Lumpfish Cyclopterus lumpus reproduction: Pituitary gene expression, physiological and morphological changes accompanying gonadal maturation

Lumpfish is the most important cleanerfish species in Atlantic salmon farming. Detailed information on the gonadal maturation process is a prerequisite for year-round production of juvenile lumpfish. In the present study we have described physiological, morphological and endocrine events associated with the annual reproductive cycle in lumpfish of both sexes. Female lumpfish matured at a body weight close to 2 kg and were capable of spawning two egg batches, one in February/March and one in May/June. Male lumpfish could be stripped for sperm most of the year. The appearance of spawning males with high 11-KT concentrations and clearly developed secondary sexual characters was synchronized with female spawning. From the onset of maturation to the periovulatory stages in females, pituitary transcript levels of gnrhr2ba1 and lhb increased in a correlated manner, together with an increase in GSI and in circulating E2 concentrations. In contrast, a high individual variation of fshb transcript levels was observed, potentially reflecting the bimodal oocyte development with oocytes at different developmental stages present in the ovary at the same time. The onset of pubertal testis growth concurred with an increase in pituitary fshb and lhb transcript levels, while plasma 11-KT concentrations increased at later stages, correlating with a gnrhr2ba1 increase. It appears that spermatogenesis and testis growth is regulated by Fsh and can proceed in the presence of low androgen levels.