International Journal of Fisheries and Aquaculture

The present study is aimed to provide information on the abundance of natural foods needed by Mugil cephalus in the Elechi Creek. The natural food of M. cephalus in the Elechi Creek was studied from stomach contents of the fish. The stomach contents were analyzed using two methods; the frequency of occurrence and numerical methods. Plant materials, diatoms, algae and dinoflagellates constituted its main food. Plant materials were found to be the most preferable food where it occurred in 67.0% of the examined fish. Annelids, fish larva, fish parts, insect parts and crustaceans comprised the food of animal origin. Sand/mud and organic matter occurred in about 11.0 and 73.0% of the examined stomachs respectively. These results indicate that M. cephalus is omnivorous plant materials were the most abundant food items by numerical and occurrence methods in the gut of the fish species.


INTRODUCTION
The study of the food and feeding habit is useful and fundamental to understand the functional role of the fish within its ecosystem.The family Mugilidae is among the commercially important fishes occurring in Elechi Creek, Rivers State.They consist of seventeen genera and eighty species which are widely distributed along the brackish estuaries and coastal lagoons.They are popular, well accepted and form a large proportion of the diets of the rural communities in coastal areas in Nigeria.Mugil cephalus occurs worldwide from approximately 42° N to 42° S Latitude (Bok, 1979;Render et al., 1995), where it inhabits estuarine intertidal freshwater and coastal marine habitats.In the western Atlantic Ocean, M. cephalus ranges from Cape Cod to Brazil, including the Gulf of Mexico, Caribbean, and West Indies (Fagade and Olaniyan, 1998).Adults and juveniles of grey mullets are hardy, euryhaline, eurythermal and not competitor for food among its species.School occurs in shallow coastal waters; they enter lagoons and estuaries to feed (Rheman et al., 2002).In estuarine waters, they feed on detritus, diatoms, algae and microscopic invertebrates (McDonough and Wenner, 2003).Bishop and Miglarese (1978) found that the principal food sources of adult mullet are detritus and epiphytic algae.However, they also observed M. cephalus feeding opportunistically on swarming polychaetes of the Nereis genus.The species is heterotrophic, adults feed primarily on detritus.Mullets constitute a large proportion of the catches by artisanal and subsistence fishermen in lagoons and rivers and generate a high market value in demand because of their taste.This paper describes the diet and feeding habits of M. cephalus based on the examination of stomachs collected.

Study area
The Elechi Creek also known as Elechi, Omo Ema Creek is located in Rivers State, Niger Delta Nigeria.The creek lies between longitude 6° 45″E and 7° 20″N and latitude 4° 38″N and 5° 5″E South-West of Port Harcourt.The creek is tide dominated embayment with little fresh water input and is characterized by extensive mangrove swamps, tidal flats, influenced by semi-diurnal tidal regime (NEDECO, 1961).
The vegetation consists of mangrove forest dominated by the red mangrove Rhizophora racemosa and Rhizophora mangle.In some areas, the white mangrove Avicennia africana is interspersed with Nypa fructicans.Its vegetation provides logs of wood for domestic and building purposes.The low inter-tidal zone is usually bare of vegetation, with clay, peat and sand deposit.The area is surrounded by numerous water fronts' residential houses.The surrounding terrestrial environment is marked by various human activities such as saw milling of timber.

Sampling procedure
The food and feeding habits of M. cephalus was studied by examining the stomach contents.Fish samples were collected with gill and cast nets.The nets used were between 30 and 60 mm mesh size.Specimens of M. cephalus were collected fortnightly from the artisanal fishermen.Fish specimens were transported to the University of Port Harcourt, Faculty of Agriculture laboratory to investigate the food and feeding habits.
The fish samples were preserved in 10% formalin solution to avoid deterioration.Each fish specimen collected was given a registration number which indicates date of capture, weight and length.Standard and total lengths (in centimeter) were measured using a measuring board while the weights (in grammes) were taken to the nearest gram using a sensitive electrical balance, MP 2003 model.The stomachs were removed intact by cutting above the cardiac and below the pyloric sphincters and preserved in a vial with 4% formalin.Dissection entailed making an incision above the longitudinal axis of the stomach and intestine.The stomach contents were emptied into a Petri dish and identified to the lowest taxonomic level.Identification entailed first sorting by eye for larger food substances.Secondly the stomach contents were dropped on the slides with the aid of a dropping pipette and observed under a light microscope (with variable magnifications up to X 40) examinations.
Food was regarded as total ingested matter and was analysed using the frequency of occurrence and the numerical methods (Bagenal, 1978;Hyslop, 1980).In the frequency of occurrence each food item was used to indicate the proportion of fishes eating a particular food item.The merits and limitations of these methods have been discussed by Dunn (1954), Crips (1963) and Fagade and Olaniyan (1972).

