Introduction

Tilapia (genus Oreochromis) is one of the most important profitable fishes in the world (Pradeep et al. 2014). The commercial production of tilapia has increasing gained spreading out in many countries all over the world because of the diversity of its capacity for farming conditions, resistance to diseases, and high survival and growth rates (Onumah et al. 2010). Red tilapia is commonly known as a hybrid species, which has derived from incessant selective breeding process consequent from the cross-breeding among male Nile tilapia (Oreochromis niloticus) and female Mozambique tilapia (Oreochromis mossambicus) (Pongthana and Ponzoni, 2010). Red tilapia are favored for aquaculture attributable to their fast growing rate, salinity patience, and attractive red coloration reminiscent of marine fish, for instance, the red snappers (Haque et al. 2016; Pongthana et al. 2010).

Photoperiod is a chief physical aspect which affects the growth, gonadal development, reproduction, maturation, and skin color of fish. Light/dark cycles deliver internal synchronization for releasing hormones and the rhythmicity elements such as melatonin which impacts the periodic physiological purpose in fish (Gines et al. 2004; Bairwa et al. 2013; Shahjahan et al. 2013).

The photoperiod (long-day or continues illumination) regimes on the growth performance enhancement was reported in several studied fish species as for juvenile Nile tilapia, Oreochromis niloticus (Rad et al. 2006, Elsbaay 2013,Wang et al. 2023), red eyed orange molly, Poecilia sphenops (Zutshi and Singh 2020), Malaysian red tilapia (Malambugi et al. 2020).

Adding to the growth, photoperiod manipulation is found to affect gonadal growth. The long day light regimes induced the gonads’ growth as gonadosomatic indices elevation as for red eyed orange molly (Zutshi and Singh 2020), pejerrey, Odontesthes bonariensis (Miranda et al. 2009), topmouth gudgeon, Pseudorasbora parva (Zhu et al. 2014) and prompted gonadal growth in white spotted rabbit fish (Siganus sutor) (Shirinabadi et al. 2013a, b). On the other hand, the application of extended photoperiod amplified fecundity in tilapia (El-Sayed and Kawanna 2004) and encouraged ovarian maturity in topmouth gudgeon (Zhu et al. 2014). Opposing, the continuous illumination induced delayed gonadal growth for 1 month in Atlantic salmon, Salmo salar (Andersson et al. 2013).

Puberty is a physiological procedure that initiated after the sex distinction and defined by the onset of the germ cell maturity and functionally completed; it finishes in the first spermiation or ovulation (Taranger et al. 2010). Different light regimes were established to control puberty timing, as in Arctic charr, Salvelinus alpinus (Liu and Duston 2018), brook trout Salvelinus fontinalis (Lundova et al. 2021).

The sex steroids were documented to be affected by photoperiodic regimes, as daily rhythms in plasma sex steroids (Biswas et al. 2005; De Alba et al. 2019; Choi et al. 2023). However, photoperiod manipulation had no significant effect on sex steroids (Sarameh et al. 2012).

The body coloration is the furthermost remarkable economic traits in fish; red tilapia with black blotches may decline its commercial value (Li et al. 2019). The effects of photoperiodic regimes on the skin color variation were studied in Malaysian red tilapia in which continuous light fish showed pale pink color, while continuous darkness induced reddish pink color (Malambugi et al. 2020). Similarly, rearing of red tilapia under continuous darkness enhanced the reddish pink coloration, which is greatly favored by consumers (Haque et al. 2016). Opposing to this assumption continuous light regime (24L:0D) showed the best skin luminosity with insignificant effect on skin color values in Nile tilapia, O. niloticus (Ali and El-Feky 2013). The application of blue light color (211.25 IU/100 g fish) for 6 weeks enhanced the body coloration of Florida red tilapia by decreasing the black spots; also, blue light prompted β-carotene content with 9-folds comparing to the white light (Aly et al. 2017).

