First documented occurrence
The first official sighting of Didemnum vexillum in the Netherlands was in 1991.
In 1998-1999, Didemnum vexillum suddenly became very common in the Grevelingen and Oosterschelde (see below and the Zeeland map), covering much of the available hard substratum. Most of the colonies die in winter (December and January), and therefore other organisms (ascidians, sponges, sea anemones, among others) are able to settle in early spring, before the didemnids begin to expand their colonies. The D. vexillum colonies are more successful (larger) in the Grevelingen than in the Oosterschelde. They are most common at depths between 3 and 12 meters in the Grevelingen and from just below low water to 14 m in the Oosterschelde. The colonies grow over all hard substratum (rocks, mussels, oysters) and also over other organisms (hydroids, tunicates) that are normally not overgrown by other species. Source: A. Gittenberger.
Overview of the Grevelingen and the Oosterschelde
The Grevelingen is a large sea water lake, which is separated from the North sea by a dike (see Zeeland map). There are no currents or tides. Every day a relatively small corridor to the North Sea is opened, enabling North sea plankton to enter the Grevelingen. Over the last 5 years the water temperature has varied between 2 degrees C in the winter to approximately 19 degrees C in the summer. The salt gradient in the Grevelingen is, on average, higher than in the North Sea.
The Oosterschelde is a large inlet on the North Sea with currents and a maximum tidal range of approximately 6 meters. Over the last 5 years the water temperature has varied between 2 degrees C in the winter to approximately 19 degrees C in the summer. The salt gradient in the Oosterschelde is, on average, the same as in the North Sea, and therefore lower than in the Grevelingen.
In the North Sea off the Netherlands, the annual water temperature varies between approximately 5 degrees C and 16 degrees C. Source: A. Gittenberger.
Web sites with imagery and text
See section on Imagery of the Family Didemnidae.
Northern Europe regional map