Ovarian development and plasma sex steroid levels in cultured female Senegalese sole Solea senegalensis

Abstract

Ovarian development was studied in cultured female Senegalese sole Solea senegalensis. Females with regressed ovaries, mainly occupied by perinucleolar oocytes, predominated throughout summer exhibiting low condition factor (K), gonadosomatic index (I(G)), and plasma 17beta-estradiol and testosterone levels. Throughout autumn and winter (ovaries at early and intermediate maturation), oocytes progressed to cortical alveoli and vitellogenic stages accompanied by increasing K, I(G), and plasma 17beta-estradiol and testosterone levels. At late winter/early spring, ovarian development reached its maximum with the predominance of females at intermediate and final maturation (the latter occupied by late vitellogenic oocytes and few early maturation oocytes) and peak values of K, I(G), and 17beta-estradiol and testosterone concentrations. Steroid levels were lower (especially testosterone) than those for naturally-spawning females, which might cause extensive atresia without final oocyte maturation (no spawning was observed). This degenerative process reduced de size of the ovary (initial and intermediate phases of regression) in association with declining K, I(G), and plasma 17beta-estradiol and testosterone levels and increasing proportions of perinucleolar oocytes. The circulating 17,20beta-dihydroxy-4-pregnen-3-one levels, the proposed maturation-inducing steroid, remained relatively constant throughout the experimental period, suggesting that oocytes were unable to respond adequately to its stimulation. We propose the inadequate seasonal thermal regime as the main cause of such dysfunction.