In order to decrease the percentage of skin black spots and improve the pink coloration preferred by market demands, the current study was set up to inspect the significance of long-day and continuous 24 h light regimes on pigmentation variances and skin coloration of red tilapia and, moreover, to investigate the growth performance, puberty timing, and reproductive characteristics of red tilapia under increasing hours of light exposure. To achieve this task, light regimes were applied on tilapia larvae for 90 days. By the end of exposure time, growth and β-carotene whole body content were evaluated and the reproductive characteristics by defining GSI%, maturity progress, fecundity, histology of gonads, and the testosterone and estradiol hormone levels in addition to sex ratio detection.

Materials and methods

Broodstock spawning

A number of 30 broodstock Florida red tilapia were brought from El-Kilo-21 hatchery located west Alexandria and placed at the marine hatchery of the National Institute of Oceanography and Fisheries. Brooders of total weight 70–220 g were retained in fiber glass tanks (1000 L) with flow-through sea water for 5 days, followed by salinity dropped 20 ppt. The temperature was 26 ± 3 °C, fish subjected to stimulated natural photoperiod and fed daily 35% crude protein formulated diet, brooders’ sex ratio was two females to one male. Fertilized eggs were hatched after 3 days from spawning and larvae had an average weight of 0.0054 g.

Experimental design

The experimental work was achieved by total number of 324 red tilapia larvae distributed in 9 glass tanks with capacity 12 L. Each tank was provided with 36 larvae of an initial body weight of 0.04 ± 0.012 g and 1.13 ± 0.12 cm total length. Three photoperiod treatment groups in triplicates were accomplished as follows: T1 = the first group was control (12L:12D), T2 = the second group was long day (18L:6D), and T3 = the third group was continuous light (24L:0D). The average values of water quality factors through the experimental time were dissolved oxygen (DO) = 9.2 (mgL−1), TDS = 35.3 (gl−1), EC 57.4 ± (ms lm−1), and salinity = (37 ± 0.2 ppt) and the water temperature was 27 °C ± 1. The experiment period was 90 days, and by the end of experimental time, samples of 10 fish each tank were taken and stored in − 20 °C for total β-carotene analysis. Blood samples (serum) were taken to measure sex steroid hormones (T and E2). Sampled fish were taken and immersed in excess aesthetic MS222 for sacrificing, then weighted, length was reported. Then, fish were dissected and sex defined macroscopically as gonads were clearly noticed, GSI detected, and the stage of maturity estimated. Also, gonad samples were reserved in 4% formal saline for histology.

Light intensity was identified at the superficial water of tanks by using a digital Lux Meter (Hanna Portable Lux Meter) and was constant at around 1000 lx throughout the experiment in all groups. To avoid light interference, the tanks were sheltered by using black polythene sheets.

Steroid hormone measurement

Samples of serum from each treatment were taken to measure testosterone (T) (total) for male fish and estradiol (E2) for female fish. The amount of serum T and E2 were evaluated by using (Cobas-C &Cobas-E (Roche) 26-instrument). Serum T was measured by means of a test kit (CAN-TE-250). Estradiol was identified using the ELISA kit (CAN-E − 430, Diagnostics Biochem Canada Inc., Ontario, Canada) according to Check et al. (1995).

Skin color and β-carotene measurements

Fish samples were homogenized in 1 mL of saline solution to quantify the total content of β-carotene. Total β-carotene was detected by using Reversed-Phase Liquid Chromatography (LC) technique in line with (Schierle et al. 2004). Fifteen fish from each tank were arbitrarily sampled and photographed with a high-resolution camera to analyze variances in fish body color. The proportion of red tilapia fish with black blotch was detected as a percentage from the total number of fish by using ImageJ software. The mean color intensity of the Red-Green-Black (RGB) was calculated on the original image. The value of RGB varied from 0 (black) to 255 (white), resembling to lower pigmentation as the number rises and RGB value was done by using ImageJ software (Mezei et al. 2011).

