Effects of 17β-estradiol and 4-nonylphenol on smoltification and vitellogenesis in Atlantic salmon (Salmo salar)

The impact of 17-estradiol (E₂) and the putative estrogenic compound, 4-nonylphenol (4-NP), on smoltification and vitellogenesis in Atlantic salmon (Salmo salar) was investigated during a 30 day period starting late April. Three groups of fresh water (FW) fish (1 year old, mixed sexes, average weight 23 g) were injected once a week with 50 µg (0.18 µmol) 17-estradiol, 3 mg (13.6 µmol) 4-nonylphenol dissolved in peanut oil, or peanut oil alone as control. Every ten days, subgroups were challenged with 28 ppt seawater (SW) for 24h, and sampled together with subgroups of FW fish. Treatment effects were examined on vitellogenic and osmoregulatory parameters. E₂ and 4-NP treatment increased the total calcium and protein level in plasma and the hepatosomatic index of FW fish, both indicating an activated vitellogenesis in the liver. The presence of vitellogenin in the plasma of 4-NP- and E₂-treated groups was further indicated by the appearance of a high molecular weight vitellogenin band (550 kDa) in electropherograms produced by native gel electrophoresis. This band appeared in exactly the same position in both the E₂- and the 4-NP-treated groups but could not be detected in controls. During the 30 day treatment period, control fish approached the peak of smoltification, as indicated by a distinct silvery appearance, decreasing condition factor, increasing levels of gill Na⁺,K⁺-ATPase and improved hypoosmoregulatory performance in the SW-challenge test. Both E₂ and 4-NP treatments significantly inhibited the progress of smoltification, as judged by a significant reduction of gill Na⁺,K⁺-ATPase activity, relative -subunit Na⁺,K⁺-ATPase mRNA expression, gill chloride cell density and a poorer hypoosmoregulatory performance of treated fish. The impaired SW-tolerance of E₂- and 4-NP-treated fish was strongly correlated with a decreased gill Na⁺,K⁺-ATPase activity. Despite a difference in relative potency, the present study shows that 4-nonylphenol and 17-estradiol may have qualitatively similar inhibitory effects on smoltification and hypoosmoregulatory physiology of Atlantic salmon. Both 4-NP and E₂ activated the vitellogenic system, and the study supports the hypothesis that sexual maturation and smoltification are antagonistic, developmental phenomena in salmon. It is suggested that the presence of estrogenic compounds in the environment may negatively influence smoltification and migration in wild stocks of salmon.     

The modulating effect of vitellogenin on the synthesis of 17β-estradiol by rainbow trout (Oncorhynchus mykiss) ovary

In vivo induction of vitellogenin (VTG) in response to the administration of 17-estradiol (E₂) was achieved and the protein was isolated by gel filtration column chromatography of plasma samples. Adult female trout were injected with the vitellogenic fraction every ten days from July to November and levels were measured by RIA from September to December. The results showed a significant decrease (p<0.01) in plasma E₂ levels in injected females compared with the controls. In December, after finishing the treatment, the plasma E₂ concentration increased, in injected females to reach a level similar to that of control females at vitellogenesis. The in vitro study showed that in early vitellogenic oocyte (from September) the presence of the vitellogenic fraction in the incubation medium causes a significant decrease (p<0.01) in the synthesis of E₂ by the oocytes. These data suggest that the concentration of the VTG into the oocyte can alter VTG production by the liver, moderating the production of E₂ by the ovary.     

In vitro hepatic monodeiodination of L-thyroxine and the temporal effect of 17β-estradiol on deiodination in rainbow trout (Salmo gairdneri Richardson)

Thein vitro hepatic monodeiodination of L-thyroxine (T4) to triiodo-L-thyronine (T3) in rainbow trout (Salmo gairdneri) was found to be pH- and temperature-dependent, and was related to the amount of homogenate in the reaction vessel, suggestive of an enzyme-regulated event. Dithiothreitol (DTT) introduced into the reaction medium stimulated T3 production in a dose-related manner, whilst 6n-propyl-2-thiouracil (PTU) inhibited T3 production, also in a dose-related manner. The conversion was stimulated in the presence of light and depressed at buffer concentrations of less than 0.1 M.Prior treatment of fish with an intraperitoneal slow-release implant containing 17-estradiol (E2), at doses which are known to induce chronic mild elevations in plasma E2 levels, elicited a biphasic response to E2 as regards hepatic T3 production from T4 with a depression of T4 to T3 conversion evident within 1–2 days after implantation, and a subsequent stimulation of T3 production evident 56 days after, implantation. This increased hepatic deiodinase activity after chronic exposure to E2 at physiological doses was accompanied by a 3.5 fold increase in Vmax without a significant change in Km, suggesting the presence of an increased amount of the enzyme.     

