Joon Yeong Kwon

A beneficial coculture: charm abalone Haliotis discus hannai and sea cucumber Stichopus japonicus

Juvenile charm abalone Haliotis discus hannai (0.35±0.12 g) and sea cumber Stichopus japonicus (5.0±1.2 g) were cocultured in a static culture system for 90 days during the overwintering period. The levels of ammonium nitrogen and nitrite in the water of coculture groups were lower than those in control group (abalone alone) throughout the experiment. No mass mortality occurred in any groups, but abalone in coculture groups showed significantly better growth performance and higher survival, suggesting that coculture of juvenile abalone with sea cucumber, a deposit-feeding species, can reduce the levels of inorganic nitrogen in the water and consequently enhance growth. This coculture appears to be beneficial and could be developed as an alternative culture method in abalone farms during the heating-aided overwintering period.

Androgen action on hepatic vitellogenin synthesis in the eel, Anguilla japonica is suppressed by an androgen receptor antagonist

Involvement of additional hormones other than estrogen in the control of vitellogenin (Vg) synthesis has been suggested in fish. However, no satisfactory explanation on the mechanism of the action of these hormones has been reported. In this study, we have exploited the possibility of androgen receptor mediation during the androgen action on the pathway of Vg synthesis. Hepatocytes were prepared from sexually immature Japanese eel Anguilla japonica and treated with estradiol-17beta (E2), 17alpha-methyltestosterone (MT), growth hormone, tamoxifen or flutamide, or in combination of these. Spent culture media were analysed by SDS-PAGE for Vg detection. Results from the chemical treatments demonstrated the necessity of E2 as the primary factor for Vg synthesis and requirement of additional hormones for the full expression of Vg. The effects of E2 and MT were effectively blocked by tamoxifen, an estrogen receptor antagonist and flutamide, an androgen receptor antagonist, respectively, indicating ER-mediated estrogen action and AR-mediated androgen action on Vg synthesis in this species.

Effects of feed restriction to enhance the profitable farming of blackhead seabream Acanthopagrus schlegelii schlegelii in sea cages

The feed intake, growth and body composition of juvenile blackhead seabream Acanthopagrus schlegelii schlegelii (Sparidae) (5.6 g fish−1) were investigated for 16 weeks in sea cages under seven repetitive feeding cycles: every day feeding (control), 6-day feeding and 1-day fasting (F6.1), 5-day feeding and 1-day fasting (F5.1), 4-day feeding and 1-day fasting (F4.1), 3-day feeding and 1-day fasting (F3.1), 2-day feeding and 1-day fasting (F2.1), and 1-day feeding and 1-day fasting (F1.1). The survival of the fish during the experimental period was not different among the feeding cycles. The greatest weight gain of fish was observed in the control, but not significantly different from that of the F6.1 and F5.1 groups (p > 0.05). Total feed intake and daily feed intake decreased with the increase of fasting frequency, however, actual feed intake, feed efficiency and protein efficiency ratio increased with the increase of the fasting frequency. Proximate composition of the whole body of fish was not affected by different feeding cycles. These results suggest that juvenile blackhead seabreams subjected to repetitive feeding cycles of 6- or 5-days feeding and 1 day fasting for 16 weeks could achieve compensatory growth, and that such mild feeding deprivation could save significant amounts of feed without causing any profit reduction that might result from a decrease in fish size or quality.

Kisspeptin2 stimulates the HPG axis in immature Nile tilapia (Oreochromis niloticus)

