Exotics in the coastal zone

Some exotic species that have ended up in Dutch coastal waters have increased greatly in numbers. Examples include the Asian tunicate, Haliclona xena and the American razor shell.

Trends in the Asian tunicate

The Asian tunicate is found along the coasts of Japan, Korea and Siberia. It was first found in Europe in 1954, near Plymouth in the United Kingdom. Later, there were sightings in Australia and on the west coast of America. This species has spread over the world in a short time, presumably because it attaches itself to the hulls of ships.

The first sighting in the Netherlands was in 1974. Since then, the species has been increasing sharply in areas such as the Oosterschelde (see figure). There are no indications that it is deleterious for other species; on the contrary, it provides an attractive substrate for native sea squirts.

Trends in the American razor shell

The American razor shell used to be restricted to the east coast of America. It was first recorded in Europe in 1979, in the Elbe estuary, where it is thought that larvae had been released when ships emptied their ballast tanks. From there the species spread to Denmark and the Netherlands. The first sightings in the Netherlands were on Schiermonnikoog in 1982. It reached Den Helder in 1982 and IJmuiden in 1985. It was first found in the Katwijk-Noordwijk area in 1986, and since then it has been increasing spectacularly along the Dutch coast.

Trends in Haliclona xena

Haliclona xena was first recorded in 1977, in the Oosterschelde, where it has since increased rapidly. It is not known where the species came from, though it is assumed that it was brought in with imported oysters. So far, it has only been found in the Netherlands, but it is assumed that it comes from elsewhere.