Basin-wide patterns of abundance of a widespread invasive herbivore in the Mediterranean Sea

Biological invasions continue to accumulate globally, yet basic ecological information is lacking for many widespread marine non-native species. The tropical–subtropical herbivorous crab Percnon gibbesi has expanded rapidly throughout the Mediterranean basin but quantitative assessments of its abundance remain geographically limited. This knowledge gap is particularly relevant given the demonstrated capacity of the species to heavily graze macrophytes under laboratory conditions and the rapid proliferation of artificial coastal structures that may further enhance its abundance. To address these gaps, we quantified P. gibbesi densities at 79 sites across the Mediterranean Basin, encompassing both natural rocky habitats and artificial hard substrates. On average, P. gibbesi was nearly twice as abundant on artificial structures as on natural substrates, and its abundance increased towards the eastern Mediterranean. Although mean densities were high (36.2 individuals per 100 m²), abundance varied markedly among sites and regions, suggesting that - assuming a positive relationship between abundance and impact - the ecological impacts of this invader are likely to be highly heterogeneous across Mediterranean landscapes. Together, these findings provide a basin-wide ecological baseline and improve understanding of how coastal urbanization and geographic gradients can shape marine invasion dynamics.