Effects of feeding (ω-3) HUFA-enriched Artemia during a progressively increasing period on the larviculture of freshwater prawns

Larvae of the freshwater prawn (Macrobrachium rosenbergii) were reared to post-larvae in a closed recirculation system. Six treatments were designed where freshly-hatched Artemia nauplii were first given to the larvae for 3, 7, 11, 15, 17 and 25 days according to the treatment. After that (-3) HUFA-enriched Artemia were given until the 28th day at which time the test was terminated and evaluated. The requirement for (-3) Highly Unsaturated Fatty Acids (HUFA) for the larval stages of M. rosenbergii was confirmed by this experiment. Moreover, the longer the period of feeding on (-3) HUFA-enriched Artemia nauplii, the better the results in terms of growth, metamorphosis rate, survival and stress resistance.     

Effect of diethylstilbestrol (DES) and 17 β-estradiol (E2) on growth, survival and histological structure of the internal organs in juvenile European catfish Silurus glanis (L.)

The effect of supplemented commercial diets with diethylstilbestrol (DES—15, 30 and 60 mg kg^?1) and 17 β-estradiol (E2—30 and 60 mg kg^?1), two chemicals commonly used in sex reversal procedure in fish, on survival and growth parameters of juvenile European catfish (Silurus glanis) was evaluated. During the two experiments, lasting 28 days each, fish were kept at temperature 25.2–26.5 °C, pH 7.4–9.3 and oxygen concentration 5.0–7.3 mg O₂ dm^?3. DES supplementation resulted in depressed growth rate of catfish. In experimental groups fed with E2, no negative effect on growth parameters was found. Both chemicals did not result in observed mortality. In all of the experimental DES groups, hepatosomatic index increased significantly, which suggests negative influence on physiological condition of catfish. DES supplementation significantly changed cytological factors of liver cells and caused hepatic alterations in parenchyma, such as vacuolization and blood congestion. Similarly, supplementation of E2 in food resulted in changes in cytological parameters of hepatocytes. However, E2 did not cause pathological changes within the liver tissue. Histological examination of the catfish gonads showed 19 and 38 % of sterile fish after treatment with 30 and 60 mg kg^?1 of DES, respectively. The results suggest that DES served in food could be ineffective in hormonal feminization process of European catfish. No disturbances of sex differentiation process after E2 treatment were observed. However, slight feminization effect in the highest level of E2 treatment group was recorded.     

Influence of 11-ketotestosterone, 17β-estradiol, and 3,5,3′-triiodo-L-thyronine on distribution and metabolism of carotenoids in Arctic charr, Salvelinus alpinus L.*

This investigation examines the influence of implants containing 11-ketotestosterone (11KT), 17-estradiol (E₂), and 3,5,3-triiodo-l-thyronine (T₃) on astaxanthin metabolism in sexually immature individually tagged Arctic charr. The fish (initial average weight 427 g) were maintained in freshwater for 40 days, and weekly implanted intraperitoneally with oil-based injections containing either 11 KT, E₂ or T₃ at levels of 0.1, 1.0 and 0.1 mg (100 g body weight (BW))^–1, respectively. The control fish were given the oil medium alone (0.2 ml 100 g BW^–1). The diet contained ca. 50 mg astaxanthin kg^–1. Carotenoid composition was monitored in plasma, fillet, liver and skin, and 11 KT, E₂ and testosterone (T) levels in plasma. All hormone treatments reduced plasma T compared to the control. E₂-treated fish had a higher (p<0.05) hepatosomatic index (HSI) than the other treatments. Hormone treatment did not influence gonadosomatic index (GSI). T₃ administration induced a silvery skin appearance. The fillet and plasma carotenoid content decreased during the experiment. 11 KT implantation reduced astaxanthin and idoxanthin concentrations of plasma and fillets, and increased the amount in liver and skin, compared to the other treatments. The relative proportion of astaxanthin to idoxanthin was higher in the control fish and T₃ implanted fish, than in fish implanted with 11 KT or E₂ (p<0.05). Fish treated with E₂ had the highest skin carotenoid concentration. Male fish had significantly higher carotenoid content in plasma, fillet and skin than female fish. This study reveals that sex hormones affect carotenoid metabolism and partitioning among body compartments of Arctic charr, effects differently displayed by the sexes.     

