Physiological studies for growth improvement of juvenile red-spotted grouper (Epinephelus akaara)

Abstract

The red-spotted grouper (Epinephelus akaara) is a subtropical species distributed in southern China, Korea, Taiwan, and southern Japan. Currently, this species is being developed as an export item in Korea. The rise of water temperature during the summer season is anticipated to affect the physiological functions of red-spotted grouper. Besides, E. akaara is a protogynous hermaphroditic fish that function as a female at a young age and undergo sex changes to become male later in life. The reproductive characteristics has slow down their growth performance and limited the availability of mature male broodstock. Hence, the present situation requires to assess the possible impacts of increased water temperature as well as to find out the effective way to accelerate the growth performance of red-spotted grouper. In the present research, the potential effects of increased water temperature on the morphology, physiology, heat shock proteins (HSPs), growth performance, and growth-related gene expression in juvenile red-spotted grouper were analyzed. In experiment 1, 200-day-old juveniles (TL: 9.4 ± 0.12 cm; BL: 12.89 ± 0.61 g) were exposed to 24 °C (control), 28 °C, 32 °C, 36 °C for 28 days; in experiment 2, 130-day-old juveniles (TL: 8.28 ± 0.10 cm, BW: 8.53 ± 0.27 g) were exposed to 25 °C (control), 28 °C, 31 °C, 34 °C for 42 days. In experiment 1, histological results showed epithelial necrosis and shortening of secondary gill lamellae in gill; cytoplasmic vacuolization, shrinkage, and coalescence of hepatocytes in liver of the 36 °C group. Biochemical observations revealed a significant elevation of glucose, glutamic pyruvic transaminase (GPT), glutamic oxaloacetic transaminase (GOT), and cortisol at 36 °C compared with the other groups (24 °C, 28 °C, and 32 °C). Highest heat shock proteins (Hsp60, 70, 90) mRNA expression was also observed in liver, gill, and muscle at 36 °C (P < 0.05). In experiment 2, hematological results showed high red blood cell (RBC) and white blood cell (WBC) count, and erythrocytic abnormalities in shape and size (ECA) at 31 °C and 34 °C water temperature. Histological results presented hyperplasia and curling in secondary gill lamellae, and swelling of primary gill lamellae in gill; the coalescence of hepatocytes, cytoplasmic vacuolization, and dilation of the sinusoid in liver of the 34 °C group. Biochemical results showed high levels of glucose, cortisol, GPT, GOT, and lactate dehydrogenase (LDH); low levels of triglyceride (TG) and total cholesterol (TCHO) in the 34 °C group compared with the others (25 °C, 28 °C, and 31 °C) (P < 0.05). Heat shock proteins (Hsp60, 70, 90 mRNA) were also highly expressed in liver, gill, and muscle at 34 °C. In addition, growth performance data showed high specific growth rate (SGR) and feed efficiency (FER) at 28 °C and 31 °C water temperature. IGF-1 and GHR mRNA expression were also highest in liver at 28 °C and 31 °C compared to 34 °C (P < 0.05). In the second part, the potential effects of 17α-methyltestosterone (MT) treatment on growth performance and sex change process in red-spotted grouper were examined. 90-day-old E. akaara juveniles (TL: 5.98 ± 0.14 cm, BW: 3.67 ± 0.24 g) were fed MT incorporated diets at a concentration of 10 and 40 mg/Kg diet for 9 week. Growth performance data showed low weight gain (WG) and down-regulation of IGF-1 and GHR mRNA levels in MT-feeding groups (10mg and 40mg MT/Kg diet) compared to control during the hormonal treatment. After completion of 9 week MT treatment, high specific growth rate (SGR) and up-regulation of IGF-1 and GHR were observed in 40mg MT/Kg diet group compared with the control and 10mg MT/Kg diet group (P < 0.05). Gonadal histology showed efferent duct-like structure and transformation of ovaries-to-testis in the MT-fed group (40mg MT/Kg diet) at the end of the experiment (36 week). MT-fed groups showed lower plasma estradiol (E2) levels compared to control during the MT treatment period and higher plasma testosterone (T) levels after completion of MT-feeding (P < 0.05). Collectively, the present research suggests that 36 °C and 34 °C is the lethal and sub-lethal temperature, respectively for E. akaara. Prolonged exposure to more than 30 °C can induce physiological stress in red-spotted grouper. Based on the physiological responses, the present study also suggests that glucose, cortisol, GOT, GPT, RBC, and WBC count can be considered as thermal stress biomarkers in red-spotted grouper. The MT treatment results suggest that MT-feeding at 40 mg/Kg diet can promote growth performance and have a high potentiality of inducing sex change from female-to-male in red-spotted grouper compared with the control and 10 mg/Kg diet.