Frequency of occurrence = X/Y x 100%
Where, X = number of stomach where each food item is present, Y = number of stomachs for the experiments.
In numerical methods, the number of individual in each category is recorded for all stomachs and the total is expressed as a proportion.

Food composition
Table 1 show the list of food items found in the stomach of M. cephalus, while the summary of the items are illustrated in Figure 1.The relative contributions of the food items are expressed by frequency of occurrence and numerical methods.A total of 100 stomachs were randomly examined.Eleven major items constituted the diet of M. cephalus, four forming the dominant stomach content.

Food composition for juvenile's M. cephalus
In the numerical analysis plant materials were dominant and composed of 30.44% of the items in the stomach, diatoms made up 24.81%, while algae made up 19.40%, dinoflagellates 17.65%, crustaceans 4.23%, fish parts and insect parts 1.73 and 0.97%.Fish larva and annelids were the least with 0.43 and 0.32% respectively.
Food items were contained in one hundred stomachs and organic matter (detritus) and sand particles were present in 25.44%, plant materials were contained in 23.35% of the stomach, algae 15.68%, dinoflagellates 11.50%, diatoms 8.01%, crustaceans 4.88%, fish and insect parts had 3.14 and 2.44%.The least food items of 1.05 and 0.70% were found in fish larva and annelids respectively.
The result of the stomach content of the various size groups is presented in Table 2. Plant materials constituted the most consumed food items fed on in the three size groups both by numerical method (30.91, 33.84 and 25.0) and by frequency of occurrence (33.33, 33.84 and 17.89) methods.Detritus (organic matter) constituted the most occurring item in the stomachs of the three size groups of M. cephalus from the Elechi Creek by occurrence (37.17, 24.32, and 17.89).Plant materials were the next in occurrence to the detritus in number of stomach where they are found in the three size groups.The medium size groups consumed more food items than the other size groups.
Variation in empty stomach by size group (Table 3) indicated that the small size group of M. cephalus had the highest number of empty stomachs (8.51%), while the medium sized group had (6.25%) and the large sized group had no empty stomach.

DISCUSSION
The food composition of the species in Elechi Creek shows a high food richness as the specimens showed high trophic flexibility by the wide variety of food items consumed.M. cephalus is diurnal feeder, consuming mainly zooplanktons, dead plant matter and detritus.In this study M. cephalus foods varied from microscopic items such as diatoms to macroscopic ones such as annelids and fish parts.This is an indication that it has advantages of using large aquatic resource to its advantage.This high trophic flexibility ensures a constant energy source which is necessary for its large population (King, 1988).Its occurrence maybe due to this reason; the fish was able to shift from one form of food to another depending on the seasons and availability of the foods.Availability of food organisms are often cyclic and maybe due to their life histories, climate, or environmental conditions.During this study, empty stomach was recorded between the months of August (12.5%)and September (5.0%).
Presence of plants presumed the species as an herbivore, while the availability of invertebrates showed it as a carnivore, fish parts as a part of its diet was an indication of its piscivorous ability.Presence of organic matter and sand/mud in reasonable number mean the species was both detritus and benthic feeder food is ingested from bottom substrate.
In relation to size, the trend among the three size groups of the stripped mullets from the Elechi Creek changes.The juveniles fed mainly on plant materials, algae, dinoflagellates, diatoms and crustaceans, while the adults predominantly fed on plant materials, algae, dinoflagellates, diatoms, crustaceans, annelids, fish parts, insect parts, sand/mud and organic matter.The high dietary importance of sand/mud and detritus may be due to the marginal mangrove vegetation growing on water-logged deposits of soft mud and clay-silt sediment.
The decomposing leaves and other biogenic materials The adults had more of sand/mud and organic matter in their stomach because they become benthic feeder as they grow.This change in diet with growth not only offers a wider range of food resources to the species but also reduces possible competition between the adults and juveniles to some extent.Seasonally variation in the feeding habit was not recorded as the study was carried out only in the rainy season.The success of mullets according to Kurian (1975) and Payne (1976) also lies in their feeding habits and the abundance of their food.Presence of detritus and sand/mud in large quantities in the stomach of the fish does not presume that these items were nutritional better than other food items that appeared in lesser number.Sand/mud has no nutritional property but they might have been picked up along with other food items.Its presence in the gizzard like stomach may assist the fish in digestion process.
In comparison with other studies, this result confirmed to the study on food and feeding habits with reviews from Akpan and Ubak (2005) and Lawson and Jimoh (2010), on the diets of mullets.However, from the study, major food of M. cephalus in the Elechi Creek where diatoms, algae, plant materials, dinoflagellates, sand/mud, organic matter, crustaceans, insect parts, fish parts and annelids.