Effects of estradiol-17β and 17α-methyltestosterone on gonadal differentiation in the coho salmon, Oncorhynchus kisutch

Methods are described for controlling sex differentiation in the coho salmon, Oncorhynchus kisutch. Eyed eggs and alevins were immersed in solutions of estradiol-17β and 17α-methyltestosterone and fed these steroids in the diet for a period of 10 weeks postswim-up. In seven out of 10 groups that received estradiol all fish resembled normal females when sampled at 4 months posthatch. In fish treated with methyltestosterone at all except the lowest dose (25 and 50 μg/l) the gonads resembled neither normal males nor normal females; these gonads were composed largely of connective tissue with only occasional germ cells. Various proportions of male, female and intersex gonads were observed in groups which received androgen or estrogen only in the diet. The implications of the work for salmon culture include the production of sterile fish and increasing the proportion of female salmon in hatchery populations.     

Effects of salinity, dietary level of protein and 17α-methyltestosterone on growth hormone (GH) and prolactin (tPRL177 and tPRL188) levels in the tilapia, Oreochromis mossambicus

In the present study, we examined the long-term effects of environmental salinity, diet (35% and 25% crude protein) and 17-methyltestosterone (MT) on corresponding levels of pituitary and serum growth hormone (GH) and prolactins (tPRL₁₇₇ and tPRL₁₈₈) in the tilapia (Oreochromis mossambicus). We observed no discernible patterns in serum GH that would suggest an effect of salinity, diet or MT. However, serum GH levels in all treatments declined at 1 and 3h after first feeding. Serum tPRL₁₇₇ and tPRL₁₈₈ were significantly higher in freshwater (FW) than in seawater (SW) and levels were significantly affected by dietary protein. tPRL₁₇₇ levels were higher in all groups fed a 35% protein diet, but tPRL₁₈₈ levels were higher only in the groups fed the MT-treated 35% protein diet; only serum tPRL₁₈₈ levels were affected by MT. Moreover, serum tPRL₁₇₇ and tPRL₁₈₈ increased throughout the sampling time-course. Subsequent work using fasted tilapia suggests that first feeding is likely to initiate the post-prandial suppression of serum GH levels. In contrast with the picture observed in blood, pituitary glands of SW animals showed higher levels of GH than FW fish. Pituitary GH was elevated by MT in both FW and SW. We also observed that pituitary tPRL₁₇₇ and tPRL₁₈₈ levels were higher in FW fish than in SW fish; tPRL₁₇₇ and tPRL₁₈₈ levels were elevated by MT only in FW animals. To assess the somatomedin activity of plasma from FW- and SW-reared tilapia, we measured [35S]-sulfate incorporation into ceratobranchial cartilage explants in vitro. Plasma from SW-adapted tilapia showed greater activity in this assay than plasma from FW-reared tilapia, suggesting that the GH-dependent IGF bioactivity of plasma is higher in SW-reared tilapia. Collectively, these studies suggest that the growth-promoting actions of SW rearing and of MT administration in tilapia may be linked to elevations in GH and/or prolactin (tPRL₁₇₇ and tPRL₁₈₈)levels.     

The dynamics of in vitro 17 β-estradiol secretion by isolated ovarian follicles of the rainbow trout (Oncorhynchus mykiss)

This study investigated the effects of human chorionic gonadotropin (hCG), 17-hydroxyprogesterone (17P) and testosterone (T) on the in vitro production of 17-estradiol (E₂) by isolated ovarian follicles of the rainbow trout (Oncorhynchus mykiss). 17P at 100 ng ml^-1, and hCG at 100 IU ml^-1 stimulated E₂ production relative to controls, whereas lower doses were ineffective. T was the most effective in stimulating E₂ production, followed by 17P and hCG respectively. The timecourse of E₂ production was investigated for both static culture, and incubations with media replacement, with follicles being exposed to hormone treatment for 30 min, 1 or 3 h, or constantly. E₂ production was observed after 30 min, 3 and 3-6 h in response to T, 17P and hCG respectively. Under static culture, E₂ levels reached maximal levels in 6 h. Longer incubations resulted in further metabolism of E₂ to E₂-glucuronide, which resulted in the blurring of treatment effects after 18 h. Incubations with media replacement resulted in higher E₂ production than in static culture. The results indicate that a 6 h incubation period is sufficient to produce significant increases in E₂ production in response to hCG, 17P and T, and that incubations longer than 12 h result in losses E₂ from the incubation media. These findings have implications for the validity of using static cultures to examine the effects of hormone treatment on the activity of steroid converting enzymes in vitro.     

