Abstract
In recent years, the Pacific brown macroalga Rugulopteryx okamurae has rapidly expanded across Mediterranean and Atlantic coasts, generating major ecological and socio‑economic impacts. In this study we examined the effects of exudates released by R. okamurae on the early developmental stages of two autochthonous sea urchin species, Paracentrotus lividus and Arbacia lixula , which are key benthic grazers in the invaded regions. Fertilized eggs were exposed for three days to different concentrations of R. okamurae exudates, and subsequent effects on survival, growth, and developmental abnormalities were determined for both sea urchin species. Acute exposure to 50 g L⁻ 1 exudates caused about 90% embryonic mortality in both species. Lower concentrations significantly reduced larval growth and increased malformations. Median effect concentrations, expressed as equivalent R. okamurae biomass, for growth inhibition were 16 g L⁻ 1 for P. lividus and 14 g L⁻ 1 for A. lixula , indicating similar sensitivity. Malformations increased significantly at exudate concentrations equivalent to 0.5 g L⁻ 1 of R . okamurae biomass in P. lividus and 5 g L⁻ 1 in A. lixula . The HPLC–MS analysis of R. okamurae exudates confirmed the presence of several bioactive polyphenols, with vanillic acid exhibiting the highest concentration (1.73 ± 0.09 µg L −1 ). Overall, our results show that R. okamurae exudates strongly inhibit the early developmental stages of native sea urchins. The presence of high biomass of this invasive alga could contribute to reduced larval survival and recruitment, driving sustained declines in their benthic populations. As these key grazers play a central role in regulating macroalgal community dynamics, their reduction would weaken grazing pressure and create conditions that further facilitate the invasion and ecological impacts of R. okamurae in the invaded Atlantic and Mediterranean rocky shores.