Milt characteristics and plasma levels of gonadal steroids in greenback flounder Rhombosolea tapirina following treatment with exogenous hormones

Fish were treated with exogenous hormones, and milt and blood samples were collected for up to 96 h post-treatment. Blood plasma samples were assayed for the gonadal steroids testosterone (T), 11-ketotestosterone (11KT) and 1 7,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta P). Milt volume, spermatocr it and sperm motility were measured from milt samples. Non-sperimated fish showed increased plasma T and 11KT in response to human chorionic gonadotro pin (hCG) but not luteinising hormone releasing hormone analogue (LHRHa). F ish did not become spermiated in response to treatment with hCG, LHRHa, 11K T, 17-hydroxyprogesterone (17P) or 17,20 beta P. Spermiated fish showed an increase in milt volume in response to hCG and LHRHa but not exogenous ster oids. Sperm motility declined to zero over 120s and was not affected by hor mone treatment or sampling time. Increased milt volume was accompanied by i ncreased plasma T and 11KT, but not 17,20 beta P levels. In a separate expe riment, LHRHa delivered by injection or pellet was equally effective at inc reasing milt volume but had no effect on plasma steroid levels. Spermatocri t declined with stripping but was not affected by hormone treatment, nor wa s sperm motility. Go-treatment of fish with 17P plus LHRHa had no additive effect on plasma steroid concentrations or milt volume. The results suggest that as in other teleosts, gonadotropin mediates milt production in greenb ack flounder.