| Abstract |
Six experimental populations of the red porgies (Pagrus pagrus) were maintained under rearing conditions in order to study the reproductive cycle and the sexual structure of species. 1022 fish originated from the wild, aged 0+ to 6+, were sampled on a monthly basis. The study was carried out by histological analysis of gonads, in vitro analysis of steroidogenesis and determination of vitellogenin and steroid hormones in the blood. Concentrations of vitellogenin in plasma were determined by an Enzyme-linked immunosorbent assay (ELISA). Plasmatic concentrations of testosterone (T), estradiol-17β (E2), estrone (E1), 11-ketotestosterone (11KT) and 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) were determined by radioimmunoassay (RIA). Thin Layer Chromatography (TLC), High Pressure Liquid Chromatography (HPLC), microchemical reactions and recrystallizations to constant specific activity were used to identify steroids produced by in vitro incubations of ovaries and testis. In females, mature fish were observed for the first time at the age of 3-4 years. More than 50% of females were mature by the age of 4-5 years. Oogenesis began in November. Vitellogenesis started during January and spawning took place from March to May. The concentration of vitellogenin in plasma increased significantly in January and reached a peak in March (405μg/ml) at the beginning of the spawning period. In vitro, ovaries at different stages of oogenesis were shown to produce mainly androstenedione (Δ4), 11β-hydroxyandrostenedione (11βΔ4), T, E2, E1, 17-hydroxyprogesterone (17P), 17,20α-dihydroxy-4-pregnen-3-one (17,20αP) and 17, 20βP4. In females, T concentrations ranged from 0.31 to 1.11 ng/ml during the reproductive cycle. T increased during the period of endogenous vitellogenesis (November-December) and remained at high levels until the end of the spawning period in May. The concentration of E2 ranged from 0.12-1.29 ng/ml. E2 levels increased during exogenous vitellogenesis. E1 levels also followed a similar pattern to that of E2 but had a lower range (0.04-0.29 ng/ml). Oestrogens (E1, E2) levels followed a similar pattern to vitellogenin. The concentrations of 11KT did not change significantly in females. Levels of 17,20βP ranged between 0.22 and 1.5 ng/ml. The changes were not related to gametogenesis. The oocytes size frequency distribution and the repeated spawnings of induced females lead to conclude that Pagrus pagrus is a multiple spawner species. Vitellogenesis is asynchronous and spawning occurs via multiple recruitments of vitellogenic oocytes to eggs. Potential fecundity counted on mature ovaries is described by the body weight (BW) and standard length (SL) due to equations: F=105BW-68550 and F=11880SL-304620. Gametogenesis takes place normally under natural photoperiod and a water temperature of 15-16.5°C. Under rearing conditions females might not spawn spontaneously and ovaries show high atresia rates during the spawning period. Spawning can be induced by the use of LHRHa at a dose of 40 μg/Kg, preferably in the form of a slow release device such as microspheres. It is possible that a lower dose might be effective, but the speed in response might be lower. The delay between treatment and first spawning was at least five days. Successful spawning of good quality eggs required that fish were at least in their fourth year of life, and that the diameter of the largest oocytes in the ovary was at least 500μm. Males matured for the first time at the age of 4-5 years. Spermatogenesis began in November. Spermiogenesis took place from December to February and spermiation lasted from March to May. Testes at stages of spermiogenesis and spermiation produced mainly Δ4, 11βΔ4, T, E2, 17,20βP, 17,20β,21-trihydroxy-4-pregnen-3-one (20βS) and 4-Androstan-19ol-3,17-dione (K4). Concentrations of T and 11KT fluctuated significantly during the male reproductive cycle and showed a similar seasonal pattern. They increased in December at the beginning of spermiogenesis, and remained at high levels during spermiation. T ranged from 0.16 to 8.5 ng/ml, and 11KT from 0.10 to 6.0 ng/ml. Concentrations of Estrogens (E2, E1) were low (E2:<0.5ng/ml, E1:<0.1ng/ml) and they did not show significant seasonal changes. The concentrations of 17,20βP ranged from 0.22 to 1.5 ng/ml. High values were reached during the resting and recovering period of the male cycle, but these seasonal changes were not related to gametogenesis. Analysis of the sexual structure in relation to the age, and sex frequency distribution in relation to the length and histological observations of the gonads, revealed the existence of protogynous hermaphroditism. The gonads were differentiated as a bisexual organ with two isolated heterosexual zones -ovarian and testicular- which develop sequentially. This development takes place asynchronously so that all the fish, before the time of sexual maturity, pass through a stage at which the ovarian zone is differentiated into an ovary. Further development proceeds through three different ways. In the firt way, immature fish present testicular tissue differentiated into testes and the ovaries degenerate before sexual maturity. These fish function as males throughout their life omitting the functional female phase (primary males). In the second way, the development of the gonad is completed with maturation of the ovarian zone, and the fish function as females (functional females). After a single, or possibly repeated spawnings, females change sex and function as males (secondary males). Finally in the last way, fish remain females without changing sex (persistent females). Rudiments of the testicular tissue remain in their ovaries, but they do not show noticeable structural and functional changes. Pagrus pagrus is a protogynous diandric species with primary and secondary males. Protogynous sex change begins with the cytological differentiation of the testicular zone during the female functional phase, and is completed with the degeneration of the ovarian zone during the male functional phase. It takes place after the age of 3-4 years, and is considered as a process of low profile, cyclic and discontinuous, which is intensified at the post-spawning period of the female reproductive cycle. Pagrus pagrus is a species of high aquaculture interest. Our study contributed to the acquisition of the basic information required for the control of reproduction and also revealed the protogynous character of this species. Histological and endocrinological data are now available to be used for further research on environmental, social and endocrinological aspects of the sex change, and of particular phases of the reproductive cycle of this species.
|
Collections
