Crassostrea gigas (Magallana gigas) - Japanese oyster

The Japanese oyster is native to Southeast Asia and Japan ^[3]. These molluscs can attach themselves with their lower shell to almost any hard surface or substrate in both marine and estuarine waters. They are also found in muddy and sandy areas, attached to assemblages of empty shells or on living beds of another species of shellfish ^[4]. Over time, a new oyster bank may develop. In this way, ‘new’ hard substrate is created in areas with previously only soft substrate.

First observation in Belgium

The cultivation of the native European flat oyster O. edulis was almost a complete failure due to the cold winters of 1962-1963. In 1969, Japanese oysters got imported from the Netherlands and released in the Sluice Dock of Ostend. These Dutch oysters had Canadian and Japanese ancestors ^[5]. Natural reproduction was thought impossible in our regions due to the cold winter temperatures of our waters. In the same year (1969), C. gigas was observed for the first time outside the cultivation areas in the Sluice Dock of the port of Ostend ^[5]. In 1980, the oyster disease bonamiasis, caused by the alien oyster parasite Bonamia ostreae, emerged. As a result, the flat oyster stock declined even further ^{[6, 7]}, while the Japanese oyster appeared resistant to this parasite ^[8].

Spreading in Belgium

The brooding of Japanese oysters outside the oyster cultivation fields caused an area expansion that was hard to stop. Nowadays, the Japanese oyster is established throughout the entire Belgian marine area. The species is found across the entire Flemish coast on all suitable – mostly artificial – hard substrates. The oysters usually settle in the area between high and low tide, provided that certain conditions of flow, silting, food supply etc. are favourable ^[9]. The Japanese oyster has large populations, especially in ports such as Nieuwpoort, Ostend, Zeebrugge and Blankenberge, but also on beach groynes and buoys at sea ^[9-11].

It is estimated that about 3.7% of the surface of the Sluice Dock of Ostend is covered with Japanese oysters. Oysters form reefs that consist mainly of empty shells. These reefs attract C. gigas larvae, which in turn recolonize the reefs ^[12]. The newly settled oysters are especially abundant near the fish farms in the southern part of the Sluice Dock of Ostend ^[12].

C. gigas is not only found near the coastline but also lives on buoys throughout the Belgian marine area. Since 2010, it has been found on wind turbines ^[13]. On offshore structures, the species is limited to the tidal zone or upper part of floating structures. On the seabed (offshore), the species does not occur, and it is unlikely that it will settle there ^[9].

There were reports of large numbers of young Japanese oysters attached to washed-up banded wedge shells Donax vittatus on the beaches from Westende to De Panne ^[14]. Living individuals of the oysters were also found on a surf clamp Spisula solida in Koksijde and shore crabs Carcinus maenas in Oostduinkerke ^[15].

Spreading in neighbouring countries

The Japanese oyster is present in large numbers in the Netherlands, Germany, England, France and is found as far as northern Denmark and southern Norway ^{[16, 17]}.

In the Netherlands, reefs with thousands of Japanese oysters are present in the Eastern and Western Scheldt, the North Sea, and the Wadden Sea. Many of the concrete dikes are overgrown with a carpet of Japanese oysters ^[7]. The first Dutch feral specimens were observed in the Eastern Scheldt in 1971 ^[18]. The rise of feral Japanese oysters began around 1983 on the Wadden Island of Texel, located in the Wadden Sea ^[7]. The species continues to expand its range in the Dutch and German Wadden Sea ^[19].

In 1965, Japanese oyster broodstock got imported for the first time in Great Britain. Since the 1990s, feral individuals of Japanese oysters have been observed in southern Britain (e.g. in the Teign estuary) and around Wales. Genetic research suggests that these feral Japanese oysters came from France ^{[16, 20]}.

Despite the species’ significant temperature tolerance ^{[10, 17]}, its spread in northern European waters is limited because reproduction is impeded due to the low temperatures. Nevertheless, the oyster was able to spread from the west coast of Sweden to southern Norway. Global warming will likely result in a further northward spread of the species ^[21].

This species was intentionally introduced for oyster culture. When the oysters reproduce, the oyster brood is carried by the prevailing sea currents, after which the young oysters settle on any type of hard substrate ^{[7, 9, 19, 22]}.

To ensure a maximum yield in aquaculture, imported species must meet a range of requirements, including tolerance to environmental stress, rapid growth and a high survival rate during transport. These species-specific characteristics also increase the success rate of colonisation ^[23].

In addition, these oysters – contrary to what was thought in the past – can easily withstand our cold winter temperatures, causing little mortality during the winter ^[10]. The Flemish ‘concrete coasts’, with their many dikes, piers and groynes, offer space and substrate for the Japanese oyster. A single oyster can produce up to 100 million eggs and, hence, can quickly colonise any type of hard surface. Once established, the oysters hardly have any enemies – apart from humans.

