•Sea urchins revealed very sensitive organisms to nZnO.•nZnO toxicity for P.lividus could not be ascribable only to the ionic Zn.•Sperm fertilization capability is not affected from tested Zn compounds.•nZnO dispersed in ASW aggregate in μm size as function of particle concentration.•Sea urchin developmental anomalies mainly evidenced nZnO toxicity. The effect of nano ZnO (nZnO) upon the fertilization and early development of embryos of the Mediterranean sea urchin Paracentrotus lividus is reported herein for the first time. Zn ion (ZnCl2) and bulk ZnO (bZnO) toxicity were assessed for comparison. The embryotoxicity tests showed a 100% effect already at 1μM of nZnO (expressed as Zn) while bZnO and ZnCl2 showed EC50s of 0.98 0.88–1.19μM Zn and 2.02 1.97–2.09μM Zn, respectively. Noteworthy, the frequency of developmental defects for the three compounds was dissimilar and a specific trend for larval skeletal abnormality produced by nZnO was observed. The sperm fertilization capability was only slightly affected by the tested chemicals while the effects were dramatic on the offspring quality of sperms exposed to ZnO compounds resulting in an early block of the regular larval development. ZnO toxicity seems related not only to Zinc ions but also to some surface interactions of particle/aggregates with target organisms and/or with the seawater.