Where is the control for the highly invasive macroalga Rugulopteryx okamurae? Assessing the role of the sea urchin Paracentrotus lividus along an invasion gradientInvasion Ecology

Alejandro Bernal-Ibáñez¹, Jorge Santamaría², Emma Cebrián², Cèlia Sitjà², Marta Castilla-Gavilán³, Esperanza G. Sumariva¹, Alexis Terrón-Sigler⁴, Francisco Luengo⁴, Alejandro Ibáñez⁴, Ismael Hachero Cruzado¹

¹IFAPA Centro El Toruño, Spain - ²Centre d’Estudis Avançats de Blanes (CEAB- CSIC), Spain - ³Nantes Université, France - ⁴Agencia de Gestión Agraria y Pesquera de Andalucía, Spain

The rapid spread and massive proliferation of the invasive macroalga Rugulopteryx okamurae in southern European coastal waters is a major ecological concern with high socioeconomic consequences. First identified as invasive in the Strait of Gibraltar (southern Iberian Peninsula) in 2014, R. okamurae is expanding along the Atlantic and Mediterranean coasts. Despite its widespread distribution, the biotic control mechanisms and long-term effects of R. okamurae consumption by native herbivores are unexplored. In this study, we present results assessing populations of the common grazer Paracentros lividus from field surveys conducted along an invasion gradient in the Alboran Sea (S Iberian Peninsula), from sites where R. okamurae had been present for 10 years, 6 years, and 0 years. Additionally, we conducted a 24-week laboratory experiment to evaluate the consumption of R. okamurae by P. lividus and its associated physiological effects. Field data revealed low population densities of P. lividus in long-term invaded sites, as well as individuals with low weight and low gonadosomatic index. In the laboratory experiment, P. lividus individuals from long-term invaded sites and fed a 100% R. okamurae diet had higher weight loss and increased mortality after 14 weeks. These negative effects were mitigated in individuals fed a mixed diet. Our findings show the long-term detrimental effects of R. okamurae ingestion on P. lividus, highlighting potential consequences for the conservation of this key species in invaded areas and the broader ecological implications for coastal ecosystems.