A place for everyone
You find a large amount of unusual living areas in the Wadden Sea. Natural forces have an obvious influence on where specific species can live. Regions flood, bringing land plants and animals in contact with seawater. On the other hand, marine life has to deal with the large extremities of a land climate, such as high temperatures versus frost and fresh rain showers versus long periods of dryness. The habitats in the Wadden Sea form transition areas between wet and dry, fresh and salt, high and low and land growth and erosion.
The plants and animals that live here are prepared for all these moments, such as the periwinkle which closes its house during low tide until the water rises again. Even the content of the water is constantly changing. Where rivers flow unhindered into the sea, you find brackish regions. When extra large amounts of river water enter the area, the brackish water can suddenly turn fresh. Starfish do not like freshwater and therefore you won't find them in these susceptible regions. However the flatfish flounder easily survives in environments that readily change between salt to fresh. It feels at home is such brackish regions.
Living organisms choose a place that fits them the best. But that does not mean that they are defenseless against the natural elements. Some can change their habitat in such a way that not only can they survive even better, but other plants and animals profit as well. Mussels, for example, attach themselves to each other to stay in place, making them less vulnerable to storms. Because they catch mud particles, they keep climbing higher up. Due to the sturdy three dimensional structure, a habitat is created for more and more plants and animals. In this manner, mussels are continually improving their own habitat for the benefit of themselves and others. Lugworms do just the opposite. Their burrowing in the seabed results in a continual movement in the Wadden Sea bottom. This keeps the sandy floor sandy so that it does not readily change into a slippery muddy surface.
Even plants are capable of affecting the Wadden Sea. Salt marshes literally evolve out of the sea, as the thick vegetation sifts sediment out of the water during flood. The Wadden Sea renews itself time and again as the waves break off pieces of salt marsh borders elsewhere.