Colonizing the open sea: population dynamics of an abundant marine invader on a natural shoreline

Ascidians have successfully colonized numerous environments around the world. Despite the broad knowledge on distribution and population dynamics of introduced ascidians in man-made environments, little is known about these same traits in natural environments. Styela clava, an ascidian originally described from northeast Asia, was introduced in San Matías Gulf, Northern Patagonia Argentina, around 2013. The aim of the present study was to examine the spatio-temporal variability of population traits and growth rates over two years, through monthly field samplings at two contrasting intertidal sites (exposed vs. sheltered) in San Matías Gulf. We also considered two environment types: tidal pools and rocky ledges. Additionally, we extended the annual samplings for another 4 years, as part of a monitoring program for the species. Population density was higher on rocky ledges than in tidal pools in the two sites studied. Annual densities of S. clava increased on rocky ledges in both sites. The length frequency distribution was uni- or bimodal at the exposed site, whereas at the sheltered site it showed a bi- or trimodal pattern. In the exposed site, two-peak recruitment occurred, while at the sheltered site, up to three peaks were observed, suggesting that S. clava could reproduce at least twice a year. The growth rates did vary spatio-temporally in a year. These results highlight the plasticity of S. clava, which allows it to colonize diverse environments, and the importance of population studies to reflect the colonization processes of introduced species.