4. Invasive Species in the Ocean

Global distribution of marine invasive species

Marine invasive species are found all around the world. They are especially common in coastal areas in and around harbours, as most of them have been transported by ships (the transport of alien species will be explained later on). The oceans which are most affected by marine invaders are the Northern Atlantic, the Northern Pacific and the Eastern Indo-Pacific (see map below). In the Northern Atlantic for example, 240 alien species have been recorded, 57% of which are harmful or invasive (Molnar et al. 2008). Marine invasive species include fish, mussels, algae, corals, herbs and seagrass.

Global map showing the number of harmful alien species by coastal ecoregions. The darker red shades indicate a greater number of species with a high impact on the local ecology. The blue colour indicates ecoregions with alien species that have been described as less harmful.
Source: Molnar et al. 2008

Some species invading new environments in the oceans are not of marine origin.

The chinese mitten crab (Eriocheir sinensis), for example, predominantly lives in freshwater but migrates seawards to breed. The mitten crab is native to Fukien Province in China to the western coastal regions of Korea.

This Chinese mitten crab was caught in the Upper Chesapeake Bay in the Eastern United States.
Photo: Gregory Ruiz / Smithsonian Environmental Research Center

In 1912 a specimen was discovered in the River Aller, a tributary of the Weser, in Germany. Since then the mitten crab has distributed all over Europe, from Northern Finland to Southern France, and on the Northern American continent. Juvenile mitten crabs can migrate as far as 1500 km while growing to adult size. They are also capable of crossing dry land to enter new river systems.
The mitten crab was probably introduced to Europe through ship's ballast water, but intentional introductions cannot be ruled out. The mitten crab is edible and may have been imported for cultivation.