Oceanographic barriers to gene flow promote genetic subdivision of the tunicate in a North Sea archipelago.

Pelagic larval development has the potential to connect populations over large geographic distances and prevent genetic structuring. The solitary tunicate has pelagic eggs and a swimming larval stage lasting for maximum a few days, with the potential for a homogenizing gene flow over relatively large areas. In the eastern North Sea, it is found in a geomorphologically complex archipelago with a mix of fjords and open costal habitats.