Long-term effects of intraperitoneal injection of estradiol-17β on the growth and physiology of juvenile stellate sturgeon Acipenser stellatus

Juvenile stellate sturgeon Acipenser stellatus were intraperitoneally injected with estradiol-17β (E₂; 0 and 5 mg/kg fish) to investigate the possibility of sex reversal and also determine the changes in biochemical parameters. Five-month-old fish (40.9 ± 1.1 g) were injected every 3-week interval during a 190-day trial. At the termination of the experiment, final weight and other growth parameters including weight gain and specific growth rate, hepatosomatic and viscerosomatic indices were not affected by repetitive injection of E₂. Hematological features of E₂-treated fish showed significant reductions in number of red blood cells, hemoglobin concentration, hematocrit value and mean corpuscular hemoglobin (P < 0.05), but no significant changes were observed in number of white blood cells, mean corpuscular volume and mean corpuscular hemoglobin concentration (P > 0.05). Calcium, phosphorus, glucose, triacylglycerol, cholesterol, total protein and estradiol concentrations were significantly increased in fish injected with E₂ (P < 0.001). Plasma progesterone and testosterone levels were noticeably lower in fish injected with 5 mg/kg E₂ rather than the control fish (P < 0.001). Histological observations of gonads showed that all fish injected with 5 mg/kg E₂ apparently feminized, while 66.6 % of the control group was female. These results revealed that the injection of E₂ is an effective method for feminization of stellate sturgeon without having significant inhibitory effects on growth and survival.     

Effects of (D-Ala6, Pro9N ethylamide) — LHRH on plasma levels of gonadotropin, 17α,20β-dihydroxy-4-pregnen-3-one and testosterone in male goldeye (hiodon alosoides Rafineque)

Male goldeye were treated with (D-Ala⁶, Pro⁹-N ethylamide) — LHRH (LHRH-A) in saline or a silastic pellet (100 µg.kg^–1 body weight) and changes in plasma gonadotropin (GtH), 17,20-dihydroxy-4-pregnen-3-one (17,20P) and testosterone (T) levels were determined by radioimmunoassay. LHRH-A increased plasma GtH levels, with the response to LHRH-A in saline being of much greater magnitude and duration than the response to silastic pellet implants. Seasonal differences were found in the response to LHRH-A. Spermiated fish were the most responsive, recrudescing fish the least, and regressed fish displayed an intermediate response. Plasma 17,20P levels were elevated in response to LHRHA in fish of all sexual stages although the magnitude of the increase was not related to the magnitude of the increase in GtH levels. Treatment with LHRH-A also resulted in a transitory increase in plasma T levels. The endocrine control of GtH and steroid secretion in goldeye is discussed in relation to studies in cyprinids and salmonids. Address for reprint requests: Department of Zoology, University of Alberta, Edmonton, Alberta T6G 2E9 Canada.     

Identification,characterization of selenoprotein W and its mRNA expression patterns in response to somatostatin 14, cysteamine hydrochloride, 17β-estradiol and a binary mixture of 17β-estradiol and cysteamine hydrochloride in topmouth culter (<Emphasis Type="Italic">Erythroculter ilishaeformis</Emphasis>)

In this study, a selenoprotein W cDNA was cloned from topmouth culter (Erythroculter ilishaeformis), and it was designated as EISelW. The EISelW open reading frame was composed of 261 base pairs (bp), encoding 86-amino-acid protein. The 5′ untranslated region (UTR) consisted of 104 bp, and the 3′-UTR was composed of 365 bp. A selenocysteine insertion sequence (SECIS) element was found in the 3′-UTR of EISelW mRNA. The SECIS element was classified as form II because of a small additional apical loop presented in SECIS element of EISelW mRNA. Bioinformatic approaches showed that the secondary structure of EISelW was a β1-α1-β2-β3-β4-α2 pattern from amino-terminal to carboxy-terminal. Real-time PCR analysis of EISelW mRNAs expression in 17 tissues showed that the EISelW mRNA was predominantly expressed in liver, ovary, pituitary, various regions of the brain, spinal cord and head kidney. Study of intraperitoneal injection showed that the levels of EISelW mRNA in brain, liver, ovary and spleen were regulated by somatostatin 14 (SS14), 17β-estradiol (E2), cysteamine hydrochloride (CSH) and a binary mixture of E2 and CSH, dependent on the dosage. These results suggest that E2, SS14 and CSH status may affect tissues of selenium metabolism by regulating the expression of SelW mRNA, as SelW plays a central role in selenium metabolism.     

