Maritime transport is considered one of the most important ways that native species are moved between marine regions. The trip can be especially successful if these species latch on to the vessel's anchors or chains, or even if they travel in the ship's ballast water tanks. Each year, between 2.2 and 12 billion tons of water are transported around the oceans of the world in these ballast water tanks which also serve as a means of transport for about 7,000 species per day. In a survey colleagues found more than 1,000 taxa of living specimens in ballast tanks of vessels arriving in European ports.
A golden rule for successful invaders is ‘the more tolerant are the more dangerous'. Therefore, migrants that survive long cross-latitudinal voyages within ballast tanks should be of particular concern as potential invaders. Identifying such species is crucial for conducting reliable risk analyses, preventing expansions and developing efficient control methods. However, many species transported in ballast water as eggs or larvae are often very difficult to identify.
In order to identify which organisms are most capable of tolerating non-native waters and are thus the most invasive, a team of researchers from Spain and Lithuania applied metabarcoding of the environmental DNA present in 70 m3 of ballast water of the scientific research vessel Polarstern. The tanks were filled in the North Sea and the vessel traveled from Bremerhaven (Germany) and Cape Town (South Africa). Over the long travel distance the ballast water was subjected to extreme temperature variations in addition to anoxic conditions.
Organisms that were alive upon entry into the tank in Bremenhaven could have been subjected to conditions of stress likely resulting in their death, thus meaning that the number of DNA molecules within the tank would decrease over the course of a trip. However, this is not what happened to a mudsnail (Peringia ulvae). The researchers found that the number sequences of a particular haplotype increased during the journey.
Although this is not conclusive evidence confirming that the little mollusk is alive but it certainly confirms the resistance of its DNA to adverse conditions. Up until now there has not been any evidence of the presence of this small snail outside of its natural habitat, although some studies have indeed described its ability to tolerate diverse ecological conditions.
The results of this study indicate the high likelihood of species survival in ballast water or ballast sediments on cross-regional voyages. It can therefore be used for the species-specific risk assessments required by the Ballast Water Management Convention and for prioritizing species of greatest management concern.