We have previously shown an increase on the ovulation rate of pacu, Piaractus mesopotamicus, using hypophysation with prostaglandin F-2 alpha (PGF(2 alpha)) (HPG protocol) rather than hypophysation (H); only, however, a persistent lack of ovulation in some females could not be further explained by PGF(2 alpha) and steroid plasma levels at time of ovulation. Considering this, here we compared the meiotic process and steroids and PGF(2 alpha) levels during spawning induction using both protocols. To that, H and HPG were treated with conventional hypophysation (0.6 mg/kg priming and 5.4 mg/kg resolving dose), and the latter received synthetic PGF(2 alpha) (2 mL/fish) at resolving dose. A negative control (saline solution), a SPG group (receiving only PGF(2 alpha)) and a PPG group (receiving priming and PGF(2 alpha)) were included. An initial PGF(2 alpha) peak between one to four hours post-resolving dose (1-4 hr) occurred in groups receiving PGF(2 alpha) (HPG, SPG and PPG). Only H and HPG showed a 17 alpha-20 beta-dihydroxy-4-pregnen-3-one (DHP) peak 4 hr, concomitant with meiosis resumption, and a final PGF(2 alpha) peak 12 hours. The HPG protocol resulted in higher PGF(2 alpha) plasma levels for longer periods (1-4 and 12 hr) than H (single peak 12 hr), occasioning in the former higher and concomitant levels of both PGF(2 alpha) and DHP at 4 h. Considering that DHP is known to induce expression of prostaglandin receptors in fish, the concomitant plasma peaks of DHP and PGF(2 alpha) may also explain the ovulation rate gain previously published, using HPG protocol, which can be further improved adjusting the priming dose and defining the exact interval of PGF(2 alpha) ovulatory action in this species.

This study aimed to investigate the effect of temperature on gonadal differentiation, growth, survival, and sex ratio of Leporinus friderici reared at 25 °C or 29 °C from 50 to 240 days after eclosion (DAE) in a water recirculation system. A total of 110 fish at 50 DAE (6.7 ± 0.1 cm and 6.1 ± 0.3 g) were equally and randomly distributed in 10 boxes (90 L) (11 fish/box, 5 boxes/temperature). One fish from each experimental unit was randomly sampled at 50, 70, 90, 110, 130, 150, 170, 190, 210 and 240 DAE. Female gonadal differentiation started at 150 DAE (11.4 ± 0.0 cm and 16.4 ± 0.0 g) at 25 °C and at 170 DAE (10.7 ± 0.7 cm and 27.7 ± 8.5 g) at 29 ºC, while testes differentiation only occurred at 29 °C from 190 DAE (12.1 ± 0.0 cm and 38.0 ± 0.0 g). Of 50 fishes sampled in each condition, 17 (12 females and five males) and three (three females) displayed gonadal differentiation at 29 °C and 25 °C, respectively. Final biometric values at 29 °C were twice those obtained at 25 °C, reaching 13.9 ± 0.65 cm and 57.3 ± 10.12 g versus 11.2 ± 0.39 cm and 28.5 ± 2.95 g, respectively. While temperature clearly influenced gonadal differentiation and growth, it had inconclusive effects on sex ratio. The higher temperature (29 °C) has direct implications for the production of this species, as it accelerates growth without causing mortality.

Histological characterization of oocyte developmental stages of suruvi Steindachneridion scriptum kept in captivity

Melatonin (MTN) plays a direct role in fish ovulation, and we hypothesized that aligning ovulation with an endogenous MTN peak levels could improve reproductive performance. To that, in experiment 1, we assessed circadian variation of plasma MTN in pacu (Piaractus mesopotamicus) females. In experiment 2, pacu females were divided into two groups: one receiving the hypophysation dose at 7 pm (dark onset) and the other at midnight (five hours after dark). Experiment 1 showed that circulating MTN levels increased at 7 pm and remained stable through the dark phase. In experiment 2, no differences in latency or reproductive performance were observed between groups. However, MTN levels at ovulation increased significantly only in the 7 pm group. Strong positive correlations between MTN levels at ovulation and reproductive parameters, such as fertility, hatching rates, and fecundity, were observed. These results confirm that PGF2α and DHP peaks at ovulation are associated with successful ovulation in pacu, consistent with literature that shows that MTN influences ovulation by acting through the Mtnr1a receptor, triggering arachidonic acid release and prostaglandin synthesis. Our findings indicate that the timing of spawning induction influences MTN levels at ovulation, which are positively correlated with reproductive success. This is the first report linking plasma MTN levels at ovulation to successful reproductive performance in fish induced by hypophysation, highlighting the relevance of spawning timing for pacu reproduction.

This study aimed to establish in vitro culture conditions for inducing ovulation in lambari (Astyanax altiparanae). All fish received a priming dose of either 0.6 mg/kg or 100 IU hCG/kg (human chorionic gonadotropin) in vivo. In experiment 1, 0.6 mg and 5.4 mg of carp pituitary extract (CPE)/ kg of ovarian fragment were tested as resolving doses. In experiment 2, prostaglandin F2α (PGF2α) analogue at 100 ng/mL was added to 5.4 mg CPE/kg as the resolving dose. Since ovulation did not occur, in experiment 3 and 4, we compared diverse forms of obtaining follicles, comparing manual follicle dissociation and collagenase at 100, 200, and 400 CDU/mL. Additionally, in experiment 4, the resolving dose of CPE and hCG was replaced by 1 µg/mL 17α,20β-dihydroxy-4-pregnen-3-one (DHP). Ovulation was successful only in Experiment 4, using mechanically dissociated follicles with a priming dose of either CPE or hCG and 1 µg/mL DHP as the resolving dose. Key findings include that 5% CO2 is unnecessary, mechanical dissociation of ovarian fragments is optimal, a priming dose of CPE or hCG is required, and DHP at 1 µg/mL is effective. These results establish a standardized protocol for in vitro ovulation induction in A. altiparanae, offering a valuable tool to study ovarian function and spawning failure in tropical species.