Ongoing expansion of the worldwide invader Didemnum 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 as Didemnum vexillum Kott, 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 of D. vexillum are 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 spring D. vexillum reaches 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 using COI sequence 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.Electronic supplementary materialThe online version of this article (doi:10.1007/s10530-015-0861-z) contains supplementary material, which is available to authorized users.

Seasonal patterns of settlement and growth of introduced and native ascidians in bivalve cultures in the Ebro Delta (NE Iberian Peninsula)

“…2015). In the Mediterranean it was recently reported from the Venice Lagoon 172 (Tagliapietra et al 2012) and in the Ebro Delta area (Ordóñez et al 2015).…”

“…During the study, it was present on the bivalve culture 338 ropes from Fangar Bay with less cover than in Alfacs Bay but still much higher than on our 339 PVC plates. Similarly, D. vexillum did not grow during the first year on the plates, in 340 contrast with the abundance and size of the colonies on bivalve culture ropes during the 341 same period, and in spite of having been initially deployed during the reproductive period 342 of the ascidian in the area (Ordóñez et al 2015). Thus, for the purpose of monitoring 343 activities for ascidian detection and abundance estimates the best strategy is to use both 344 clean and colonized experimental surfaces.…”

“…It thrives in artificial habitats, but it 182 can also colonise adjacent natural substrates Ordóñez et al 2013a). 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 230 For the study of cover over time, we selected the species with at least 100 occurrences, 231 with three exceptions: Didemnum vexillum was included even if it appeared late in the 232 study (23 occurrences in summer and fall 2016) because it is abundant in the nearby 233 oyster ropes and is a well-known nuisance in these cultures (Ordóñez et al 2015); 234 Diploso...…”

Origin and Dispersal History of Two Colonial Ascidian Clades in the Botryllus schlosseri Species Complex

“…Significant attention is focused on the species that are thriving or will thrive in these new ecosystems in order to predict how these new ecosystems will function [7–12]. …”

Real‐time PCR detection of Didemnum perlucidum (Monniot, 1983) and Didemnum vexillum (Kott, 2002) in an applied routine marine biosecurity context

“…; Ordóñez et al . ), and therefore, the method is expected to be useful in detecting D. perlucidum at introduced worldwide locations. However, as with many other tropical introduced ascidians in Australia (Kott ; Zhan et al .…”