Biodiversity: Exotic Species

Razor clam

We have observed a great many changes in our marine flora and fauna in recent years. Global warming is having a knock-on effect on marine biodiversity.

  • Several northern species are now less frequently observed in the southern North Sea: shrimp, cod, haddock and halibut, the lion's mane jellyfish.
  • Several southern species are extending their environment to the north; this is particularly the case with invertebrates: the grey swimming crab, volcano barnacle, small hermit crab, Pennant's swimming crab, velvet crab, razor clam, limpet, whelk. There are also growing numbers of southern fish species such as the wrasse, seahorse, sardine, red mullet and anchovy.
  • It would appear that species from tropical and subtropical seas that reach our coast through shipping or aquaculture (known as exotic species), benefit from the effects of rising temperatures. Some have multiplied so exponentially that they now represent a dominant element of our marine flora and fauna. The American razor clam was first observed here in 1987, but today its shells are washing up on our beaches by the million. The Japanese oyster first established itself here in the early 1990s, but is now found on breakwaters and harbour walls in huge numbers. Two recent Asian introductions, the Asian shore crab (2003) and the Pacific crab (2006) are now very common indeed (2014). At present we know of 120 exotic species in the North Sea as a whole, but the true number is undoubtedly higher than this.
Japanese oyster

Endemic species are losing some of their geographical distribution area through competition with exotic species. The numbers of some exotic species have even grown into plague proportions; in these cases we describe the species as invasive, the common slipper shell for example. This shellfish came over from America with oysters in the late 19th century and became a real pest for local oyster farmers. However, as yet we know of no species endemic to the North Sea that has become extinct due to the introduction of an exotic species. But the phenomenon has been observed in fresh water and estuaries. Exotic species can also have an adverse effect on the reproduction of our endemic species. The American and endemic crab can mate, for example, but their offspring are sterile.

Exotic species can have the effect of increasing biodiversity. But the newcomers are mostly fast-growing species, more resistant to disturbance and pollution. They are at home in harbour and coastal areas with artificial, hard substrates and in areas that have become impoverished through overfishing. There is a danger that the world's marine flora and fauna will become homogenous and uniform. So, although introductions might enrich local biodiversity, they could also bring about worldwide impoverishment.

There is also a real danger that exotic species bring organisms and disease along with them, which harms endemic species . The oysters from Japan, for example, brought a single cell parasite that damages the endemic oysters. Introduced phytoplankton species can cause a toxic plankton bloom, rendering the oysters and mussels unfit for consumption.

Orange barnacle

The deliberate introduction of exotic species is forbidden under Belgian law (Art. 11 of the Act concerning the Protection of the Marine Environment (MMM Law, 1999). Measures such as the Marine Strategy Framework Directive (MSFD 2008/56/EC) are important policy instruments at a European level. Around the world, techniques are being sought to treat ballast water and so inhibit the import of species in this way. Aquaculture is a growing source of species introduction and so measures should also be put in place in this area.

The OD Natural Environment has years of expertise in the identification and study of marine introductions. It represents Belgium in two working groups for the International Council for the Exploration of the Sea (ICES), which are concerned with the introduction and transfer of marine organisms and the role of shipping and other vectors in their spread.

Go to top