Growth performance

Growth factors included the following.

Weight gain (WG) was assessed by equation WG (mg) = Wf − Wi, where Wf is the final body weight (mg) and Wi is the initial body weight.

Specific growth rate SGRw %/day1 = 100 × (lnWf − lnWi)/t, where ln is the natural logarithm and t is the time by number of days (Mourad et al. 2022).

Gonadosomatic index (GSI) = gonad weight/gutted weight*100.

Hepatosomatic index (HIS) = liver weight/gutted weight*100.

Fecundity

Ovaries of sampled fish by the end of the experiment were kept in 4% buffered formalin, and the subsamples of ripe ovaries were weighed and mature ova were counted for three times each sample. And the relative fecundity/g = number of whole mature eggs/g total weight of the fish (Ismail et al. 2023).

Histological procedure

The dissection of gonads took place from every fish to small portions and reserved in 4% formal saline. Gonadal samples then were cleared in 70% ethyl alcohol, dehydrated by ethyl alcohol series, and inserted in paraffin wax as described by Assem et al. (2019); sections were performed at 6–9 μm. The staining of sections was done by hematoxylin and eosin; sections were examined and microscopic images were pictured with a Leica digital system.

Statistical analysis

Data are presented as mean ± standard error. Significant alterations between the tested groups and the control were verified through one-way analysis of variance (ANOVA) at p < 0.05; subsequently, the Duncan test was employed for multiple comparisons. The analysis was done by means of the SPSS® version 22.0 package (SPSS 1998).

Results

Growth performance

The present data of growth performance in male red tilapia (final body weight (FBW), WG, SGR, final total length) and survival rate (%) (Table 1) demonstrated insignificant alterations for the three different groups (T1, T2, and T3 groups (p < 0.05), as presented in Table 1).

Table 1 Growth performance and survival rate (%) of male red tilapia for 90 days in different photoperiod groups n = 24 (mean ± SE)
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Growth performance in female red tilapia demonstrated that (Table 2) T3 group significantly increased (p < 0.05) to reach 14.52 ± 0.79, 14.48 ± 0.79, 6.51 ± 0.06, and 9.17 ± 0.21 (final body weight (FBW), WG, SGR, and final total length, respectively). Nevertheless, insignificant alterations were reported among the T1 and T2 groups in FBW, WG, SGR, and TL (Table 2); also, insignificant variations in survival rate were found between the three different light regime groups (T1, T2, and T3 groups) (p < 0.05), as presented in Table 2.

Table 2 Growth performance and survival rate (%) of female red tilapia for 90 days in different photoperiod groups (mean ± SE), n = 24; means with unlike letters are significantly diverse (p < 0.05)
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Gonadosomatic index (GSI%)

Male GSI% of T3 group significantly rose (p < 0.05) to reach 0.59 ± 0.47 (Fig. 1A), while insignificant variation (p < 0.05) among the male GSI% of the T1 and T2 groups (0.33 ± 0.06 and 0.35 ± 0.04, respectively) was detected. The GSI% of female fish of T2 group significantly augmented to peak as 4.28 ± 0.43 (p < 0.05). There was an insignificant variation (p < 0.05) among the female GSI% of the T1 and T3 groups (2.28 ± 0.48 and 2.80 ± 0.34 correspondingly) though (Fig. 1B).

Fig. 1
figure 1

Gonadosomatic indices (GSI %) of male (A) and female (B), hepatosomatic indices (HIS %) (C), and relative fecundity (D) of red tilapia after 90 days in different photoperiod groups (mean ± SE), n = 24 for A, B, and C and n = 10 for D. Values accompanied by unlike letters are significantly diverse (p < 0.05)

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Hepatosomatic index (HSI%)

Male fish displayed insignificant variances in HSI% among the three groups: T1 (control), T2, and T3 (3.38 ± 0.22, 3.25 ± 0.24, and 3.08 ± 0.23), respectively (p < 0.05).

