Why are mussels still common in Japan?

Pacific oyster

You can see more and more Pacific oysters in the Wadden Sea, the Oosterschelde and Grevelingen. Although they taste great, not everyone is happy with their appearance. The bowls are razor-sharp, you can easily cut yourself on them. And in some places so many oysters are now growing that they are displacing other mussels. Other soil animals, on the other hand, find a safe environment and solid ground. Although many birds struggle to open the Pacific oysters, some species of birds seem to be just discovering this treat.

Pacific oysters for restaurants and fishmongers

The Pacific oyster has been grown on a relatively large scale in the Oosterschelde since the 1960s. In the trade, the Pacific oyster is known as ‘Creuse’ (in the Netherlands). Pacific oysters are good for breeding. They reproduce when they are one year old and grow very quickly. After two years, the creuse weighs around 100 grams (without the shell) and is then suitable for consumption. This takes much longer for the European oyster: a European oyster only weighs around 75 grams after four years.

Distribution and habitat of the Pacific oyster

The Pacific oyster is native to the Asian coast of the Pacific. They are now also native to North America, Australia, Europe and New Zealand through humans. The first Pacific oysters were brought to the Netherlands from Portugal as early as 1883 as a test to see whether they could be farmed here. They were called Portuguese oysters. These have a slightly different color and shape than the Pacific oysters from Japan. Since the 1960s, the Pacific oysters from Japan have been brought to Zeeland in large quantities by oyster farmers. The European oyster suffered badly from oyster disease and the breeders thought the Pacific oyster would be stronger. The breeders expected that the oyster would not reproduce by itself in the cold Dutch waters and that they could bring the imported oyster safely to the Netherlands. Unfortunately, the cold did not prove to be a major problem, as the oysters multiplied en masse in the mid-1960s.

In 1983 the animal was introduced in the Wadden Sea. The Pacific oyster is now found in the Wadden Sea and in the delta area. The number of Pacific oysters in these areas is still increasing. The oysters need solid ground in order to settle, but this area does not necessarily have to be large. An empty oyster shell is a good basis for a new oyster reef. The largest reefs are found on sand plateaus and dykes, which fall dry at low tide, but they can also occur up to 80 meters deep.

The new reefs of Pacific oysters

In the Oosterschelde there are large quantities of Pacific oysters and in the Wadden Sea they are increasing. The Pacific oysters eat a lot more algae, take up a lot more space and, by chance, also eat larvae from other mussels. Biologists feared that the competition from the Pacific oyster would make the common mussels, such as cockles and mussels, much rarer. This can get into trouble for shell-eating birds such as eider ducks and oystercatchers.

But the oyster reefs are also a kind of rocky subsoil for many marine animals, such as sea anemones, barnacles and small fish, which otherwise would not find a suitable habitat in the world of sand and silt. In the past, this function was taken over by the mussel beds, but they are much less stable than the oyster reefs and, moreover, have become rare as a result of the mussel fishing.

The oyster reefs, in turn, are part of the solution to the mussel problem, as the somewhat older reefs are a suitable underground for mussels. This is how mixed mussel beds have emerged in many places.

Birds and oysters

It is not that easy for a bird to open Pacific oysters. Especially not if they are all stuck together or have attached themselves to a stone or a dike. But in 2007, scientists observed that oystercatchers and seagulls began to learn how to crack this pesky prey. Oystercatchers stick their bills into oysters that are a little bit open. You can then open the shell wider and eat the oyster. Seagulls are more rigorous. They take oysters lying around and then drop them out of the air onto the sea dike. At first it didn't work that well (30% successful), but now it almost always works (90%). If you ride your bike over Texel's Wadden Sea dykes, you will find broken oyster shells everywhere.

Pacific oysters on Texel

As early as the 1970s, a Pacific oyster could sometimes be found in the tidal flats and on the dykes of Texel. Since the late 1990s, they have become numerous in the Texel mudflats. There are now large oyster reefs in the Mokbaai, near Zeeburg and De Cocksdorp. The oysters are collected in small quantities. You can order them in Texel's restaurants.

Properties of Pacific oysters

  • Dimensions:
    up to 20 centimeters, sometimes much larger
  • Colour:
    white to grayish
  • Food:
    plankton
  • Enemies:
    almost no one, but recently gulls and oystercatchers, diseases, parasites, humans
  • Reproduction:
    sexually

Names

  • Never: Japanse oester (creuse, wilde kreuse)
  • Lat: Crassostrea gigas
  • Closely: Pacific oyster (Japanese oyster)
  • Deu: Pacific oyster (rock oyster)
  • Danish: ze Latijnse naam (Øster)