Effects of temperature on the survival,feeding, and growth of pearl gentian grouper (female Epinephelus fuscoguttatus × male Epinephelus lanceolatus)

Fisheries Science - Pearl gentian grouper (Epinephelus fuscoguttatus ♀&nbsp;×&nbsp;Epinephelus lanceolatus ♂) is an important commercial and economic fish from the...     

Long-term study of the reproductive timing of the Neotropical catfish Iheringichthys labrosus (Lütken, 1874): Influence of temperature and river discharge

Unravelling the effect of climate variability on species biology has been one of the main goals of ecological studies. Environmental factors such as river discharge and temperature have being proposed as triggers of reproductive cycle in fish. In temperate climates, fish reproduction is affected mainly by temperature, while is influenced by flood pulses in large tropical rivers. We evaluated the influence of temperature and river discharge as triggers of Iheringichthys labrosus’ reproduction. We studied the following reproductive variables: gonadosomatic index (GSI), hepatosomatic index (HSI) and condition factor (K). Females of I. labrosus were examined in a time series of ten years along three sites located in Lower Uruguay River at the beginning (last week of November-first week of December, spring) and by the end of the reproductive period (middle-end of April, autumn). Generalised linear models detected a positive effect of mean winter temperature and site on the spring mature female GSI. Site was significant in addition to the interaction between temperature and site. Moreover, the effect of winter temperature was stronger than water discharge in triggering reproductive timing of I. labrosus, contrary to the model proposed for other species in tropical systems. Additionally, a literature review suggested a latitudinal gradient on reproduction, occurring earlier and prolonged with decreasing latitude (and increasing temperature). In this study, we stated the effect of temperature on the life history in a Neotropical fish using a multifaceted approach, particularly important in the current context of global climate warming.     

Pathological effects of Cichlidogyrus philander Douëllou, 1993 (Monogenea,Ancyrocephalidae) on the gills of Pseudocrenilabrus philander (Weber, 1897) (Cichlidae)

Histopathological changes of Cichlidogyrus philander Douëllou, 1993 on the gills of Pseudocrenilabrus philander (Weber, 1897) were studied using light and scanning electron microscopy. Observations revealed that C. philander attaches to its host by alternating the prohaptor (for temporary attachment or feeding) or haptor (using haptoral parts for firm and secured attachment). The sharp terminal ends of the anchors are inserted basally into the gill lamella, between two adjacent secondary gill lamellae and the marginal hooklets assist by superficially penetrating, holding and lifting epithelial tissue in the proximal region of the secondary gill lamella. The attachment of C. philander resulted in compression, rupturing of the interlamellar epithelium, change in the organization of epithelial cells in both primary and secondary gill lamella, displacement of the extracellular cartilaginous matrix, occasional rupturing of blood vessels and erythrocytes and some cells becoming ill-defined. At the site of attachment, the host response comprises of hyperplasia, increase in the number of mucous cells and infiltration with neutrophils. It was concluded that the effect of C. philander is mild in natural conditions.     

Effects of Dietary β-Hydroxy-β-Methylbutyrate on Growth and Survival of Nile Tilapia,Oreochromis niloticus,Vaccinated Against Streptococcus iniae

Abstract

β-hydroxy-b-methylbutyrate (HMB), a leucine catabolite, has been shown to cause increased disease resistance and growth in animal production. A vaccine produced from formalin killed bacteria and concentrated extracellular products of the ARS-98-60 Streptococcus iniae isolate has been used for the prevention of streptococcal disease in Nile tilapia, Oreochromis niloticus. In the present study, the effects of feeding HMB were determined in tilapia vaccinated by intraperitoneal (IP) injection of the S. iniaevaccine or unvaccinated (controls). Nile tilapia were fed diets containing either 0, 12.5, 25, or 50 mg HMB/kg diet for 14 days. The mean daily growth rate and feed efficiency showed no significant (P> 0.05) differences between the treatment groups. Dietary HMB supplementation did not enhance antibody production in unvacci-nated Nile tilapia following challenge. Dietary HMB supplementation did not enhance the survival of vaccinated Nile tilapia following challenge injection with 1 X10⁸ CFU of S. iniae.     

