Gametogenesis and seminatural reproduction of the Amazon twospot astyanax Astyanax bimaculatus (Linnaeus, 1758) cultivated in an enriched environment.

Environmental enrichment is used to provide well-being to the animals, such as fish, in captive conditions, mimicking their natural habitat. It may influence fish behavior, physiology, and survival. In terms of reproduction, however, the relationship between environment enrichment and successful reproduction in captivity is still poorly explored in fish species. Aiming to understand any possible benefits of structural enrichment on fish reproduction, 10-days-hatched larvae of the twospot astyanax Astyanax bimaculatus were raised for 18 weeks in tanks with different elements of structural environmental enrichment (PVC pipes, stones, and artificial plants). In the 5th month of life, those animals were hormonally induced to reproduce to assess gamete formation and offspring quality. Animals raised in a sterile-reared environment (non-enriched) showed earlier spawning than the enriched one, presenting significant quantities of Postovulatory follicle complexes (POCs) and cells in atresia in female ovaries, indicating possible reproductive dysfunction or stress, as well as a greater quantity of empty testicular lumen in males, indicating great release of sperm. On the contrary, animals cultivated in enriched environments showed gonads filled with semen in males and vitellogenic oocytes in females. Furthermore, offspring from the sterile-reared group presented significant rates of larval abnormality compared to the enriched group. In conclusion, the results of this study show that environmental enrichment can interfere with the reproduction of fish in captivity, mainly by preventing early maturation of gametes, which can result in low-quality offspring and, consequently, low production of fish species.

Embryonic development of the fire-eye-tetra Moenkhausia oligolepis (Characiformes: Characidae).

This study describes the embryonic development of Moenkhausia oligolepis in laboratory conditions. After fertilization, the embryos were collected every 10 min up to 2 h, then every 20 min up to 4 h, and afterwards every 30 min until hatching. The fertilized eggs of M. oligolepis measured approximately 0.85 ± 0.5 mm and had an adhesive surface. Embryonic development lasted 14 h at 25ºC through the zygote, cleavage, blastula, gastrula, neurula, and segmentation phases. Hatching occurred in embryos around the 30-somites stage. The present results contribute only the second description of embryonic development to a species from the Moenkhausia genus, being also the first for this species. Such data are of paramount importance considering the current conflicting state of this genus phylogenetic classification and may help taxonomic studies. Understanding the biology of a species that is easily managed in laboratory conditions and has an ornamental appeal may assist studies in its reproduction to both supply the aquarium market and help the species conservation in nature. Moreover, these data enable the use of M. oligolepis as a model species in biotechnological applications, such as the germ cell transplantation approach.

Seminal characterization of the Amazonian fire-eye tetra Moenkhausia oligolepis (Günther, 1864).

The seminal characteristics of Moenkhausia oligolepis are described. Three males were induced with a single dose of carp pituitary. Semen was collected 6 h after induction, and diluted in dibasic sodium phosphate extender solution. For motility analysis, 1 µl of diluted semen was added to 10 µl of distilled water to achieve gamete activation. The average duration of total motility was 76.67 s; while the average sperm motility rate at intervals of 15 s was 95.3, 85.3, 59.6, 31.7, 13.0, 4.6 and 1.2%. To determine sperm concentration in samples, 0.5 µl of semen was diluted with 500 µl of glutaraldehyde. An aliquot of 10 µl of this dilution was utilized for cell counting. An average count of 4.97 × 109 ± 3.46 sperm/ml was obtained. Morphological analyses were performed using eosin-nigrosine dye; 20.33% of the sperm were observed to be dead. Live sperm, comprising the other 79.67%, had an average length of approximately 30 µm, with a head diameter of 4.488 ± 0.7 µm; and a flagella plus mid-piece length of 26.071 ± 12.4 µm. Of those sperm, 69% had a normal morphology, while 31% had primary and secondary abnormalities. The observed abnormality rate did not have a detrimental effect on artificial fertilization potential for the species. The description of the seminal characteristics of a species is one of the most important sets of information required for artificial reproduction of fish in captivity. It also contributes significantly to the total biological knowledge of the studied species.