Hisashi Chuda

Reproductive parameters of the chub mackerel Scomber japonicus estimated from human chorionic gonadotropin‐induced final oocyte maturation and ovulation in captivity

Final oocyte maturation and ovulation of captive chub mackerel Scomber japonicus with fully yolk-accumulated occytes were induced by a single injection of human chorionic gonadotropin. Reproductive parameters, including spawning frequency and batch fecundity, which are required to estimate spawning biomass in pelagic fish by the daily egg production method, were analyzed. Germinal vesicle migration (GVM) occurred at 18–24 h post-injection, and the hydration and ovulation of oocytes were completed at 30 and 36 h post-injection, respectively. The results of the maturation process suggest that fish with GVM-stage ovaries captured in the daytime from the field are capable of spawning on the night following their capture. The oocytes used in the oocyte sizefrequency distribution method for batch fecundity estimates should be at late GVM and more advanced stages. The results of sequential artificial insemination showed that the quality of ovulated eggs held in the ovarian lumen rapidly deteriorated as time progressed after ovulation. This indicates that the fertilization window for the ovulated eggs of chub mackerel lasts only a few hours, and spawning behavior should be performed within a few hours after ovulation in the wild population.

Improvement of the survival in the seven-band grouper Epinephelus septemfasciatus larvae by optimizing aeration and water inlet in the mass-scale rearing tank

The water flow in larval rearing tanks has been indicated to cause mass mortality of the seven-band grouper Epinephelus septemfasciatus larvae. Therefore, a new aerating method was tested in an actual scale intensive rearing tank (8.0 m in diameter, 1.87 m of water depth, 100 m3 of volume), in which an aerator was positioned at the center of the rearing tank surrounding cylindrical drain (1.2 m in diameter) to generate the flow field, and seven larval rearing trials were performed. The survival rate with the former aeration methods were compared, in which several aerators were located in the rearing tank. The survival rate at 10 days after hatching with the new aeration method (61.5±5.1%, n=7) was approximately three times higher than the former methods (21.2±13.7%, n=6). The flow environment of rearing tanks was also examined by quantifying the flow field, and the relationship between the flow field in the rearing tank, behavior of larvae and survival discussed. It was confirmed that the vertical circulating flow was observed in rearing tanks, and determined effectively the survival and the behavior of grouper larvae in patchiness.

Maturation-inducing Hormone of the Tiger Puffer, Takifugu rubripes (Tetraodontidae, Teleostei): Biosynthesis of Steroids by the Ovaries and the Relative Effectiveness of Steroid Metabolites for Germinal Vesicle Breakdown In Vitro

Abstract Yolk-free follicle cells during final meiotic maturation were incubated with a radioactively labeled steroid precursor, and the maturation-inducing activity of the steroid metabolites produced was tested in an in vitro assay based on GVBD. Of the metabolites obtained by TLC, whose purity and final characterization were confirmed by recrystallization to constant specific activity, two steroids, 17,20β-P and 20β-S, exhibited maturation-inducing activity in vitro. However, 20β-S was overwhelmingly more effective and faster at inducing GVBD in vitro than 17,20β-P. During final oocyte maturation in the tiger puffer, a shift in steroidogenic enzymes, a decrease in C17,20-lyase activity, and an increase in 20β-HSD activity, were found, as previously reported in salmonids and medaka. In addition to this shift in steroidogenic enzyme activity, however, high and increasing 21-hydroxylase activity throughout all the oocyte developmental stages was a distinctive feature in tiger puffer follicles. This 21-hydroxylase activity likely enables ovarian follicles to accumulate enough 11-deoxycortisol during vitellogenesis for 20β-S production on a massive scale during GVBD. Thus, this study provides not only evidence on the physiological role of 20β-S as a MIH in the tiger puffer, but also fact on enzymatic kinetics in 20β-S biosynthesis.

Characterization of ovarian membrane receptor for 17, 20β-dihydroxy-4-pregnen-3-one, a maturation-inducing hormone in yellowtail (Seriola quinqueradiata)

In our previous studies, we tentatively identified 17,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) as a maturation-inducing hormone (MIH) in yellowtail (Seriola quinqueradiata) through in vivo and in vitro experiments. In this study, we investigated the binding sites for radioactive 17,20 beta-P and characterized the receptor binding to the ovarian plasma membrane in yellowtail undergoing first stage of maturation (FSM). Equilibrium binding sites for 17,20 beta-P have been detected within 1h incubation and the binding dissociated completely within 50 min at 4 degrees C and was pH dependent (optimum pH 7.8). Scatchard analyses of specifically bound 17,20 beta-P showed the evidence of a single class of high affinity binding sites (K(D)=22.9 nM), with limited capacity (B(max)=2.1 pmol/g tissue) to the ovarian membrane of yellowtail undergoing FSM. Competition results revealed that ovarian membrane receptor was highly specific for 17,20 beta-P. There was no other steroid competed strongly with the binding sites of [3H]17,20 beta-P, except 17,20 beta-P itself. On the other hand, 17,20 beta-P did not bind to the membrane prepared from maturationally incompetent (MI) and ovulation (OV) stages of oocytes. As the time proceeded after the stimulation of HCG, binding activity increased significantly (0.389+/-0.036 pmol/g tissue) in the ovarian membrane of maturationally competent (MC) oocytes by 12h postinjection. The binding activity was further significant (0.868+/-0.032 pmol/g tissue) at FSM by 24h postinjection and reached its peak (0.920+/-0.115 pmol/g tissue) temporarily at second stage of maturation (SSM) by 36 h postinjection and then sharply declined to the prestimulation levels during OV stage by 48 h postinjection. In addition to our previous findings, the present results indicate that 17,20 beta-P is the MIH in yellowtail.