Expansion of the invasive alga Rugulopteryx okamurae (Dictyotaceae, Ochrophyta) in the Mediterranean Sea: first evidence as epiphyte of the cold-water coral Dendrophyllia ramea (Cnidaria: Scleractinia)

[EN] The Mediterranean Sea is one of the main hotspots for the introduction of nonindigenous species in the world. The invasive alga Rugulopteryx okamurae was detected in 2015 along the coasts of the Strait of Gibraltar, the Atlantic entrance to the Mediterranean Sea. This brown seaweed of Asian origin, with an enormous expansion capacity and high survival rate, has spread rapidly across the Alboran Sea (western Mediterranean). Nevertheless, this species continues to show its highest abundance in areas close to the Strait of Gibraltar. We recorded R. okamurae in Punta de la Mona (Granada, northern coast of the Alboran Sea), one of the zones with the highest marine biodiversity in the Alboran Sea. This record is the easternmost point within the Spanish coasts where abundance data for this invasive species are available. Rugulopteryx okamurae demonstrated its highest abundance values at 48 m, an unusual record for this species in the Mediterranean Sea. We found R. okamurae growing on rocky bottoms, multiple coralligenous organisms, artificial substrates and epiphyting the coral Dendrophyllia ramea (an important habitat-forming species). Cold-water corals provide substrate, shelter and habitat for other species and play a critical role in deep water ecosystems. It is crucial to study and quantify the possible effects of invasive species on habitat-forming species, especially in the Mediterranean Sea, where the arrival of exotic organisms is striking; a better understanding of their dispersal capacities is urgently needed, as they can affect key organisms. The results obtained in this study validate the prolific capacity of R. okamurae to colonize new areas, affecting both littoral and deeper coastal areas. This behaviour, coupled with the ability to grow on a wide variety of suitable substrates, creates a strong threat to natural environments.This work was supported by the project S.O.S. Corales and NGOs Coral Guardian, Coral Soul and Equilibrio Marino, and ESMARES2 from the Instituto Español de Oceanografía (IEOCSIC), under the framework ordered to the IEO-CSIC by the Ministerio de Transición Ecológica y Reto Demográfico (MITECO) of the Spanish government for the application of the Marine Strategy Framework Directive (MSFD) in Spanish waters. We thank the scientific divers from S.O.S. Corales who assisted us in sampling expeditions and the underwater photographers Rafael Camacho (Figure 4A), Lolo García (Figure 4B) and Javier Sánchez (Figure 4C). We would also like to thank the BioInvasion Records reviewers who have contributed to the improvement of this article.Peer reviewe