For female HSI%, the T1 fish group displayed significant promotion (p < 0.05) to be 3.78 ± 0.22 compared to T2 (3.08 ± 0.16) and T3 (3.10 ± 0.11) groups, while no significant variation (p < 0.05) was identified between T2 and T3 groups (Fig. 1C).

Relative fecundity

The relative fecundity of red tilapia of T2 light regime group significantly increased to reach 19.51 ± 1.88 per gram for total weight which ranged from 10.25 to 16.95 g (p < 0.05). On the other hand, an insignificant difference in relative fecundity (p > 0.05) was detected between photoperiod T1 and T3 groups (14.16 ± 0.71 and 13.77 ± 1.24 per gram for total weight which ranged from 9.12 to 38.1 g (T1) and ranged from 9.1 to 22.46 g (T3), respectively (Fig. 1D)).

Total β-carotene content

The total β-carotene content of red tilapia of T3 photoperiod regime group significantly elevated to range 2.44 ± 0.87 (p < 0.05). There was an insignificant variation (p > 0.05) among T1 and T2 photoperiod regime groups though (0.28 ± 0.04 and 0.17 ± 0.09), respectively (Fig. 2A).

Fig. 2
figure 2

Total β-carotene (µg/g) (A), total testosterone (B), estradiol (E2) (C), and sex ratio (%) (D) of red tilapia after 90 days for different photoperiod groups (mean ± SE), for A, B, and C n = 9. Values supplemented by unlike letters are significantly varied (p < 0.05)

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Steroid hormone levels

The T levels of male fish T3 group was significantly high as 14.63 ± 0.32 (ng/mL) (p < 0.05), while there was an insignificant variation (p > 0.05) in T levels for males betweenT1 and T2 groups (6.25 ± 1.13, 7.81 ± 0.25) (ng/mL), respectively (Fig. 2B). The E2 of female fish T2 group significantly increased to reach 2667 ± 296 (pg/mL) (p < 0.05). Conversely, there was an insignificant variation (p > 0.05) in E2 levels for females between T1 and T3 groups (1948 ± 196, 1995 ± 0.9) (pg/mL), respectively (Fig. 2C).

Sex ratio

Sex ratio was studied including all survived fish after the 90-day experimental period. T3 group showed significant increase in male sex ratio percentage (p < 0.05) compared to T2 group, whereas insignificant alternation was observed among T1 group and T2 group and for T1 group and T3 group, as presented in Fig. 2D. On the other hand, T2 group showed a significant increase in female sex ratio (p < 0.05) comparative to T3 group, though insignificant differences were detected among the T1 group and T3 group likewise for T1 group and T2 group, as presented in Fig. 2D.

The Red-Green-Black (RGB) and black spots

The average color intensity was measured on the original images. There were insignificant alterations in RGB values between the three groups: T1, T2, and T3 (128 ± 2.07, 130 ± 1.82, and 129 ± 0.33, respectively (p > 0.05)), as shown in Fig. 3A. The black spots of red tilapia of T3 group reported significant decline to reach 5.79 ± 0.59% (p < 0.05). However, there was an insignificant variation in black spots (p > 0.05) between T1 and T2 groups (14.95 ± 1.67 and 15.09 ± 1.31%, respectively, as shown in Fig. 3B).

