Sexual dimorphism in the sea urchin Paracentrotus lividus: morphological and biochemical analysis

As most echinoderms, the sea urchin Paracentrotus lividus is a gonochoric species without an evident sexual dimorphism. This characteristic obviously represents a disadvantage in those experimental contexts, where could be helpful to distinguish the sex in vivo, without biopsies or spawning induction (these latter strategy being useful only when gonads contain mature gametes). The present study aims to demonstrate the presence of sexual dimorphism at the level of the genital papillae by using different microscopy techniques: stereomicroscopy, light microscopy, and scanning electron microscopy. Possible differences between sexes were also investigated looking at sexual hormones (testosterone and estradiol) levels within coelomic fluids. Our results confirmed the presence of morphologically different genital papillae: in particular males presented a short conical papilla, internally lined by a ciliated epithelium, whereas females presented a flattened papilla, slightly sunken in the body surface and without ciliation. Biochemical analysis confirmed the existence of sex specific differences, in particular referred to estradiol levels (higher in females). The results obtained so far suggested the presence of morphological and physiological adaptations in both sexes and provided a non invasive tool for in vivo sex recognition in this important species.