title: Ongoing expansion of the worldwide invaderDidemnum vexillum(Ascidiacea) in the Mediterranean Sea: high plasticity of its biological cycle promotes establishment in warm waters.
abstract: Non-indigenous ascidians are of particular concern to aquaculture industry and, paradoxically, the activities associated with it represent an important way to translocate these species worldwide. In 2012 a non-indigenous ascidian was found covering the oyster crops in the Ebro Delta (Western Mediterranean). We have identified the ascidian genetically and morphologically asDidemnum vexillumKott, 2002. This finding indicates that the species is currently expanding its distribution in the Mediterranean Sea, as it has recently been found in the eastern basin (Venice, Adriatic Sea). Introduced populations ofD. vexillumare found in temperate and cold waters worldwide, and a successful establishment in the Mediterranean implies a remarkable capacity of adaptability to warm, subtropical conditions. We assessed the life cycle (growth and reproduction) of the ascidian at the studied site. The species has a marked seasonal cycle, with regression in the warmest months and reappearance during winter. In springD. vexillumreaches its maximum abundance, followed by a peak in reproduction just before regression. This cycle is reversed with respect to the one observed in colder waters, highlighting a plastic biological cycle of this invader and an hitherto unknown ability to establish itself in warm waters. We also analysed the genetic structure of the population of the Ebro Delta and the one established in the Lagoon of Venice usingCOIsequence data. The low genetic diversity in our samples (three haplotypes) was consistent with what is observed in the introduced populations worldwide. It is likely that the ascidian was introduced with oyster stock from bivalve cultures in the Atlantic French coasts, where the same three haplotypes have been reported. The high boating activity in the Ebro Delta makes further human-mediated transport of the species highly likely, and nearby fishing grounds can be severely affected if invaded. It is urgent to implement measures to prevent the continuous expansion of this ascidian pest in the Mediterranean.
authors.affiliation: Department of Genetics and IRBio, University of Barcelona, Barcelona, Spain.
authors.forename: V
authors.initials: V
authors.lastname: Ordóñez
authors.affiliation: Department of Genetics and IRBio, University of Barcelona, Barcelona, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Pascual
authors.affiliation: Institute of Agriculture and Food Research and Technology (IRTA), S. Carles de la Ràpita, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Fernández-Tejedor
authors.affiliation: Australian Institute of Marine Science (AIMS), Townsville, Australia.
authors.forename: M C
authors.initials: MC
authors.lastname: Pineda
authors.affiliation: CNR - National Research Council of Italy, ISMAR - Marine Sciences Institute, Arsenale - Tesa 104, Castello 2737/F, 30122 Venice, Italy.
authors.forename: D
authors.initials: D
authors.lastname: Tagliapietra
authors.affiliation: Center for Advanced Studies of Blanes (CEAB-CSIC), Acces Cala S Francesc 14, 17300 Blanes, Girona Spain.
authors.forename: X
authors.initials: X
authors.lastname: Turon
keywords: Aquaculture; Ascidian; COI; Growth rate; Introduced species; Maturity index