It has been suggested that kisspeptin influences reproduction and onset of puberty in fishes. Unlike mammals, which produce only one kisspeptin (Kiss1), some teleosts have two, Kiss1 and Kiss2, both thought to be involved in the stimulation of gonadotropin (GTH) secretion. In Nile tilapia (Oreochromis niloticus), however, only Kiss2 has been identified so far. The effect of Kiss2 on GTH release varies significantly depending on species and reproductive stage. Furthermore, its physiological function in this species is not clearly defined. In this study, kiss2 gene expression profiles were examined using quantitative real-time PCR (qRT-PCR) in the brain, pituitary, and gonads of Nile tilapia at different reproductive stages (male: immature, pre-spermiation, post-spermiation; female: immature, pre-spawning, post-spawning). The kiss2 mRNA expression profiles of the brain, pituitary, and gonads of both sexes shared a similar pattern their expression was significantly higher at the immature stage than at the mature or post-spawning stages, implying it is involved in early gonadal maturation in this species. To investigate the effect of kisspeptin on the hypothalamus-pituitary-gonad (HPG) axis in vivo, synthetic kisspeptin2 (FNYNPLSLRF) was injected into immature male and female tilapia intraperitoneally (i.p.) at a dose of 200pmol/g body weight. The results showed that synthetic Kiss2 administration increased the expression of GnRH I, fshβ and lhβ mRNA in the brain and increased 17β-estradiol (E2) and 11-ketotestosterone (11-KT) levels in the plasma. These results suggest that Kiss2 stimulates the expression of GnRH and GTH genes in immature Nile tilapia.

Neurokinin B-related Peptide Suppresses the Expression of GnRH I, Kiss2 and tac3 in the Brain of Mature Female Nile tilapia Oreochromis niloticus

Neurokinin B (NKB) and neurokinin B related peptide (NKBRP) belong to tachykinin peptide family. Theyact as a neurotransmitter and/or neuromodulator. Mutation of NKB and/or its cognate receptor, NK3R resulted in hypogonadotropic hypogonadism in mammals, implying a strong involvement of NKB/NK3R system in controlling mammalian reproduction. Teleosts possess NKBRP as well as NKB, but their roles in fish reproduction need to be clarified. In this study, NKB and NKBRP coding gene (tac3) was cloned from Nile tilapia and sequenced. Based on the sequence, Nile tilapia NKB and NKBRP peptide were synthesized and their biological potencies were tested in vitro pituitary culture. The synthetic NKBRP showed direct inhibitory effect on the expression of GTH subunits at the pituitary level. This inhibitory effect was confirmed in vivo by means of intraperitoneal (ip) injection of synthetic NKB and NKBRP to mature female tilapia (20 pmol/g body weight [BW]). Both NKB and NKBRP had no effect on the plasma level of sex steroids, E2 and 11-KT. However, NKBRP caused declines of expression level of GnRH I, Kiss2 and tac3 mRNAs in the brain while NKB seemed to have no distinct effect. These results indicate some inhibitory roles of NKBRP in reproduction of mature female Nile tilapia, although their exact functions are not clear at the moment.

Differential expression of two distinct aromatase genes (cyp19a1a and cyp19a1b) during vitellogenesis and gestation in the viviparous black rockfish Sebastes schlegelii

Abstract Oviparous fishes are known to possess two types of aromatase, which are ovary type (cyp19a1a) and brain type (cyp19a1b). These aromatases play important roles in the reproduction of oviparous fish. However, little information is available on the role of aromatases in the reproduction of viviparous fish. This study has been carried out to examine the changes of aromatase gene expression prior to and during vitellogenesis and during gestation in a marine viviparous fish, black rockfish, Sebastes schlegelii. The coding sequence of cyp19a1b gene cloned from rockfish brain was 1497 nucleotides (nt). Rockfish cyp19a1b was more similar to cyp19a1b from other fish species than to rockfish cyp19a1a in amino acid sequence. In addition, phylogenetic analysis showed that aromatases of this viviparous rockfish were more similar to those of oviparous fishes than those of viviparous mammals. Cyp19a1a mRNA was expressed most strongly in the ovary and weakly in the testis, brain, and spleen. Cyp19a1b mRNA was detected only in the brain at cortical alveolus stage (CA) but, additionally, detected in the ovary during vitellogenesis and gestation. The expression of both aromatase mRNAs was the highest at the vitellogenic stage (V) and the expression of cyp19a1a mRNA far exceeded that of cyp19a1b mRNA in the ovary of all reproductive stages studied here. Results from this study suggests that viviparous rockfish also has two distinct aromatase genes as oviparous fish do, and that the expression of cyp19a1a, rather than cyp19a1b, in the ovary is closely related to the vitellogenic activity in this species.

