Reproductive performance and propagule pressure: Key drivers of Rugulopteryx okamurae (Ochrophyta, Dictyotales) invasion of a Posidonia oceanica meadow

Abstract

The invasive seaweed Rugulopteryx okamurae has rapidly spread along the Mediterranean and Atlantic coasts, displacing native species and causing unprecedented environmental and socioeconomic impacts. One of the threatened habitats are Posidonia oceanica meadows. In this habitat the vegetative performance of R. okamurae is already known, but the reproductive mechanisms that allow its continued dominance in space and time are still not known. Thus, the objective of this study is to investigate the reproductive performance of R. okamurae invading a P. oceanica meadow in Southern Spain, identifying and quantifying the reproductive structures, their temporal and intrapopulation changes and, the potential environmental factors triggering them. The results do not rule out the presence of sexual reproduction (only meiotic tetraspores have been observed, but no gametangia), suggesting that the population is mainly maintained by clonal mechanisms based on the formation of vegetative propagules and mitotic monospores that form throughout the thallus on both sides, more abundantly in larger individuals. Due to the greater ontogenetic maturity of the propagules and their larger presence throughout the year (only absent in January), we can say that the populations are mainly maintained thanks to them, since unicellular monospores were only observed in May and July. Estimates of propagule pressure indicate that it exceeds the carrying capacity of the species' benthic habitat and that, therefore, new individuals are exported to the water column. This fact would explain the enormous amount of biomass in suspension, facilitating the dispersal and colonization of the species and the colonization of neighboring areas, where it produces severe the socio-economic impacts.