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Wednesday, May 27, 2020
BERLIN, Apr 4 2011 (IPS) - Exotic species pose a threat to biological diversity in many parts of the world. But the invasion of the Baltic Sea by an oyster native to the Pacific coast of Asia is rather atypical in several ways.
More than 30 years later, as the waters of the North Atlantic have grown warmer from the effects of climate change, this exotic (non-native) bivalve has now spread north up the coasts Europe as far as Germany and Ireland.
Also known as the Japanese oyster or Miyagi oyster, the Pacific oyster differs from the European flat oyster (Ostrea edulis) in that its valves are narrower and curved and its flavour is more distinctive. Thanks to its hardy constitution it has made itself at home in the seas of northern Europe, competing with and displacing local species.
But unlike other invasions of exotic species, it seems that the Pacific oyster invasion has created new opportunities for other species and contributed to diversifying the fauna and flora of the North Atlantic and the Baltic Sea.
“The Pacific oyster acclimatised perfectly to our region,” said Achim Wehrmann, a geologist from the Department of Marine Research at the Senckenberg Research Institute in Wilhelmshaven, some 300 kilometres northwest of Berlin on the German coast of the Wadden Sea (Wattenmeer, in German).
The Wadden Sea, which lies between the Frisian Islands, the North Sea and the coasts of the Netherlands, German and Denmark, is an intertidal zone forming a shallow body of water with wetlands and tidal flats, which can be crossed by foot when the tide is out. Its floor, visible for several hours a day, is rich in nutrients and home to thousands of species.
According to Wehrmann, the Pacific oyster was identified in the Wadden Sea for the first time in 1998. Thirteen years later, around 15,000 tons of the oyster are harvested here every year.
The Pacific oyster’s migration revolutionised regional habitats.
“At the beginning of the invasion, the Asian oyster was content with occupying the areas of the Wadden Sea that are temporarily underwater,” explained biologist Christian Buschbaum, who works for the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, close to Wilhelmshaven. But as timed passed, the Pacific oyster migrated to the areas permanently underwater, the native habitat of the blue mussel (Mytilus edulis), also known as the common mussel. Because Pacific oysters form clusters of hundreds of individual oysters, grow more rapidly, and are bigger than the local mussels, they came to outnumber them.
However, “curiously, this invasion has not provoked any major harmful effects,” Buschbaum told Tierramérica.
“The local species accepted it. Although the oysters and mussels both feed on plankton, and now compete for it, the two species coexist well. The local mussel is a bit smaller than it was before the arrival of Crassostrea gigas, but, other than that, there have been no other negative impacts,” he said.
Moreover, the Asian shellfish brought with it a type of algae known as Japanese brown seaweed (Sargassum muticum), which has also spread throughout the North Atlantic and Baltic Sea.
The exotic algae has become a source of food for Haliichthys taeniophorus – a fish from the same family as seahorses – which for many years was considered to be threatened with extinction, added Buschbaum.
Nevertheless, Wehrmann cautioned that “the speed with which this oyster reproduces is a problem. Another problem is that it can be dangerous for humans,” he told Tierramérica. The oyster’s valves have very sharp edges and can cause painful cuts to tourists who walk barefoot during low tide, as well as to unaccustomed consumers.
“At the institute we are studying the oyster’s heavy metal content and other pathogenic risks, especially in terms of cholera, to develop parameters for comparison with other species,” explained Wehrmann.
Until now, the studies conducted have not uncovered any risks, although European health authorities have still not authorised their consumption as food, he added.
The growth of the Pacific oyster population contrasts with the fate of its local cousin, the European flat oyster, which is “very close to extinction” as a result of diseases, overharvesting, and the spread of the Pacific oyster itself, said Karin Dubsky of the environmental organisation Coastwatch Europe.
While Ostrea edulis is of no particular environmental importance, its protection “is a moral issue,” she said.
“Just as the whole world is concerned about the fate of the panda, the survival of the European flat oyster should be protected as well,” she stressed.
The fate of Ostrea edulis is shared by many other oyster species across the globe. According to an article published in February in BioScience journal, 85 percent of oyster reefs worldwide have been lost over recent decades.
An international research team led by marine biologist Michael Beck of the University of California, Santa Cruz examined oyster reefs across 144 bays and 44 ecoregions around the world. They concluded that oyster reefs in most bays (70 percent) and ecoregions (63 percent) are at less than 10 percent of their former abundance, based on comparisons with records from between 130 to 20 years ago.
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