Onychostoma rara is a fish species distributed only in the Yuan River system in the middle Yangtze River and the Xijiang River system in the Pearl River basin of China. It is locally popular due to its tender meat, delicious taste, and high nutritional value. However, stressors including overfishing and environmental damage have led to a sharp decline in the number of O. rara and it is currently listed as an endangered fish species in the "Red List of Species in China." The artificial breeding and release of endangered fish can aid in protecting them from extinction. To achieve this, wild O. rara were captured from the Yuan River system in Huaihua from 2016–2019. In 2019, numerous artificially reared O. rara reached sexual maturity, and the F1 generation was obtained by artificial cultivation and manual stimulation with oxytocin. Our findings indicate that male F1 individuals attain sexual maturity for the first time at the age of 2 years, whereas female F1 individuals achieve sexual maturity at 3 years. By the age of 4 years, 90% of the F1 population has reached sexual maturity. In 2023, we successfully achieved the controlled reproduction of cultured O. rara. Until Det 2023, there were 100 wild parents, 2000 cultured parents, and 2.02×10⁵ newly hatched larvae. One hundred male and one hundred female parents were sequestered in two separate net cage for the subsequent artificial oxytocin experiment.” Artificial oxytocin was injected with a mixture of luteinizing hormone releasing hormone A₂, chorionic gonadotropin, and carp pituitary gland extract. Each female received two injections, while each male received one that was half the dose of the female's second dose. After the response period was reached, which was approximately 11 h after the second injection, the fertilization was performed using the artificial dry method. The sequence and morphological characteristics of the developing fertilized eggs were observed using a microscope and described in detail. The newly spawned eggs of O. rara were yellow, sinkable, and weakly adhesive, with an average diameter of (2.21±0.06) mm. After absorbing water and approximately 30 min after fertilization, the average diameter of the fertilized eggs expanded to (2.88±0.07) mm and became non-adhesive. When the water temperature was 19.5-25.5 ℃, the time from fertilization to hatching was 45.5 h, and the accumulated temperature from fertilization to hatching was 1024.39 h·℃. The newly hatched larvae showed a light-yellow color, with no other pigments visible throughout the larva body. On the second day, melanosis in the eyes and red blood vessels in the heart and abdomen were clearly visible. When the water temperature was decreased, the time of the embryo hatching from the membrane was extended, and the embryonic development of the prophase stage of incubation was more complete; furthermore, the eye pigments were clearly visible, and red blood flow could be seen in the heart and abdomen. The morphological characteristics of the prophase stage of incubation were basically consistent with those of the larvae on the second day, with higher water temperatures. This suggests that temperature is crucial for the hatching and development of O. rara embryos.
In general, the egg traits of the O. rara are basically the same as those of other fish in the genus Onychostoma, which may be related to their typical habitats such as rivers with strong currents and gravelly bottoms. In such environments, fertilized eggs can be dispersed with water flowing downward into the spaces between rocks to avoid predation and improve survival rates. However, due to the difference between species and environmental factors, the degree and stage of organ development and the time from fertilization to hatching are different. Using artificial breeding and artificial oxytocin technology, this study realized the artificial reproduction of O. rara for the first time and carried out a preliminary study on the development of O. rara embryos and larvae, providing basic data and an important reference for the future implementation of large-scale breeding of this species.