Robin P. Gauff ¹ Francesco Mugnai ² Francesco Paolo Mancuso ^3* Francesca Porri ⁴ Federica Costantini ^5* Laura Airoldi ¹

• ¹ Chioggia Hydrobiological Station “Umberto D’Ancona”, Department of Biology, University of Padua, Padua, Veneto, Italy
• ² Dipartimento di Scienze Biologiche, Geologiche e Ambientali (BIGEA), UOS Ravenna,, University of Bologna, Bologna, Emilia-Romagna, Italy
• ³ Dipartimento di Scienze della Terra e del Mare (DiSTeM), University of Palermo, Palermo, Sicily, Italy
• ⁴ South African Institute for Aquatic Biodiversity, Grahamstown, South Africa
• ⁵ Department of Biological, Geological and Environmental Sciences, University of Bologna, Bologna, Italy

Marine coastal habitats are often characterized by strong gradients of anthropogenic disturbance such as pollution, typically most severe at urban waterfronts. These variations create stress for local organisms leading to their distribution along the disturbance as a function of their tolerance. nonindigenous species (NIS) are considered more tolerant to anthropogenic disturbances than their native counterparts, thriving in urban areas, where native species are sparser. It is however not yet entirely clear if these distribution patterns are due to larval behavior and preferential settlement or postsettlement processes. In the present study, we investigated the abundance of adults and settlers of two native (Ostrea edulis, Mytilus galloprovincialis) and two non-indigenous (Magallana gigas, Xenostrobus securis) bivalves along a strong marine urban environmental gradient. Oysters had sparse abundances of both adults and settlers at all sites, with no obvious distributional gradients. The two mussel species showed different settler-adult distributions along the gradient. Both settlers and adults of the native mussel M. galloprovincialis strongly decreased moving from the outermost periurban site to the innermost urban site, consistent with preferential settlement of larvae according to environmental conditions. The non-indigenous adult mussel X. securis showed a distribution pattern opposite to that of M. galloprovincialis, markedly increasing in abundance from the outermost to the innermost site. This was not paralleled by the distribution of X. securis settlers, which established over a larger area, including sites where adults were essentially absent. The mismatch between settler and adult distributions for X. securis suggests that post-settlement factors dictate spatial adult distribution.