Sea urchin (Paracentrotus lividus) cryopreserved embryos survival and growth: effects of cryopreservation parameters and reproductive seasonality.BACKGROUND:  The cryopreservation of embryos can be a powerful biotechnological tool to supply all year-round biological material for sea urchin aquaculture production. This study investigates different methodological and biological factors that may affect the result of the cryopreservation process of sea urchin (Paracentrotus lividus) embryos. Our data indicate that neither embryo density nor the use of different cryopreservation containers presented effect on the cryopreservation outcome. Contrary to other marine invertebrates, for sea urchin embryo cryopreservation ultrapure water cannot be used as CPA solvent, yielding zero survival. After studying the reproductive parameters along the reproductive season, we found a positive correlation between both male and female Condition Index (C.I.), and between the oocyte weight and C.I. Both the histology study of female gonads and the C.I. variation, suggest that the sea urchin natural spawning period in the Ría de Vigo occurs between June and July. We found no correlation between any of the reproductive parameters monitored and the cryopreservation outcome.