Conflict of Interest
The authors have not declared any conflict of interest.

INTRODUCTION
Presently, aquaculture is a fast growing food production industry that contributes nearly 50% of the annual fisheries production (FAO, 2012).In the Philippines, tilapia is the second most cultured fish species after milkfish.However, annual production of Nile tilapia is usually affected by episodes of high mortality mostly due to bacterial infections that could be attributed to very intensive culture practices and sometimes aggravated by seasonal effects of low environmental temperatures.One of the opportunistic bacteria that infect cultured Nile tilapia * is Aeromonas hydrophila which causes septicemic infections in the fish (Cipriano, 2001).These pathogens are ubiquitous and are thus found in a variety of aquatic environments.To control these bacterial infections, antibiotics are mainly used.However, antibiotic use may lead to the emergence or development of antibiotic-resistant bacteria, thus safer and more effective alternatives should be used (Pachanawan et al., 2008).Studies on the use of immunostimulants derived from natural bioactive products are presently gaining importance as an option instead of using chemotherapeutants and antibiotics.Immunostimulants enhance the non-specific immune responses (innate immunity) as well as the specific immune response mechanisms (adaptive or acquired immunity) of a certain organism (Anderson, 1992).Other immunostimulants like chitin, chitosan, and levamisole have been reported to enhance the non-immune responses of common carp (Cyprinus carpio) which led to a higher percentage survival of the fish (Gopalakannan and Arul, 2006).
Corn silk (Zea mays) is traditionally used in the treatment of cystitis, edema, gout, kidney stones, nephritis, and urological disorders (Ebrahimzadeh et al., 2008).It is also used as an anti-diabetic agent since it reportedly counteracts hyperglycaemia (Guo et al., 2009).It has phenolic compounds such as anthocyanins, vanillic acid, and protocatechuic acid which are reportedly responsible for its antioxidant capacity (Ebrahimzadeh et al., 2008).Corn silk extract has also shown great potential when it comes to prevention of diseases involving overproduction of radicals (Liu et al., 2011).Administration of natural products may be done via bathing, injection, or oral administration, the latter of which is the least stressful of all the three routes (Harikrishnan et al., 2011).
This study was therefore undertaken to investigate the protective role of dietary corn silk supplements against A. hydrophila infection and induced hepatic damage by measuring some nonspecific immune parameters and lipid peroxidation activity as indicator of oxidative stress.

