Spontaneous precocious vitellogenesis of female Anguilla rostrata in captive seawater cage

Northern hemisphere temperate Anguillid eels, including Anguilla rostrata, A. anguilla and A. japonica, typically exhibit ovarian development arrested at the oil droplet stage prior to seaward migration, with final maturation occurring during oceanic spawning migration. Although tropical anguillid eels can reach early to mid-vitellogenesis before the onset of spawning migration, spontaneous early vitellogenesis of captive-reared populations has not been documented. Here, we report large-scale spontaneous vitellogenesis in A. rostrata reared in a nearshore sea cage. Over 40% of the 2-year-old A. rostrata developed to the early vitellogenic (EV) stage with a mean follicle diameter of 353.9 ± 42.3 μm and a gonadosomatic index exceeding 4%. In these fish, plasma levels of estradiol and 11-ketotestosterone, as well as key genes in the brain-pituitary-gonad-liver (BPGL) reproductive axis-including gnrh1, fshb, lhb, lhr, cyp19a1, and vg1/2-were significantly higher than those in fish arrested at the early oil droplet (EOD) stage. Transcriptomic analysis comparing brains from eels at the EOD and EV stage identified 14 significantly upregulated genes and 12 significantly downregulated genes. Among these, the significant upregulation of neuroendocrine factors such as galp, cyp19a1, and dexras1 was confirmed by qPCR. These neuroendocrine factors likely integrate physiological and environmental cues to activate the BPGL axis and facilitate precocious vitellogenesis in A. rostrata. Furthermore, eels that had undergone spontaneous vitellogenesis required only 6-7 weekly hormone injections to be successfully induced to maturation, suggesting that they were more suitable for artificial reproduction. Overall, our findings advance the understanding of reproductive initiation in A. rostrata and support strategies to improve ovarian development under aquaculture conditions.