Early Gonadal Differentiation of the Protogynous Red Spotted Grouper, Epinephelus akaara.

Red spotted grouper, Epinephelus akaara is a popular aquaculture species in many Asian countries. This species is a protogynous hermaphrodite that first differentiates into female and changes to male later. Due to this reproductive characteristic, stable supply of male and female gametes is a key to the success of seed production in this species. Thus, understanding early gonadal differentiation is required to develop effective sex control techniques. Red spotted grouper were reared in indoor tanks and sampled every 5 days from 40 days post-hatch (DPH) to 130 DPH. Changes of gonadal tissues were examined and analyzed by means of histology. A pair of gonadal primordium has already existed underneath the kidney in the posterior part of the body cavity at 38 DPH when this study began. Gonadal primordia of 38, 40 DPH consisted of germ cells surrounded by a few somatic cells. The blood vessel was observed in the gonadal primordium at 45 DPH. The number of somatic cells and size of gonadal primordium increased age-dependently up to 60 DPH. Formation of ovarian cavity was obvious by two protuberant aggregations of somatic cells at 65 DPH. Completed ovarian cavity and oogonia were first observed in the gonad of one fish sample at 105 DPH. Based on these histological observations, it can be suggested that induction of primary male differentiation could be more successfully applied at around 60 DPH in this species.

Involvement of androgens and growth hormone in the synthesis of vitellogenin in Japanese eel (Anguilla japonica)

Immature eels positively responded to estradiol-17β (E2) injection in terms of vitellogenin (Vg) synthesis but not to growth hormone (GH) or 17α-methyltestosterone (MT) injection. However, injection of MT or GH combined with E2 strongly increased Vg synthesis. In in vitro experiments, eel hepatocytes treated with E2, GH, or MT alone did not produce detectable amount of Vg, whereas the combination of E2 with GH or MT, or both greatly increased Vg synthesis in the hepatocytes.

Effect of melatonin on GnIH precursor gene expression in Nile tilapia, Oreochromis niloticus

Abstract Sexual maturation and gonadal development of fish is greatly influenced by photic information, an external environmental factor, and melatonin mediates this information to regulate gonadotropin (GTH) secretion and gonadal activation. The relationship between gonadotropin inhibiting hormone (GnIH) and melatonin in fish, however, has not been studied to date. Here, the GnIH expression pattern and daily change of melatonin levels were compared to each other in mature tilapia (body length 16.1 ± 0.2 cm, body weight 77.7 ± 3.43 g), and the effect of melatonin injection on GnIH gene expression was investigated. GnIH gene expression increased at night when the secretion of melatonin increased, whereas gene expression decreased during the day when melatonin secretion decreased. Injecting tilapia intraperitoneally with melatonin increased GnIH gene expression and decreased the expression of gonadotropin releasing hormone (GnRH) and GTH. Furthermore, the injection decreased the 11-KT concentration in male tilapia. These results indicate that melatonin is likely to suppress the hypothalamus-pituitary-gonad (HPG) axis via the action of GnIH in this species.

Effects of exogenous melatonin on the reproductive activities of Nile tilapia, Oreochromis niloticus

Abstract Gonadal development and sexual maturation of fish are often related to photic conditions. We attempted to utilise the direct application of melatonin to elicit similar effects in the absence of photoperiod manipulation. We found no significant differences in somatic growth between melatonin-treated groups and controls, indicating that melatonin administration did not negatively affect the growth of the experimental fish. Treatment with low-dose melatonin (0.3 mg/kg BW) resulted in a decrease in spawning frequency, number of spawned eggs and gonadosomatic index in female tilapia. In male tilapia, low-dose melatonin was associated with a decrease in sperm count, spermatocrit and spermatozoa activity index, implying that exogenous melatonin might mimic the effects of shortened light photoperiod, which suppresses reproductive activity in this species. These results suggest the possibility that an optimised dose of melatonin treatment could be used to control the spawning behaviour of this species.