Fish and experimental design
Mixed sex Oreochromis niloticus obtained from the Southeast Asia Fisheries Development Center, Freshwater Fisheries Station, Binangonan, Rizal and Philippines were used.Prior to the experiments, the fish were acclimatised in rectangular tanks with recirculating water system.They were fed daily at a feeding rate of 3% of the total body weight with commercial grow-out tilapia pellets.Regular water change was done during the pre-experimental period.For the experiments, three groups of tilapia, 20 fish each (58.9 ± 1.76 g, 180 fish total), were designated: a) negative control, which consisted of healthy tilapia fed with commercial tilapia pelleted feeds; b) positive control, which consisted of tilapia infected with A. hydrophila and fed with commercial tilapia feed pellets; and c) corn silk treated group, which consisted of tilapia infected with A. hydrophila and fed with the corn silk-coated tilapia feed pellets.
In order to determine the antioxidant property of corn silk, another experimental set-up consisted of four groups of tilapia (42.5 ± 0.98 g; 80 fish) that were designated as: a) negative control, which consisted of tilapia fed with commercial pelleted tilapia feeds; b) positive control, which consisted of tilapia fed with feeds coated with paracetamol; c) paracetamol and silymarin treated group, which consisted of tilapia fed with a mixture of paracetamol and silymarin-coated fish food; and d) paracetamol and corn silk treated group, which consisted of tilapia fed with a mixture of paracetamol and corn silk extract-coated fish food.

Fish food preparation
The powdered corn silk (1.750 g) produced by Britmix Wellness, Inc. was coated onto 500 g of commercial fish feed pellets.The powdered corn silk was mixed into a 22% gelatin solution and this mixture was coated onto the fish feed pellets.The coated feeds were air dried for 48 h and both the coated and uncoated feed pellets were stored in the refrigerator (4°C) to prevent bacterial and fungal contamination.The corn silk-coated fish food pellets were used for the fish in the first experiment.For the second experiment, four types of fish diets were used: a) for the negative control group, unsupplemented commercial fish food, b) for the positive control group, powdered paracetamol (1000 mg) was coated using gelatin onto 500 g of fish feed pellets, c) for the paracetamol-silymarin reference group, 1000 mg of paracetamol and 450 mg of silymarin (Steinbach Products, Inc.; Mandaluyong, Philippines) were coated onto 500 g fish feed pellets, and d) for the paracetamol-corn silk group, 1000 mg of paracetamol and 1000 mg of corn silk were coated onto 500 g fish feed pellets.The fish were fed at 3% body weight daily during the experimental period.

Preparation and Injection of bacteria
A. hydrophila culture was obtained from the National Institute of Molecular Biology and Biotechnology (BIOTECH) of the University of the Philippines -Los Baños.The bacterial innoculant was prepared through serial dilution and spread plate technique.Final bacterial concentration was adjusted to 6.2 × 10 5 CFU ml -1 .A 0.1 ml of the bacterial suspension was injected intramuscularly near the lateral line of the pectoral region of the fish from the positive control group and corn silk treated group of the first experimental set-up.Phosphate buffer saline (PBS) pH 7.2 with the same volume was injected into the negative control tilapia group.Bacterial infection was done after a 30 day feeding period.

Sample collection
For the immune response experiments, the fish in each treatment tanks were sacrificed at day 7 post-infection.Each fish was immobilized with a blow in the head.Blood was extracted from the midventral caudal peduncle.Plasma was obtained from the extracted blood by centrifuging at 10,000 rpm for 5 min at 4°C.Head kidney were dissected out and immersed in Petri dishes with cold supplemented fish physiological saline (FPS).For the antioxidant experiments, fish were sacrificed from each of the four tanks at day 16 of the experimental period.The livers were removed and immersed in PBS to be used in lipid peroxidation assay.

Lysozyme assay
Dilutions of hen egg white lysozyme (HEWL) was used as standard and prepared in phosphate citrate buffer (pH 5.8).Micrococcus lysodeikticus solution (75% w/v) buffered to pH 5.6 in phosphate citrate was mixed to HEWL standard dilution or tilapia plasma in a microtiter plate 7:1 ratio.Absorbance at 450 nm was read after 15 min using the ELISA plate reader.Using the standard curve of Vmax rates, plasma lysozyme concentrations were determined.Protein concentrations of the plasma samples were also determined.

