Role of estradiol in the reproductive biology of echinoids

Despite the extensive use of sea urchins in embryology, the hormonal mechanisms regulating echinoid reproductive processes are scarcely known. To overcome this gap of knowledge, in this research we focused on sex steroids and particularly on estradiol (E2), whose presence and seasonal variations were actually reported in different echinoderm tissues by past studies. We administered (via peristomial injection) three different concentrations (2 ng/mL, 20 ng/mL and 200 ng/mL) of 17ß-estradiol to specimens of the sea urchin Paracentrotus lividus for 2 and 12 weeks. The lowest concentration was close to the physiological one, previously measured in field specimens. Despite the actual increase of circulating E2 in the coelomic fluids, both short- and long-term hormonal treatment did not induce marked variations in the considered reproductive parameters. The Gonad Index appeared to be more influenced by the feeding ratio than by the E2. Similarly, the maturative stage of the gonads was not markedly affected by the hormone, although some slight sex-specific differences could be observed: treated females never reached the maximum maturative stage compared to the controls as instead happened in males injected with the lowest E2 concentration. The activities of the enzymes involved in estradiol synthesis and metabolism suggested the presence of homeostatic mechanisms to maintain the endogenous E2 levels within a certain threshold. Overall our present results suggest that E2 does not markedly influence echinoid reproduction and, particularly, it does not promote female maturation, as on the contrary reported for vertebrates and also suggested for asteroid echinoderms.