Effects of 3,5,3′-triiodo-L-thyronine (T3) and 6-n-propyl-2-thiouracil (PTU) on growth of GH-transgenic coho salmon, Oncorhynchus kitsutch

GH-transgeniccoho salmon (Oncorhynchus kitsutch) juveniles were fed diets containing 3,5,3-triiodo-L-thyronine (T₃; 30 ng/g fish) or 6-n-propyl-2-thiouracil (PTU; 20 ug/g fish), to assess the effect of these drugs on the physiology, growthand survival in comparison with untreated transgenicand non-transgenic salmon. After 84 days, food intake, feed efficiency, survival, growth, hepato-somatic index (HSI), viscera-somatic index (VSI), plasma L-thyroxine (T₄), T₃and growth hormone (GH) levels,and cranial morphological abnormalities were determined. Growth of transgenic salmon was significantly faster than the nontransgenic salmon,and was increased by exogenous T₃and reduced by PTU. Food intake of transgenic salmon was higher than that of the nontransgenic group, but was reduced by exogenous PTU administration. Food conversion efficiency of transgenic salmon was lower than that of nontransgenic salmon,and also was increased by T₃ but reduced by PTU in the transgenic fish. The survival rate in all transgenic groups was significantly higher than that of nontransgenic,and transgenic T₃and PTU treatment groups showed higher survivals than the transgenic-control group. The HSIand VSI of the transgenic fish were higher than the nontransgenic fish;and both parameters in the transgenic salmon were increased by PTU, but reduced by T₃. The plasma T₄ level in transgenic salmon was approximately 1.5-fold higher relative to the nontransgenic fish, whereas no difference was observed among the transgenic groups. Plasma T₃ levels in transgenic salmon were also approximately 2-fold higher relative to the nontransgenic fish. However, the plasma T₃ level in transgenic animals was increased by exogenous T₃ administration, but was reduced by exogenous PTU to that observed in nontransgenic salmon. The plasma GH level of transgenic fish was higher than that of the nontransgenic salmon,and the level was increased by the exogenous T₃, whereas exogenous PTU did not reduce significantly GH levels in transgenic salmon. Transgenic fish also displayed cranium, jawand opercular abnormalities typical of the effects of this gene construct incoho salmon, indicating that some imbalance in growth processes has been induced. However, these abnormalities (especially cranial disruptions) were diminished by administration of exogenous PTU. In conclusion, exogenous T₃and PTU treatments can induce hyperthyroidismand hypothyroidism, respectively,and have inverse effects on growthand skeletal abnormalities of transgenic salmon constitutively expressing GH.     

Binding characteristics and regulation of the 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S) receptor on testicular and sperm plasma membranes of spotted seatrout (Cynoscion nebulosus)

The binding characteristics of 17,20,21-trihydroxy-4-pregnen-3-one (20-S) to plasma membranes prepared from the testes and sperm of spotted seatrout (Cynoscion nebulosus) were investigated using a filtration method to retain the bound 20-S. A single class of high affinity (K_d = 17.9 nM), low capacity (B_max = 0.072 nM g^-1 testes) binding sites was identified by saturation and Scatchard analyses on testicular membranes of spermiating spotted seatrout. A corresponding receptor (K_d = 22.17 nM, B_max = 0.00261 nM ml^-1 milt) was also detected in spermatozoan membrane preparations. The rates of 20-S association and dissociation were rapid, both had T_halfs of less than 1 min. Competition studies indicated that the receptor was highly specific for 20-S. 17,20-dihydroxy-4-pregnen-3-one, which had the highest affinity of the other steroids tested, had a relative binding affinity (RBA) of 14.3%. Progesterone, 11-deoxycortisol and testosterone competed with an order of magnitude less affinity (RBA's of 7.4, 1.8 and 1.1%, respectively). Estradiol displayed low affinity for the receptor (RBA = 0.4%) and cortisol did not cause any displacement at 1000-fold excess concentration. Specific 20-S receptor binding was detected in plasma membranes from testes of both spermiating and non-spermiating seatrout and on spermatozoa. Prolonged incubation of testicular fragments from a spermiating fish with gonadotropin (15 IU ml^-1 human chorionic gonadotropin) or forskolin (10 µM) caused a 2–3 fold increase in membrane receptor binding. Previous studies have shown that gonadotropin-induced upregulation of the 20-S plasma membrane receptor in seatrout ovaries is required for the oocytes to become responsive to 20-S and undergo final maturation. The existence of a 20-S membrane receptor on sperm and its upregulation in the testes by gonadotropin raises the possibility that final maturation of spermatozoa in male seatrout may be regulated by a similar mechanism.