Macrophage phagocytic activity assay
The head kidney samples were homogenized using a screen mesh and suspended in 3 ml supplemented Leibovitz-15 medium (L-15).The cells were centrifuged at 400 rpm for 5 min in room temperature.The pelleted cells were washed twice with supplement L-15.After washing, the cells were suspended in FPS and the cell count and viability was determined by staining the cells with trypan blue (1:9), utilizing the trypan blue exclusion method.The cell concentration was adjusted to 10 6 viable cells ml -1 .Opsonized yeast cells were used as feeds to phagocytes, approximately with 10 8 yeasts ml -1 in PBS, prepared and mixed with one (1.0)ml of 0.8% Congo red dye.The yeast suspension was then autoclaved and washed with an equal volume of PBS until the excess Congo red dye was removed.
The suspension was centrifuged at 400 rpm for 5 min.The cells obtained were suspended again in PBS.The prepared yeast cells were added to the cell suspension from each sample at 2:1 ratio.An aliquot sample of 20 l was smeared onto a glass slide after one hr of incubation at room temperature.The smears were air dried and fixed with 95% ethanol.After 24 h, the films were stained with 1% eosin for one min, rinsed with distilled water, and dipped in Giemsa stain for two minutes.These were air dried for 24 h.Cover slips were mounted onto the glass slides using Entellan.The smears were observed under the oil immersion objective.Percentage of active phagocytes was recorded from a hundred phagocytes that were counted in representative areas of the slide.Data were expressed as mean percentages of active phagocytes.

Reactive oxygen species (ROS) production assay
The assay for the assessment of superoxide anion produced outside the mitochondria was done according to the protocol of Zelikoff (1996).Head kidney macrophages were obtained from the previous preparation and adjusted to a cell concentration of 4 x 10 6 cells ml -1 .Four microcentrifuge tubes were labeled (1 to 4), and cells (125 l of 10 6 cells.ml -1 ) were added to each of these four tubes containing 250 l ferricytochrome solution (final concentration = 2 mg ml -1 ) and 62.5 l of bovine superoxide dismutase was added to the second and fourth tubes.
Ten microliters of phorbol 12-myristate 13-acetate was added to the third and fourth tubes at a final concentration of 2 g ml -1 .Fish physiological saline solution (FPS) was added to each tube to bring up the total volume to 0.5 ml.An additional tube containing all the reagents but without the cells served as the blank.Each tube was vortexed for approximately 30 s and 200 l aliquot placed into the wells of a 96-well microtiter plate.The absorbance was measured at 492nm for up to an hour.Time points used for measurement include: 0, 15, 30, 45, and 60 min.The plates were incubated at room temperature, in a humid environment, between readings.The rate of superoxide anion radical production was determined from measurements taken over time, while OD readings at a single time point were used to make comparisons between different exposure groups.Change in absorbance was calculated by subtracting the mean of the "blank" wells and the wells containing SOD from the absorbance measured in the non-SOD-containing wells.By multiplying the change in absorbance by 15.87 the nmol concentration of SOD-inhibitable superoxide anion radical was computed.Data are expressed as nmol O2/2x105 cells/unit time.

Lipid peroxidation assay
The thiobarbituric acid reactive substance assay (TBARS assay) was used to measure lipid peroxidation in hepatic tissues obtained from tilapia.It measure malondialdehyde (MDA), one of the compounds formed by lipids after oxidative processes.Briefly, using a ground glass homogenizer, one gram of liver tissue was homogenized in 2 ml of PBS.An aliquot of 0.5 mL of the homogenate was transferred in clean test tubes and 2.5 ml of trichloroacetic acid (TCA) and 1 ml of thiobarbituric acid (TBA) was added to all test tubes and then mixed through vortexing.
The samples were then subjected to a hot water bath for 30 min.The samples were allowed to cool and then 4 ml of butanol was added to each of the test tubes.The samples were vortexed and the organic layer was removed and placed in centrifuge tubes.The organic layer was centrifuged at 3000 rpm for 10 min at room temperature.The absorbance was read at 532 nm using tetramethoxypropane was used as the standard.Data were expressed in μmol MDA.mg protein 1 .

Protein content determination
Protein content was determined with the use of the BIO-Rad protein assay kit.The dye was prepared by diluting one part of the concentrated dye with four parts of deionized water.The filtrate was then collected.Dilutions of BSA were used as standard.10 μl of each standard and sample solution were added to 96 well plates.200 μl of the diluted dye was then added to all wells.The plate was incubated for 5 min at room temperature and absorbance was read at 595 nm with the use of an ELISA plate reader.Data were expressed as mg ml -1 .

