Marine organisms are rich sources of natural products that, among other activities, help to maintain the species equilibrium. Samples of the marine brown alga Canistrocarpus cervicornis (Kützing) De Paula & De Clerck were collected in Búzios, Rio de Janeiro, Brazil, in June 2006. The extract was obtained in CH2Cl2 and subjected to fractionation by chromatographic methods in order to isolate and purify the compound (4R, 7R, 14S)-4α, 7α-diacethoxy-14-hydroxydolastane-1(15), 8-dien. Then, the effects of the extract and the dolastane diterpene on zygotes and gametes of the sea urchin Lytechinus variegatus were evaluated. The exposure of male and female gametes to the C. cervicornis extract promoted, respectively, a reduction of 10-30% in fertilization and a 20 to 70% decrease in the number of eggs. Furthermore, the exposure of zygotes to the extract inhibited their development up to 86.7 ± 1.6% (at a concentration of 250 µg mL-1), as well as generating abnormalities in 39-50% of zygotes. The results of the dolastane diterpene showed no evidence of inhibition in the zygotes’ development, thought it was proved to induce anomalies. At higher concentrations (25 and 50 µg mL-1), it was possible to observe cell lyses.