled graph. The volume producing 50% hemolysis was considered for determining the hemolytic activity of the serum samples and was expressed as U ml−1.

The lysozyme levels in the blood serum were determined using a turbidimetric assay according to the method of Ellis (1990) by measuring the lytic activity of the C. carpio juvenile serum against lyophilized Micrococcus lysodeikticus (Sigma, St Louis, MO, USA). A volume of 135 μl of M. lysodeikticus at a concentration of 0.2 mg ml−1 (w/v) in 0.02 M sodium citrate buffer (SCB), pH 5.8, was added to 15 μl of serum sample. As a negative control, SCB was replaced instead of serum. Results were expressed in milligram of lysozyme per milliliter of serum. Hen egg white lysozyme (Sigma) in phosphate-buffered saline was used as a standard. A unit of lysozyme activity was defined as the amount of serum causing a reduction of absorbance of 0.001 per minute at 450 nm at 22 °C.

Serum total immunoglobulin (Ig) was measured using the method described by Siwicki et al. (1994). Primary separation of immunoglobulins from the serum was achieved by precipitation with polyethylene glycol (PEG), and the resulting supernatant was analyzed. To perform the assay, 100 μl of serum was combined with 100 μl 12% PEG and incubated at room temperature for 2 h in continuous agitation. Following the incubation time, the mixture was centrifuged (400×g, 10 min at room temperature), and total protein concentration in the supernatant was determined by the Biuret method. The total Ig levels were calculated considering total protein values less the quantity of protein in the supernatant.

Serum biochemical analyses

Serum biochemical parameters were analyzed by means of an autoanalyzer (Mindray BS-200, China) using commercial clinical investigation kits (Pars Azmoon and ZistChimi Kits, Tehran, Iran). Biochemical measurements were conducted for total protein (TP) and albumin (ALB), glucose (GLU), triglyceride (TG), total cholesterol (CHO), high-density lipoprotein (HDL), and low-density lipoprotein (LDL). Moreover, the content of total globulin (GLO) was estimated by subtracting albumin from total protein (Kumar et al. 2005).

Statistical analyses

The results (mean ± SE, standard error) were evaluated by one-way analysis of variance (ANOVA) followed by Duncan to compare the means between each individual tested groups using SPSS (Version 16; SPSS Inc., Chicago, IL, USA) at P < 0.05 level.

Results

During the feeding trial, the fish survival was 100% in all groups. Fish fed nano-Se diet had the highest WG (97.2 ± 10.8%), FI (21.9 ± 1.5 g fish−1), FER (42.4 ± 0.8%), and PER (1.3 ± 0.0%) (Table 3). Fish fed Se-supplemented diets had higher VSI values than the control group; however, other somatic parameters (HSI and K) did not change among experimental groups.

Table 3 Growth performance of common carp fed the diets containing different sources of Se for 56 days (mean ± SEM, n = 3)
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Intestinal histomorphometric analyses of the foregut and midgut sections revealed that different dietary Se sources significantly affected perimeter and height of villi (Table 4). The perimeter of the villi was significantly increased in the foregut section of fish fed SeMet and nano-Se diets, compared with respective values for the control and sodium selenite groups. Fish fed Se-supplemented diets had greater villi perimeter in midgut section than the control group. Intestinal villi height was significantly taller for fish fed nano-Se diet than the control group in both intestinal sections, and other groups showed intermediate values.

Table 4 Morphological changes in intestinal sections of common carp fed different sources of Se for 56 days (mean ± SEM, n = 3)
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In this study, serum GPx (Fig. 1a) and SOD (Fig. 1b) activities were significantly higher in nano-Se and SeMet groups than the control and sodium selenite groups; however, serum CAT (Fig. 1c) activity did not differ among different dietary treatments. Moreover, serum MDA (Fig. 1d) was at highest (16.5 ± 1.1 nmol mL−1) and lowest (11.1 ± 0.8 nmol mL−1) levels in the control and nano-Se groups, respectively.

Fig. 1
figure 1

Serum antioxidant enzymes activities including GPx (a), SOD (b), and CAT (c) as well as MDA level (d) in common carp fed different dietary Se sources

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In the present study, fish fed Se-supplemented diets had higher RBC, Hct, Hb, and MCV values than the control (Table 5). Moreover, fish in nano-Se and SeMet had higher WBC counts and neutrophil percentage than the other groups, but fish in the control had the highest lymphocyte percentage (P < 0.05). Serum lysozyme activities were higher in fish fed nano-Se and SeMet diets as compared to fish fed other experimental diets (Fig. 2a). On the other hand, fish fed nano-Se diet had the highest serum ACP activity (474.3 ± 17.1 U mL−1; Fig. 2b) and total Ig content (73.1 ± 7.0 mg L−1; Fig. 2c).

