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Simple Summary: The meagre (scientific name: Argyrosomus regius) is a fish species inhabiting the Mediterranean Sea and the eastern Atlantic Ocean. Consumer appreciation and good market quotations induced the aquaculture industry to invest in its domestication. However, when reared in tanks, meagre males produce milt of low quality and quantity, and females do not finalise oogenesis and do not spawn spontaneously. The reproductive dysfunctions affecting meagre reared in tanks, can be alleviated through the use of hormone administration or through the association of hormone treatments with photo-thermal control. In this study, the reproductive maturation of meagre, reared in sea cages under routine farming condition, was assessed. It was observed that female and male meagre produced in captivity (hatchery), transferred as juveniles to sea cages located in the Gulf of Taranto (Italy), and reared until sexual maturity under routine commercial conditions, are able to produce and release gametes spontaneously. Meagre reproduction, under routine farming conditions, may represent an opportunity for the expansion of meagre EU aquaculture production; this actually relies on limited amounts of fingerlings, produced by few hatcheries. The meagre Argyrosomus regius (Asso, 1801) is a promising aquaculture species that shows reproductive dysfunctions when reared in tanks. The aim of this study was to assess the capacity of meagre, reared in cages under routine farming conditions, to mature gonads and reproduce spontaneously. Meagre adults, reared in a fish farm located in the Gulf of Taranto (Italy), were sampled from March to July 2021. The gonadosomatic index and sex steroid plasma concentrations increased from March–April to June, and then decreased in July. In March–April, most of the females showed perinucleolar or cortical alveoli oocytes as the most advanced stages in the ovaries, and most of the males had testes at early spermatogenesis stage. In June, most of the sampled females had oocytes at late vitellogenesis or early post-vitellogenesis stages, and males had seminiferous tubules filled with spermatozoa. In July, most of the females had signs of previous spawning, and males showed scarce amounts of luminal spermatozoa. The present study demonstrated the capacity of meagre, reared in sea cages under commercial conditions, to carry out gametogenesis and spontaneously release gametes. Meagre reproduction, under routine farming conditions, may represent an opportunity for the expansion of meagre aquaculture production. [ABSTRACT FROM AUTHOR]

Simple Summary: The meagre, Argyrosomus regius, is a commercial fish species of which aquaculture production is limited due to technological bottlenecks such as inconstant reproduction in captivity. In addition, there are only a few broodstocks in European farms, which rises the problem of limited genetic variability of the cultured meagre population. The development of germ cell xenotransplantation technology might increase aquaculture production through the production of fertile meagre gametes in a host species for which farming technology is well established. Spermatogonial stem cells have the capacity to proliferate and to proceed through the spermatogenesis process, eventually giving rise to mature spermatozoa. In the present study, meagre spermatogonial stem germ cells were identified, isolated, cultured in vitro, and cryopreserved. Isolated spermatogonial stem cells proliferated, maintained their stem properties in culture, and were still viable after cryopreservation and thawing. This work represents a first step towards the development of a xenotransplantation technology that might facilitate the production of this valuable species in captivity. The meagre, Argyrosomus regius, is a valued fish species of which aquaculture production might be supported by the development of a stem germ cell xenotransplantation technology. Meagre males were sampled at a fish farm in the Ionian Sea (Italy) at the beginning and end of the reproductive season. Small and large Type A undifferentiated spermatogonia were histologically identified in the germinal epithelium. Among the tested stemness markers, anti-oct4 and anti-vasa antibodies labeled cells likely corresponding to the small single Type A spermatogonia; no labeling was obtained with anti-GFRA1 and anti-Nanos2 antibodies. Two types of single A spermatogonia were purified via density gradient centrifugation of enzymatically digested testes. Testes from fish in active spermatogenesis resulted in a more efficient spermatogonial stem cell (SSC) yield. After cell seeding, meagre SSCs showed active proliferation from Day 7 to Day 21 and were cultured up to Day 41. After cryopreservation in dimethyl-sulfoxide-based medium, cell viability was 28.5%. In conclusion, these results indicated that meagre SSCs could be isolated, characterized, cultured in vitro, successfully cryopreserved, and used after thawing. This is a first step towards the development of a xenotransplantation technology that might facilitate the reproduction of this valuable species in captivity. [ABSTRACT FROM AUTHOR]