Data analysis
All of the data were analysed for normality of distribution using Shapiro-Wilk test and homogeneity of variance prior to one way ANOVA test at P<0.05.Kruskal-Wallis test was used as nonparametric test for data with non-normal distribution.Comparison of the data was employed by using Least Significance Difference test for homogenous data sets and Games-Howell test for nonhomogeneous data.MDA concentration values were analysed using Mann-Whitney U test.

RESULTS AND DISCUSSION
The efficiency of an immunostimulant is evaluated by testing its ability to protect the fish against pathogens, and also by measuring the immune response produced (Galindo-Villegas and Hosokawa, 2004).In the present study, the protective effects of dietary corn silk supplementation were determined in Nile tilapia.In order to determine the immune enhancing property of corn silk, fish were fed for 30 days with corn-silk coated feed pellets and then inoculated with A. hydrophila.Subsequently, phagocytic activity, plasma lysozyme levels and production of reactive oxygen species were measured as indicators of nonspecific immune response.Experiments were also undertaken in order wherein hepatic injury was induced through paracetamol-corn silk treatment to determine if corn silk could ameliorate oxidative stress.
Corn silk is composed of stigmas and styles of the Zea mays (maize) plant, and it has been used in traditional Chinese medicine for the treatment of various ailments (Ren et al., 2009).It has been reported that corn silk is an excellent source of many bioactive compounds such as flavonoids, saponin, alkaloids, tannins, phytosterols, allantoin, vitamin E and K, etc. (Hu and Deng, 2011;Ren et al., 2013).It is possibly due to these bioactive components that research studies in corn silk were undertaken to identify its relevance to human health, which includes its reported immune enhancing effects.Similarly, fish health researchers have continually searched for bioactive compounds that could be used to enhance fish immunity and protection against various pathogens.Best known immune stimulants are glucans and lipopolysaccharides, and synthetic compounds, animal and plant extracts and vitamins that usually target the nonspecific immunity of cultured fish species (Ardo et al., 2008).
Based on the results, higher lysozyme level was exhibited by the group of fish which were fed with corn silk coated feed pellets and infected with A. hydrophila (Figure 1) but the increase was not significantly different from that of the positive control group.It is likely that the concentration of the powdered corn silk added may not have been potent enough to raise the lysozyme levels in tilapia significantly.Alternately, the duration of the feeding period prior to the bacterial challenge could have been longer in order to boost the immune system.
However, it is likely that this increase in lysozyme level could also aid in the destruction of A. hydrophila.It is also worthy to note that bioactive compounds from corn silk could also inhibit bacterial infection.In a study by Nessa et al. (2012), antimicrobial activities of corn silk extracts and bioactive compounds were compared with that of gentamycin, and they found out that the extracts and flavonoids were significantly more sensitive against a number of bacteria.
Phagocytosis is the most primitive immune response mechanism, and basically involves the ingestion of a pathogen by macrophages or neutrophils.Phagocytic activity has been significantly enhanced by the corn silk treatment in this study.The corn silk treated group has statistically significant phagocytic activity than the positive and negative control groups (Figure 2).These results likewise indicate a potential immunostimulatory effect of corn silk in A. hydrophila infected tilapia.This response has also been exhibited by some fish species when treated with bioactive extracts.Yin et al. (2009) reported increase in phagocytosis in carp when treated with Ganoderma lucidum and Astragalus radix.They attributed these mainly to the polysaccharides, monosaccharides, flavonoid and alkaloid contents of both herbs.
Production of ROS is considered as an important  microbial killing mechanism (intracellular and extracellular) in vertebrates.Animals have inherent enzymes (antioxidants) to detoxify these anions to counteract the possible adverse effects in normal cells and tissues.Antioxidants include superoxide dismutase, catalase, glutathione S-transferase, glutathione peroxidase, vitamin E components such as -tocopherol and -tocopherol.One study suggested that -terpineol, citronellol, and eugenol, are some of the main compounds involved in the antioxidant action of corn silk (El-Ghorab et al., 2007).More recent studies relate flavonone glycosides as potent antioxidants (Hu and Deng, 2011;Ren et al., 2013).
This study confirmed the ROS scavenging capacity of corn silk especially in unstimulated head kidney cells (Figure 3) where ROS production was greatly inhibited.This effect was also shown by lower ROS levels obtained in PMA-stimulated cells from corn silk-treated fish.The scavenging activity and antioxidative capacity (anti-lipid peroxidation) have been recently confirmed also by Hu and Deng (2012) and Ren et al. (2013).It should be taken note of that ROS are actually produced by macrophages as microbial killing mechanisms.Kim et al. (2005) reported that corn silk activates murine macrophages to produce cytokines and enzymes that are important in regulating normal physiological functions inflammatory response in macrophages.The data from this study likewise showed the immunostimulatory role of corn silk in macrophages from tilapia head kidney.As for oxidative stress results, the negative control fish had significantly lower MDA levels than the positive control fish as shown in Figure 4.The paracetamol-corn silk treated group has a lower concentration of MDA compared to the paracetamol-silymarin treated group, but this was not statistically different.The results showed that corn silk had comparable antioxidant effects as silymarin, and these treatments were able to alleviate, although not significantly, the oxidative effects of paracetamol on the hepatic cells.Paracetamol hepatotoxicity is highly dependent on the cytochrome P450 enzyme system.As paracetamol is metabolised at the cytochrome P450 system, it is converted to the toxic metabolite N-acetyl-Pbenzenequinoneimmine (NAPQI).NAPQI is a toxic free radical that binds to proteins and fatty acids and reacts with glutathione, a natural antioxidant, eventually depleting it.Most importantly, paracetamol leads to the formation of reactive oxygen and nitrogen species that induce oxidation of cellular membranes.

