The dispersive nature of the marine environment and extensive maritime trade mean that many species were introduced into the gulf fairly early, but the rate accelerated after the Second World War, similarly to reports on various other areas along the European coastline.
Given the lack of major port infrastructures, it seems as though the gulf was not a major point of entry for species introduced into Europe or France; instead it appears to have been a region colonised during the expansion stages of species introduced via a natural route (larval dispersal) or via human activities, particularly shellfish farming.
The historical and geographical context is needed not just to see the contemporary impacts that species introduction has on a biological compartment or human activities, but also to understand the dynamics of this introduction and its repercussions at different spatial and temporal scales, and to prepare suitable management plans if necessary.
Among the species presented, some have remained quite geographically confined (F. enigmaticus) while others have undergone proliferation phases since they were introduced into the gulf, some of which were quite dramatic (common slipper limpet, Pacific oyster). The perception of these species has changed over time. The common slipper limpet gives a good example of the a priori ‘positive’ consequences for local benthic diversity, but when it is very proliferic within a site, it may reduce regional diversity by homogenising communities and may present a risk for certain human activities.
Furthermore, although the introduction of some species can be spectacular in some parts of the world, others may be introduced in a completely localised and non-widespread manner, like F. enigmaticus, which multiplies to such an extent in certain South American areas that it clogs power station cooling systems.
Other species, such as the Manila clam, are also confined to certain areas of the gulf, but their very recent introduction and expansion probably cannot be used to draw any conclusions about their possible future distribution areas.
Finally, the invasion of other species (such as E. modestus and M. sextonae) remains almost imperceptible to most actors as it is tricky to identify them and they are small in size.
Evaluation of the risk presented by the introduction of species in the marine domain is therefore complex and must be carried out individually for each species, each region and each stage of colonisation in time and space. Moreover, the current definition of the good environmental status of coastal zones with regard to the Marine Strategy Framework Directive (MSFD) is partly based on this evaluation.