Table 5 Hematological parameters of common carp fed the diets containing different sources of Se for 56 days (mean ± SEM, n = 3)
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Fig. 2
figure 2

Serum immunological parameters including lysozyme (a) and ACP (b) activities as well as Ig content (c) in common carp fed different dietary Se sources

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Regarding serum biochemical parameters, fish fed nano-Se diet had the highest serum TP (4.1 ± 0.1 g dL−1) and ALB (2.3 ± 0.1 g dL−1) levels (Table 6). In addition, CHO (94.3 ± 4.9 mg dL−1) and LDL (13.1 ± 2.3 mg dL−1) concentrations were lower in fish fed nano-Se diet as compared to other groups (P < 0.05). There were no significant differences in serum GLO, GLU, TG, and HDL levels among different treatments.

Table 6 Blood biochemical parameters of common carp fed the diets containing different sources of selenium for 56 days (mean ± SEM, n = 3)
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Discussion

The results of the present study showed that fish fed nano-Se-supplemented diet had the highest WG and FE, indicating that nano-Se was the most bioavailable Se source to common carp juveniles. In contrast, Lorentzen et al. (1994) demonstrated that dietary supplementation of inorganic (sodium selenite) and organic Se (SeMet) (1 and 2 mg kg−1 diet, respectively) could not enhance WG in Atlantic salmon (Salmo salar). The species-specific variations in the intestinal Se absorption rate; the Se concentrations in rearing water, bioavailability, and different metabolic pathways for different Se sources; and the amount of dietary vitamin E (Abdel-Tawwab et al. 2007; Hao et al. 2014) may result in discrepancies of optimal dietary Se level in various fish species.

In the current study, fish fed nano-Se and SeMet diets had greater perimeter and height of intestinal villi than control and sodium selenite groups. These results suggest that nano-Se and SeMet could maintain the integrity of the intestinal tract more efficiently as indicated by taller villi, and also protect the intestinal epithelial cells covering the villi. This maintenance effect may possibly be attributed to improved antioxidant enzyme activities and redox status in the intestine, which could result in less oxidative stress. On the other hand, the improved WG in nano-Se group could partly be related to increased ability of fish to assimilate the nutrients because of greater intestinal surface area, increasing the brush boarder enzyme secretion as well as reduction in enterocyte cell death and/or enterocyte turnover rates associated with oxidative stress (Read-Snyder et al. 2009; Nugroho and Fotedar 2015). In this context, it has been reported that the protein content and GPx activity of the intestinal epithelial cells of crucian carp increased with increasing concentrations of nano-Se in the in vitro culture medium (Wang et al. 2013). Moreover, the authors of the previous study showed that nano-Se supplementation resulted in more consistency and integrity of the intestinal epithelial cells. Thus, better WG and FE of the common carp fed on nano-Se diet in our study may be explained by higher protein contents of the intestinal cells that could result in better metabolism of absorbed nutrients. In addition, Nugroho and Fotedar (2015) demonstrated higher numbers of longer microvilli with more integrity in crayfish (Cherax cainii) fed on organic Se-supplemented diet.

The antioxidant effect of Se is accounted for its incorporation in selenocysteine, which is part of the active center of the GPx (Kohrle et al. 2000). Glutathione peroxidase has a protective role against oxidative radicals in body cells and enhances the body’s cell resistance such as immune cells against peroxidative damage (Burk et al. 2003). The results of this study showed higher serum GPx and SOD activities in nano-Se and SeMet groups as compared to the sodium selenite and control groups, indicating that nano and organic forms of Se could efficiently strengthen the antioxidant system against oxidative stress. A serum concentration of MDA is an index of lipid peroxidation and oxidative stress, and its levels depend upon the antioxidants. In the present study, fish fed on Se-supplemented diets had lower serum MDA levels compared to the control. Also, the lowest serum MDA level was observed in fish fed on nano-Se diet. In agreement, other studies demonstrated that nano-Se and organic sources of Se could enhance GPx activity, antioxidant capacity, and oxidative stress resistance in different fish species (Lin and Shiau 2005; Zhou et al. 2009; Liu et al. 2010; Le and Fotedar 2014; Ashouri et al. 2015; Liu et al. 2016). In contrast, Cotter et al. (2008) demonstrated that sodium selenite (0.4 mg kg−1) resulted in higher GPx activity in hybrid striped bass than Se-yeast when supplemented.

The potent antioxidant capacity of the Se (Molnár et al. 2011; Ashouri et al. 2015; Khan et al. 2016) might increase stability of the RBCs membranes and their survivability by protecting them against oxygen free radicals, causing membrane damage, cell hemolysis, and anemia. In this context, Le et al. (2014) reported that increasing dietary Se (Se-yeast) levels (from 0 to 2 mg kg−1) led to an increase in RBC’s GPx activity in yellowtail king fish, which could protect RBCs from oxygen free radicals. In this study, different sources of Se increased the hematological parameters (RBCs count, Hb level, Hct%, and MCV) in juvenile common carp in comparison with control diet, indicating healthier status of fish fed on Se-supplemented diets. Similar to the finding of the present study, several studies have proved the role of Se in improving the hematological indices in different fish species such as Hct% in hybrid tilapia (El-Hammady et al. 2007); RBC counts in Nile tilapia (Oreochromis niluticus; Molnár et al. 2011); and RBC counts, Hb, and Hct% values in African catfish (Clarias gariepinus; Abdel-Tawwab et al. 2007) and golden mahseer (Tor putitora; Khan et al. 2016, 2017).