Fig. 3
figure 3

The Red-Green-Black (RGB) (A) and black spots (B) of red tilapia for 90 days in different photoperiod groups (mean ± SE), n = 45. Values supplemented by unlike letters are significantly varied (p < 0.05)

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Maturity stages

The maturity stages of male fish exhibited that the T2 fish group had a greater proportion of mature stage than the T1 and T3 groups (38, 29, and 23%, correspondingly), whereas the T3 group exhibited the highest proportion of nearly ripe stage (Fig. 4A). Male fish exhibited higher proportion of ripe stage for T1 group rather than T2 and T3 groups (53, 43, and 50%, correspondingly) (Fig. 4A). For female fish, the T1 group defined the highest ratio of mature stage, while the T2 and T3 groups did not display any mature females (Fig. 4B), and the less amount of nearly ripe females was recorded in T2 and T3 groups and the highest ratio of nearly ripe females was recorded in T1 group. T3 and T2 groups reported the higher percentage of ripe stage (70 and 70%, respectively), while the less ratio of ripe stage is recorded in T1 group (44%) as shown in Fig. 4B.

Fig. 4
figure 4

The percentages of maturity of male (A) and female (B) of red tilapia after 90 days for different photoperiod groups (mean ± SE)

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Gonadal histology

The testicular tissue of red tilapia did not show perceptible changes in all photoperiod treatments (Fig. 5A–C). Testicular tissues of all treatments showed well-developed mature testis with clusters of variable spermatogenetic stages (Fig. 5A–C). But the testicular tissues of T3 group displayed large magnitude of spermatozoa relative to T1 and T2 groups (Fig. 5C). The ovarian configuration of red tilapia did not show noticeable changes in all photoperiod treatments (Fig. 5D–F). The ovarian tissue showed that the three light regime groups were more progressive stages and having more developed vitellogenic and ripe oocytes (Fig. 5D–F).

Fig. 5
figure 5

Gonadal tissue of red tilapia. A Testis tissues of T1 group with advanced mature testis with nests of spermatogonia (spg), primary spermatocytes (1ry.spc), secondary spermatocytes (2ry.spc), spermatids (spd), and packed with spermatozoa (Spz). B Testis tissues of T2 group in advanced phase enclose different spermatogenic stages as T1. C Testis tissues of T3 group with greater amount of spermatozoa. D–F Ovarian tissues of T1, T2, and T3 groups, respectively, with ripe stage, cytoplasmic growth (cg), follicular epithelium (FE), primary oocyte (p) and progressive primary oocyte (pm), yolk granules (yg) and ripe ova (*), vacuoles (V). Hematoxylin–eosin stain

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Discussion

In this study, the influence of long-day light regime on the growth of red tilapia was investigated. Male red tilapia growth performance after 90 days demonstrated insignificant variations between tested photoperiod treatments, while a growth-enhancing effect of continuous light regime on female growth performance parameters demonstrated significant increase (p < 0.05), indicating that 24L:0D (continuous illumination) photoperiod induced the somatic growth in female red tilapia. A number of research work have reported contradicting results on the effect of long-day and continuous light regime on somatic growth and survival, as for haddock (Melanogrammus aeglefinus) larvae (Downing and Litvak 2000). Also, when Nile tilapia was reared for 160 days in closed recirculating system and subjected to long-day photoperiods (18L:6D, 24L:0D), no significant variance was detected in the growth rate among the photoperiodic groups (Wang et al. 2023). Contrasting to our results, extended photoperiod (18L:6D) has a benefic influence on growth rate in Nile tilapia males (Carlos et al. 2015).

The responses of fish to photoperiodic management including the growth stimulation have been recorded by many authors, e.g., in Malaysian red tilapia when fish cultured under both continuous light and darkness (Malambugi et al. 2020). Also, Ali and El-Feky (2013) presented that tilapia growth was augmented with increasing light hours. Prolonged photoperiod was more effective for ideal growth performance, survival of Nile tilapia fingerlings rather than the other experienced cycles (Rad et al. 2006; Ali and El-Feky 2013). Moreover, the 24L:0D cycle was suggested for optimal performance of Nile tilapia (Elsbaay 2013) and for Caspian roach (Shahkar et al. 2015). Under long-day photoperiods, Nile tilapia reached their energy for somatic growth and prompt greatest feed proficiency (Veras et al. 2013). It is proposed that long-day and continuous illumination photoperiod cycles induced faster growth rate as a consequence of factors like enhanced appetite and developed food intake, greater feed conversion efficiency, and increase in swimming activity (Veras et al. 2013). It is suggested that the increasing fish swimming activity attributable to experiencing more illumination time possibly enthused the deposition of amino acids, which is required for the creation of muscle’s protein, that lead to improved growth, as the protein deposition is accountable for the mainstream of weight gain (Biswas et al. 2005; Veras et al. 2013).

