Rendez-Vous at the Baltic? The Ongoing Dispersion of the Black-Striped Pipefish, Syngnathus abaster

Syngnathus abaster is a pipefish whose distribution is commonly referred as encompassing the Mediterranean and Black Sea, as well as the Atlantic Coast northwards up to the Bay of Biscay. Given the species ability to endure large variation in water salinity, this euryhaline pipefish may now be encountered in a wider variety of aquatic environments, not only marine or brackish but also in purely freshwater habitats. During the last decades, information on the biology of the black-striped pipefish grew, and new data emerged. As this information derives from populations that are geographically distant, it is important to review the available data across the distribution in order to understand the more general processes occurring. While summarizing the distributional information, collected data suggests that S. abaster is expanding its spatial distribution in two very distinct ways, either by slowly moving north through the Eastern Atlantic Coast, or by successively establishing viable populations in freshwater ecosystems, in the east. Available data on reproduction suggests that all the sampled populations reproduce at roughly identical temperature ranges, advocating that this species is mainly a spring and summer spawner, at least when inhabiting marine environments. Nevertheless, important inter-populational differences exist in several reproduction-related variables, such as the maximum egg number carried by pregnant males, hinting that the current taxonomy is in need of an extended revision. Open questions related to the causes and implications of freshwater colonization are also further discussed.