An abnormal specimen of threatened Ryukyu-ayu (Plecoglossus altivelis ryukyuensis) with an additional pelvic fin

The present study describes a fish individual of Plecoglossus altivelis ryukyuensis with three pelvic fins collected from a hatchery-reared population in Amami-oshima Island, Japan. The additional pelvic fin had the same number of rays as the neighboring pelvic fins (eight fin rays), and its girdle was fused with that of the neighbor. The comparison of morphometric measurements and meristic counts of the abnormal specimen with those of the normal specimens revealed that this abnormality was not a syndrome when all other characteristics were in the expected ranges. The condition factor of abnormal fish was close to the ideal value, suggesting that this abnormality did not severely affect the activities of the fish under rearing conditions. Well-managed rearing conditions might contribute significantly to the survival and growth of abnormal fish. The cause of this abnormality is unclear, but the limited diversity in the genome of the subspecies is assumed to be a probable factor

Artificial breeding and larval rearing techniques to conserve ayu, Plecoglossus altivelis (Plecoglossidae)

Ex-situ conservation, which is important to protect critically endangered species, requires special artificial breeding and rearing techniques during the early life stages of a species. This study describes the artificial breeding and larval rearing techniques to conserve ayu (Plecoglossus altivelis), an important fish that is endangered in some locations. Fish eggs and milt were acquired from mature fish by gently pressing the fish abdomen; subsequently, artificial insemination was performed by gentle mixing. The fertilized eggs were then incubated in running freshwater (for approximately 14 days). After hatching, the larvae were placed in brackish water (ca. 10‰ salinity; 7-8 mg L-1 dissolved oxygen) to maintain live food to the larvae. From the 1st to 35th day after hatching (DAH), the larvae were fed with live rotifers. Subsequently, from the 15th DAH onwards, the larvae were fed with brine shrimp nauplii along with rotifers and were familiarized with artificial food. After the fish gained a body length of 4-5 cm and started consuming 0.5-0.8 mm formulated food grain, they were transferred to a fingerling nursery with freshwater for further cultivation. This study also noticed that the susceptible stages of ayu larvae were at approximately 14-16th and 40-45th DAH. This work provides necessary information to design a conservation plan for the ayu population in the southern distribution range of this species where it is currently facing threats of extinction, such as in Vietnam. Additionally, the study describes the rotifer culturing technique that can assist in conducting larviculture

Interploidy gene flow involving the sexual-asexual cycle facilitates the diversification of gynogenetic triploid Carassius fish

クローン繁殖フナは稀に有性生殖をしながら繁栄 --遺伝的に多様なクローンフナが存在する謎を解明--. 京都大学プレスリリース. 2021-11-19.Asexual vertebrates are rare and at risk of extinction due to their restricted adaptability through the loss of genetic recombination. We explore the mechanisms behind the generation and maintenance of genetic diversity in triploid asexual (gynogenetic) Carassius auratus fish, which is widespread in East Asian fresh waters and exhibits one of the most extensive distribution among asexual vertebrates despite its dependence on host sperm. Our analyses of genetic composition using dozens of genetic markers and genome-wide transcriptome sequencing uncover admixed genetic composition of Japanese asexual triploid Carassius consisting of both the diverged Japanese and Eurasian alleles, suggesting the involvement of Eurasian lineages in its origin. However, coexisting sexual diploid relatives and asexual triploids in Japan show regional genetic similarity in both mitochondrial and nuclear markers. These results are attributed to a unique unidirectional gene flow from diploids to sympatric triploids, with the involvement of occasional sexual reproduction. Additionally, the asexual triploid shows a weaker population structure than the sexual diploid, and multiple triploid lineages coexist in most Japanese rivers. The generated diversity via repeated interploidy gene flow as well as an increased establishment of immigrants is assumed to offset the cost of asexual reproduction and might contribute to the successful broad distribution of this asexual vertebrate

An abnormal specimen of threatened Ryukyu-ayu (Plecoglossus altivelis ryukyuensis) with an additional pelvic fin

The present study describes a fish individual of Plecoglossus altivelis ryukyuensis with three pelvic fins collected from a hatchery-reared population in Amami-oshima Island, Japan. The additional pelvic fin had the same number of rays as the neighboring pelvic fins (eight fin rays), and its girdle was fused with that of the neighbor. The comparison of morphometric measurements and meristic counts of the abnormal specimen with those of the normal specimens revealed that this abnormality was not a syndrome when all other characteristics were in the expected ranges. The condition factor of abnormal fish was close to the ideal value, suggesting that this abnormality did not severely affect the activities of the fish under rearing conditions. Well-managed rearing conditions might contribute significantly to the survival and growth of abnormal fish. The cause of this abnormality is unclear, but the limited diversity in the genome of the subspecies is assumed to be a probable factor

Historical effect in the territoriality of ayu fish.

Ayu fish form algae-feeding territories in a river during a non-breeding (growing) season. We build a cost-benefit theory to describe the breakdown and formation of territory. In the early stage of a growing season, all fish hold territories at low densities. Once all territory sites are occupied, excess fish become floaters. When fish density further increases, a phase transition occurs: all the territories suddenly break down and fish form a school. In contrast, when the fish density is decreased, territories are suddenly formed from the school. Both theory and experiments demonstrate that ayu should exhibit a historical effect: the breakdown and formation processes of territory are largely different. In particular, the theory in formation process predicts a specific fish behavior: an "attempted territory holder" that tries to have a small territory emerges just before the formation of territory.autho