In the present study, there were insignificant changes in the HSI% for the three photoperiod regimes of male red tilapia, while significant decline in the HSI% was reported for the long day treatments of female fish. In agreement with our results, the application of various photoperiods for tilapia males did not show substantial effect on HIS% (Bizarro et al. 2019). On the other hand, Taranger et al. (2006) stated that both male and female Atlantic cod, Gadus morhua, under constant light express higher HSI% ratio than the natural light.

Photoperiod manipulation induced significant changes in the GSI% of mature tilapia in this study; male GSI% of continuous photoperiod group significantly augmented. Similarly, long-day photoperiod induced the gonadal growth and GSI in male fish, P. sphenops (Zutshi and Singh 2020), while, contrasting to our results, tilapia males under various photoperiodic regimes did not show significant alterations in GSI% (Bizarro et al. 2019). The long-day photoperiod application showed more tendencies in increasing the GSI% than continuous illumination during female gonadal maturation in the present study. Parallel to our results, long-day photoperiod (18L:6D) induced an increase in the GSI of female fish, P. sphenops (Zutshi and Singh 2020), of female pejerrey (Miranda et al. 2009), and also in both male and female topmouth gudgeon (Zhu et al. 2014). Contrasting to our results, prolonged photoperiod (18L:6D) negatively affects growth of freshwater matured female rohu, Labeo rohita, after 30 days of exposure (Shahjahan et al. 2020). Moreover, photoperiodic manipulation did not stimulate the GSI% of Nile tilapia (Veras et al. 2013). Conclusively, the impact of photoperiodic manipulations on GSI% of tilapia is still inconsistent.

In this study, the relative fecundity of mature female red tilapia reared under long-day photoperiod significantly increased. Correspondingly, the female E2 reported the highest levels on long-day photoperiod. The continuous illumination in current results has no influence on either fecundity or E2 peak. Parallel to this finding, number of egg/g did not show any significant difference under continuous light in pikeperch, Sander lucioperca (Sarameh et al. 2012). Also, 18L:6D photoperiod was also found to induce an increment in the E2 levels in S. dumerili females (Nyuji et al. 2018) and for S. japonicus females (Choi et al. 2023). The constant illumination diminished E2 levels of brook trout, Salvelinus fontinalis, and delayed female gonadal development (Lundova et al. 2021). Moreover, our results showed significant elevation for the male serum T for fish group reared under continues light regime (24L:0D) for 90 days. Opposing to the present results, the continuous illumination showed suppressed T levels in both male and female brook trout, Salvelinus fontinalis (Lundova et al. 2021), whereas insignificant effects were described on the T levels in pikeperch (Sander lucioperca) under continuous illumination for 40 days (Sarameh et al. 2012).

