17α-20ß-dihydroxy-4-pregen-3-one levels during the spawning period in wild amberjack (Seriola dumerilii).

Seriola dumerilii, (Carangidae, Perciformes) is a gonochoric teleost occurring in the Mediterranean Sea. Despite the growing interest for this species, little is known on sex steroid pattern during the reproductive season. Aim of this study was to verify if 17-20ß-dihydroxy-progesterone (DHP) could be involved as maturation inducing substance (MIS) in S. dumerilii as in other species. 244 females of wild S. dumerilii were collected in South Mediterranean Sea (Pelagie Islands), during three spawning seasons and histologically staged. Blood samples were withdrawn by puncturing the gill artery and plasma aliquots were frozen at -20°C for steroid RIAs. DHP plasma values were measured in wild females at the end of vitellogenesis and during final maturation (FOM). DPH values were related both to 17ß-estradiol and testosterone values and to morphological features of oocytes. All specimens with ovaries characterised by a population of oocytes undergoing FOM, showed a significant increase (P<0.01) in DHP value. Furthermore, we analysed DHP plasma values in relation to three different stages occurring during FOM. Early FOM, including GV migration and lipid droplets coalescence, showed the first increase in DHP values (xxpg/ml±xx). Middle FOM corresponding to GVBD was characterised by the higher values of DHP (xxxpg/ml±xx). In late FOM (hydration), DHP started to decrease (xxxpg/ml±xxx). These results suggest that DHP could be considered as the MIS in S. dumerilii