The Waddenseadike along the south coast of the inhabited part of Terschelling, one of the Frisian islands, the Netherlands, is protecting the lowlands of that island from inundation by the Waddensea. The history of Terschelling shows us numerous events that parts of the dike collapsed during extremely stormy weather. Until fairly recently such dike breaches in the form of round, fairly deep pools (locally called ‘wiel/wielen’) just behind the dike, could still be traced at six places along the Waddenseadike. Now only two pools and a tiny part of a third one are still present. Two of these pools are of the brackish water type (the Pool near Seeryp and the Formerumerwiel), while the still existing pool (Ponswiel) and the three, most eastern pools (Ronde Wiel, Westerwiel and Oosterwiel), which disappeared from the landscape of Terschelling due to dike enforcements in 1968/70, may be classified as freshwater habitats. So far 16 molluscan species have been found in these pools. One is a brackish water species ( Hydrobia ventrosa and another an amphibian land snail (Oxyloma elegans).