The testicular tissue of red tilapia under continuous light photoperiod displayed large amount of spermatozoa comparing to other light regimes, whereas females did not display variations in ovarian tissue among the three groups. Current observations regarding maturity showed increasing ratio of nearly ripe and ripe females in long-day photoperiod, indicating enhancement in sexual maturation, while no changes in male maturity stages between the three photoperiodic treatments were detected. Counterparts of these findings were reported in white spotted rabbit fish with significantly increasing gonadal development under long-day light regimes (Shirinabadi et al. 2013a, b) and also for male and female goldfish which practiced the long-day cycle (19L:5D) (Sarkar and Upadhyay 2011) and male and female P. sphenops under a long-day cycle (18L:6D) (Zutshi and Singh 2020). On the other hand, a long-day photoperiod delayed spawning and inhibited vitellogenesis by decreasing the proportions of vitellogenic oocyte in rohu (Shahjahan et al. 2020). Earlier researches suggested that photoperiod is the crucial factor in fish sexual maturity and reproduction (Bromage et al. 2001; Garcia et al. 2018). According to Amano et al. (2004), the photoperiod has an essential role on gonadal maturation through regulation of gonadotropin releasing hormone (GnRH) production. Altogether, current data associated to performance of reproduction support that the long-day light regime improved the female gonadal maturation during broodstock manipulation.

The coloration of red tilapia is one of the most favored traits; the alteration of the coloration of cultivated fishes can prompt technical and profitable problems and may reduce the marketability of the cultured fish (Haque et al. 2016). There are various factors affecting the coloration and β-carotene content of fish, including nutritional, environmental, light color, light intensity, stressors, genetic, and neuro-hormonal aspects (Fujii 1993; Templonuevo and Cruz 2016; Aly et al. 2017). In the present study, the continuous illumination treatment reported significant elevation in the total β-carotene content in addition to a lesser number of black spots, while the RGB analysis displayed no significant changes between the three fish groups. Parallel to the present results, long-day photoperiod prompted brighter skin color with higher values of carotenoid in red-eyed orange molly (Zutshi and Singh, 2021), while continuous exposure to light induced stress that causes faded body coloration in the freshwater ornamental common molly (Moghan Prasad and Velmurugan 2020). In the current research, the continuous light exposure reduced the amount of black spots with higher content of body β-carotene which conveyed to more pink coloration of red tilapia. In a similar approach, when Florida red tilapia is exposed to blue light, it displayed less black spot percentage with higher content of carotene in its body (Aly et al. 2017). The low RGB value indicating higher pigmentation, Malambugi et al. (2020) reported that for Malaysian red tilapia the fish raised under continuous darkness showed less RGB values indicating higher pigmentation compared to those raised under natural photoperiod consistent with lower pigmentation as increasing the value.

In this study, continuous light treatment showed significantly increased in male ratio value (p < 0.05). Conversely, long-day photoperiod (18L:6D) and control (12L:12D) groups showed more female ratio. Parallel to our findings, the extended photoperiod may affect the production of all male Nile tilapia phenotypically and genotypically (Carlos et al. 2015). Opposing to current results, the ratio of females was greater under the long-day photoperiod, and the short-day photoperiod management resulted in the most male-biased ratio in Leuresthes tenuis (Brown et al. 2014). For Nile tilapia, the optimum photoperiod cycle for female differentiation was reported as 14L:10D for 2–3-month exposure time (Biswas et al. 2005). In hypothesis for how continuous light has masculinization effects on pike silverside Chirostoma estor, Martínez-Chávez et al. (2014) suggested a relation among continuous illumination, oxidative stress, and environmental sex determination. Pike silverside reared for 12 weeks under continuous light sex ratio was biased towards male, in which continuous illumination can perform as a ROS-generating stressor that might trigger antagonistic defense mechanisms, which could finally induce masculinization (Corona-Herrera et al. 2018).

To conclude, this study displays that photoperiod influences the growth performance, reproductive parameter, sexual maturity, and pigmentation of red tilapia. The rearing of red tilapia larvae under continuous illumination induced the somatic growth, while long-day photoperiod (18L:6D) induced reproductive capacity in female fish. Male fish has more sex ratio and displayed enhancement in the reproductive performance under continuous light. Also, continuous illumination boosted the total β-carotene content with less number of black spots without fading in the red-pink pigmentation. These results validated that, in the culture water during early life of red tilapia, long-day and continuous light applications are effective in growth, gonadal development, and coloration of this species, proposing the use of long-day light regime during early larval stages to get better quality female brooders.