Biological invasions can promote secondary contacts between related species and thus provide excellent case studies for investigating the joint ecological and evolutionary trajectories of interfertile taxa. Here, we studied two tunicates living in sympatry, and sometimes in syntopy, in the English Channel, Ciona intestinalis species A (presumed native to the NW Pacific) and species B (native to the N Atlantic). In addition to monitoring their co-existence over time, we examined the level of interspecific gene flow, a process that may increase the invasiveness of the non-native species. The sampling scheme was repeated twice a year for 3 years (six distinct generations) to determine the relative abundance of the two species in 11 localities along the coasts of the English Channel and the Iroise Sea (covering 1600 km) in Brittany, France. We examined 23,000 individuals, including 5315 specimens for which reproductive status was determined. Four speciesdiagnostic molecular markers traced interspecific gene flow on a random subset of 3048 individuals. Regardless of the sampling date, the two species co-occurred in most of the study sites, with species A showing higher frequency in the autumn. The regional pattern of seasonal variation in relative abundance of the two congeners appears to correspond to different thermal growth optima. Abrupt variations in environmental parameters can act synergistically and may favor the non-native species locally. Despite syntopy, synchronous gamete production and successful in vitro interspecific crosses, only 4.3 % individuals Electronic supplementary material The online version of this article (showed an admixed genome (i.e. footprint of present-day or past introgression events), most of them with a species A maternal lineage, of which only one was a putative first generation hybrid. Altogether, efficient barriers seem to prevent interspecific crosses between the two interfertile congeners in the wild: present-day hybridization may have less impact than competitive interactions on the fate of the two study species over their sympatric range.
A comprehensive expert consultation was conducted in order to assess the status, trends and the most important drivers of change in the abundance and geographical distribution of kelp forests in European waters. This consultation included an on-line questionnaire, results from a workshop and data provided by a selected group of experts working on kelp forest mapping and eco-evolutionary research. Differences in status and trends according to geographical areas, species identity and small-scale variations within the same habitat where shown by assembling and mapping kelp distribution and trend data. Significant data gaps for some geographical regions, like the Mediterranean and the southern Iberian Peninsula, were also identified. The data used for this study confirmed a general trend with decreasing abundance of some native kelp species at their southern distributional range limits and increasing abundance in other parts of their distribution (Saccharina latissima and Saccorhiza polyschides). The expansion of the introduced species Undaria pinnatifida was also registered. Drivers of observed changes in kelp forests distribution and abundance were assessed using experts' opinions. Multiple possible drivers were identified, including global warming, sea urchin grazing, harvesting, pollution and fishing pressure, and their impact varied between geographical areas. Overall, the results highlight major threats for these ecosystems but also opportunities for conservation. Major requirements to ensure adequate protection of coastal kelp ecosystems along European coastlines are discussed, based on the local to regional gaps detected in the study.
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