Selenoproteins, especially GPx, protect neutrophils and macrophages from superoxide radicals derived from respiratory burst activity (Hodgson et al. 2006). In this study, WBCs counts and neutrophil percentage were increased in nano-Se and SeMet groups which could be due to the enhancement of fish health status. In fish, it has been reported that nano-Se or SeMet is more readily absorbed, and more potent in terms of bioavailability and effects on health, than sodium selenite (Wang et al. 1997; Le and Fotedar 2014). Similar to our results, Zhou et al. (2009) reported that a diet supplemented with nano-Se or SeMet significantly increased WBC counts in crucian carp as a consequence of increasing GPx activity in plasma and tissue. On the other hand, in the current study, lymphocyte percentage decreased in fish fed on Se-supplemented diets. In this context, it should be mentioned that the leukogram changes and immune responses would be different depending on species, individual differences, nutritional condition, and animal welfare (Nandra 1997).

Se can affect fish immune system via enhancing the activation of antioxidant enzymes (i.e., GPx, SOD and CAT) (Han et al. 2011; Le and Fotedar 2014; Naderi et al. 2017a), regulation of cell signaling molecules (i.e., nuclear factor kappa B and interleukin 2 (IL-2)), regulation of the function of immune cells (i.e., lymphocytes, natural killer cells, and neutrophils) by activating high-affinity IL-2 receptor (Rayman 2004), and anti-stress effects (Naderi et al. 2017b), which can finally lead to immune competence promotion. The results of this study showed that lysozyme activity was higher in nano-Se and SeMet groups compared to other groups, indicating that nano-Se and organic Se had higher bioavailability than inorganic Se for lysozyme activity. Enhanced lysozyme activity in nano-Se and SeMet groups could be associated with increased WBC counts, especially neutrophil percentage, which is the main producer of the lysozyme in blood. Similarly, dietary Se supplementation led to an increase in serum lysozyme activity in yellowtail king fish (Le et al. 2014; Le and Fotedar 2014), rainbow trout (Naderi et al. 2017a), and gold mahseer (Khan et al. 2017). In contrast, Cotter et al. (2008) showed that different Se sources had no effects on lysozyme activity in hybrid striped bass. Moreover, the increased serum SOD activity in nano-Se and SeMet groups may represent higher non-specific cellular immune responses in these groups, since the activity of SOD for detoxifying superoxide anion is associated with the respiratory burst activity of neutrophils and macrophages (Lin et al. 2011). In addition, fish fed nano-Se-supplemented diet had relatively higher serum ACP activity and Ig level as compared to other groups, suggesting that nano-Se was more effective than other Se sources in improvement of immune responses in common carp juveniles. In this context, it has been reported that dietary Se supplementation alone (Le and Fotedar 2014) or in combination with vitamin E (Le et al. 2014) led to an increase in serum antibody titration and bactericidal activity in yellowtail king fish.

Plasma or serum total protein, which is mainly synthesized by liver parenchymal cells, has been used as a broad clinical indicator of health, immune competence, stress, and nutritional condition in fish (Riche 2007). In addition, it is believed that the absorbed Se is bound to albumin and transported to the liver, where it could be used for selenoprotein synthesis (Suzuki et al. 2010). Based on the results of this study, the highest concentrations of TP and ALB were observed in fish fed on nano-Se, suggesting better health and nutritional status in this group. Similar results were reported in African catfish (optimum Se requirement = 0.3–0.5 mg kg−1 organic Se; Abdel-Tawwab et al. 2007) and common carp (Ashouri et al. 2015) as a consequence of increasing Se concentration in the liver. Based on our data, dietary nano-Se supplementation caused significant decrease in serum CHO and LDL concentrations, as reported previously in common carp (Ashouri et al. 2015) and rainbow trout (Naderi et al. 2017a). Previous studies have demonstrated that dietary Se supplementation increased LDL receptor activity (Dhingra and Bansal 2006a) but decreased 3-hydroxy 3-methylglutaryl co-enzyme A (HMG-CoA) reductase expression in rat (Dhingra and Bansal 2006b), which can lead to decrease in serum LDL and CHO levels (Yang et al. 2010).

In conclusion, the results of this study showed that dietary nano-se supplementation increased growth performance and feed efficiency in common carp juveniles, possibly as a consequence of improving integrity and consistency of intestinal epithelial cells, antioxidant capacity, immunological responses, and serum biochemical health indices.