CS
In the present study, silymarin was used as reference substance since it is known to have four different flavonoids that make it a good antioxidant.Its main flavonoid component is silbin with isosilibinin, silydianin, and silychristin.It has been proven to protect against carbon tetrachloride toxicity, acetaminophen (paracetamol), phalloidin, galactosamine, and thioactemide (Pradhan and Girish, 2006;Pradeep et al., 2007).Similarly, corn silk contains many isolates of flavonoids like myricetin, fisetin, quercetin, naringin and luteolin which have been shown to possess antioxidant and prooxidant properties of varying degrees.
However, some studies showed different flavonoid synergists as most effective in hepatoprotection.Moreover, it has been reported that corn silk effectively increases antioxidant enzyme levels such as sodium dismutase and glutathione peroxidase (Hu and Deng, 2011;Nurhanan et al., 2012).In a study by Liu et al. (2011), two flavones glycosides were isolated from the nbutanol extracts of corn silk that exhibited very high antioxidant and free-radical scavenging activities.The present study confirmed the potential of corn silk as a potent antioxidant in a cultured fish species.This could be relevant in feed formulation where high concentrations of lipids or fatty acids are used, and thus necessitates the addition of antioxidants.
However, the appropriate dietary dosage in fish still need to be further investigated.Likewise, further immune-based trials are needed to maximize fully the protective property of corn silk against fish pathogens.

Figure 1 .
Figure 1.Summary of the stomach contents of M. cephalus from the Elechi Creek.

Table 3 .
Variation in empty stomach by size of M. cephalus from Elechi Creek.swamp ensure a constantly enriched nutrient pool for algae often seen growing extensively on the exposed mud surfaces.

Figure 2 .
Figure 2. Percentage phagocytic activity of head kidney phagocytes.Data are presented as mean ± SEM. *-CS, corn silk treated group significantly different (P < 0.05) from the positive control.

Figure 4 .
Figure 4. MDA concentrations in Nile tilapia at day 16 of paracetamol treatment.Data are presented as mean values ± SEM. * (-) group significantly different from the (+) and CS groups (P < 0.05).(-), negative control fish, fed with unsupplemented diet; (+), positive control fish, diet with paracetamol; para-sily, paracetamol and silymarin-treated group, diet with paracetamol and silymarin; para-cs, paracetamol and corn silk-treated group, diet with paracetamol and corn silk.