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Massive DNA Sequencing of Ballast Water in Ice-Breaking Research Vessel Detects Cold-Tolerant Invasive Species: Up to 12 Billion Tons of Water Transported in Ship Ballast Tanks Each Year; 7000 Different Species Carried Each Day

The German ice-breaking research vessel “Polarstern” (photo) travels thousands of miles between the Northern and Southern hemispheres in search of biological samples. In addition to the members of its crew and staff of scientists, this ship has some additional on-board passengers: namely, organisms that can adapt to extreme water temperatures and could potentially invade the new waters where this ice breaker takes them. By analyzing the DNA present in this vessel's ballast water, a team of scientists has obtained the first molecular evidence of the persistence of DNA belonging to a tiny sea snail that is capable of tolerating adverse conditions. The new work has been published in the November 2015 issue of the Journal of Molluscan Studies. The article is titled “Environmental DNA Evidence of Transfer of North Sea Molluscs Across Tropical Waters Through Ballast Water.” 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 European report that analyzed 15 samples of ballast water, live specimens of more than one thousand species were discovered in ship tanks that arrived to European ports. These taxa, however, must face very harsh conditions upon arrival: darkness, temperature changes, salinity, murky waters, turbulence, and a lack of oxygen. Not all of the species will survive, and the ones that do become potential invasive species. In order to identify which organisms are most capable of tolerating non-native waters and are thus the most invasive, a team of researchers led by the scientists at the University of Oviedo in Spain, and including colleagues from Klaipeda University in Lithuania, analyzed the environmental DNA present in the 70 square meters (~753 square feet) of ballast water in the tank (filled with water from the North Sea) of the scientific research vessel “Polarstern.” This ship traveled between Bremerhaven, Germany and Cape Town, South Africa between October 2012 and December 2012. [BioQuick Editor’s Note: Spain’s University of Oviedo was founded in 1608.]

"Seeing as this ballast water has traveled from the north to the south and has even crossed the tropics, it has thus been subjected to extreme temperature variations in addition to anoxic conditions [a total lack of oxygen]," explains Alba Ardura, Ph.D. Dr. Ardura is the first author of the new study and presently a researcher at the University of Perpignan in France. At the time of the work, Dr. Ardura was affiliated with the University of Oveido.

IS THIS SNAIL A POTENTIAL INVASIVE SPECIES?

While the filling up of the ballast water tank, the organisms that were alive upon entry into the tank in Bremenhaven might have been subjected to conditions of stress which could result in their death, thus meaning that “the number of DNA molecules would decrease over the course of the trip," points out Dr. Ardura.

Nonetheless, this is not what happened to the laver spire shell snaill, also called the mudsnail (Peringia ulvae).

"The number of one of the haplotypes (variations in DNA within the same species) increased during the trip," Dr. Ardura notes.

Animals like this invertebrate leave behind traces of their presence in the waters where they live. These traces can include dead cells that have sloughed off or fluids. In this case, the mudsnail left behind traces in the ballast water.

It is possible to extract DNA from a sample of water in order to determine what species are living there. "We can thus have a wide range of information about the species which are present in the environment we are analyzing without having to carry out individual sampling one by one," observes the expert.

Although finding evidence of this mollusc does not confirm that it is alive, “it does confirm the resistance of its DNA to adverse conditions," says Dr. Ardura.

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 previously described its ability to tolerate diverse ecological conditions.

MASSIVE SEQUENCING OF ENVIRONMENTAL DNA

For the researchers, environmental DNA and its massive sequencing represent a "very promising" tool for rapid biodiversity analysis and the detection of potentially invasive species that are present in ships' ballast waters.

It should be pointed out, however, that presently "the tool has its limitations which need to be remedied in order to develop an effective and robust method for applications in this field," according to Dr. Ardura.

IMAGE

The image shows the German research vessel “Polarstern” during its trip between Bremerhaven, Germany and Cap Town, South Africa in 2012. (Credit: Anastasija Zaiko).

[Press release] [Jounrnal of Molluscan Studies abstract]