Abstract
Spatio-temporal changes in genetic structure among populations provide crucial information on the dynamics of secondary spread for introduced marine species. However, temporal components have rarely been taken into consideration when studying the population genetics of non-indigenous species. This study analysed the genetic structure of Styela plicata, a solitary ascidian introduced in harbours and marinas of tropical and temperate waters, across spatial and temporal scales. A fragment of the mitochondrial gene Cytochrome Oxidase subunit I (COI) was sequenced from 395 individuals collected at 9 harbours along the NW Mediterranean coast and adjacent Atlantic waters (> 1,200 km range) at two time points 5 years apart (2009 and 2014). The levels of gene diversity were relatively low for all 9 locations in both years. Analyses of genetic differentiation and distribution of molecular variance revealed strong genetic structure, with significant differences among many populations, but no significant differences among years. A weak and marginally significant correlation between geographic distance and gene differentiation was found. Our results revealed spatial structure and temporal genetic homogeneity in S. plicata, suggesting a limited role of recurrent, vessel-mediated transport of organisms among small to medium-size harbours. Our study area is representative of many highly urbanized coasts with dense harbours. In these environments, the episodic chance arrival of colonisers appears to determine the genetic structure of harbour populations and the genetic composition of these early colonising individuals persists in the respective harbours, at least over moderate time frames (five years) that encompass ca. 20 generations of S. plicata.
Highlights
The increase in maritime traffic and aquaculture activities in recent decades has fostered the spread of non-indigenous marine species (NIS) across the globe (Carlton, 1996; Galil, 2000; Grosholz, 2002; Zenetos et al, 2012)
Large ports with commercial and overseas cruise activities are interspersed among our sampling areas (Barcelona, Tarragona, Valencia, Cartagena, and Algeciras have the highest volumes of traffic) and can act as initial entry points of introduced species from other seas
Through the analysis of the genetic structure of populations of an introduced ascidian over ca. 1,200 km of coastline at two time points five years apart, we have found significant spatial differentiation, but negligible temporal variation
Summary
The increase in maritime traffic and aquaculture activities in recent decades has fostered the spread of non-indigenous marine species (NIS) across the globe (Carlton, 1996; Galil, 2000; Grosholz, 2002; Zenetos et al, 2012). In highly urbanized coastal areas, dense networks of harbours and artificial structures can act as steppingstone strongholds for the propagation of introduced species (Glasby et al, 2007; Dafforn, Glasby & Johnston, 2012; Lopez-Legentil et al, 2015; Airoldi et al, 2015) These networks provide unique opportunities for the study of dispersal mechanisms and processes occurring during secondary spread of NIS. Populations in small harbours and marinas are likely subject to high stochasticity in the arrival of individuals, resulting in the low levels of genetic diversity previously reported (Dupont et al, 2007a; Perez-Portela, Turon & Bishop, 2012; Rius & Shenkar, 2012) Unravelling the genetic signatures of these processes will provide insight into the secondary spread of NIS in the area and contribute to the broader knowledge of introduced species dynamics in highly urbanized coasts
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