The recent warmer summers caused a strong population growth of the Japanese oyster along the North Sea coasts ^[24]. To reproduce, the species requires a minimum temperature of 16-18°C. At the same time, cold winters have become scarce, and adult animals easily survive winter ^[10].

However, hot summers are responsible for massive mortalities among Japanese oysters. This was observed in some regions, including the Sluice Dock of Ostend. It is still uncertain if temperature itself played a role in this or whether the mortalities were the result of related hypoxic conditions or another (unknown) cause ^[12].

The Japanese oyster only occurs in water with salinity higher than 10 PSU ^[7]. By comparison, the North Sea has an average salinity of 35 PSU. This limits its spread to rivers. In the Wadden Sea, this oyster spreads remarkably slower than in the Eastern Scheldt, due to a lack of hard surfaces ^[7].

Populations of Japanese oysters are usually large and densely populated. In the competition for space and food, native species such as the common blue mussel Mytilus edulis and the common cockle Cerastoderma edule may be disadvantaged ^[19]. Despite the fears of mussel farmers that their mussel cultures are failing, there is no conclusive evidence that the increasing number of Japanese oysters is directly responsible for the downward trend in mussel culture. It is possible that other factors, such as climate change, play a more important role ^[25]. The Japanese oyster filters larvae of native species from the water as a source of food so that in addition to competition, one can also speak of predation ^[7]. As a precaution, mussel farmers in the Eastern Scheldt are, therefore, removing developing oyster beds. Although experiments are being done regarding the cultivation of shellfish along the Belgian coast (e.g. in wind farms) and concrete plans are made for cultivation facilities, there are currently no permanent, large-scale cultivation plots. So, the impact of the Japanese oyster cannot yet be observed there ^[9].

The Japanese oyster is not only a threat to native molluscs. Other species can also be under pressure when this oyster is present. In 2007, living individuals of the shore crab C. maenas were found with Japanese oysters attached to their carapace ^[15]. Some carapaces were almost completely covered with these heavy oysters. It was assumed that the presence of the oysters complicated the mobility of the crabs, lowering their probability of survival ^[15].

These disadvantageous effects are also visible at higher levels in the food chain. Coastal birds, such as the oystercatcher and the hibernating red knot, feed mainly on mussels ^[26]. When Japanese oysters outcompete the mussels, the birds are left with an empty stomach. The oysters are too big and strong to be opened and, initially, seemed like they were not a potential food source for these birds ^[7]. Since 2007, researchers have observed that oystercatchers and gulls learned to crack these difficult prey, for example, by dropping them on a hard surface ^[9].

In some places, the Japanese oyster is a potential food source and suitable substrate for other species. For example, the non-indigenous Japanese shore crab Hemigrapsus sanguineus can feed on C. gigas. In places where the crab is common, such as in and around the ports of Ostend and Nieuwpoort, it can form a potential threat to the oyster ^[27]. On the other hand, the non-indigenous Japanese shore crab Hemigrapsus takanoi takes advantage of the hard substrate provided by the Japanese oyster ^[28]. These examples illustrate that the spread of several non-native species often goes hand in hand.

When an oyster bed can develop undisturbed for several years, the oysters start growing over each other. These clusters form a razor-sharp, irregular-looking ‘oyster carpet’ that is dangerous for recreationists ^[7]. Where oyster beds are present, wearing appropriate footwear can be an effective measure, especially in the central Sluice Dock of Ostend, where oysters grow vertically and pose a hazard to water sports enthusiasts ^[9]. On piers, oysters do not grow vertically. Therefore, they are less dangerous here. In harbours, the oysters prevent the sluice gates from closing properly, which is why divers have to remove them in the harbour of Ostend ^[9]. There appears to be no significant damage to recreational boats in marinas ^[9].

Japanese oysters cannot be harvested; they are often too big, fused and irregularly shaped to be sold on the market. Extensive oyster reefs along the Dutch and Belgian coast offer a solid substrate, which – before the extensive development of the trawl fishery – were also naturally present at the Flemish coast as contiguous banks of O. edulis. In the Eastern Scheldt, the Japanese oyster is used as a ‘reef builder’ to prevent the erosion of sandbanks ^[29].

Each ecosystem structure is inhabited by specific species and, therefore, changing the system’s structure can significantly alter the species composition ^{[13, 30]}. Oyster beds provide a home for many animal species, and from this point of view, the Japanese oyster is the basis of a habitat that can be of high ecological value ^[31]. Intentionally introduced molluscs often carry a wide variety of sessile organisms, pathogens and parasites with them. These are inadvertently introduced into the new environment and receive the opportunity to establish themselves. Associated organisms, living in the transported water and sediment, are consequently also introduced into the new habitat ^[32]. For example, the introduction of C. gigas was accompanied by the introduction of more than 20 other species, of which some have successfully established themselves in our regions ^[33]. Examples include the Clinging jellyfish Gonionemus vertens ^[34], the parasitic copepod Mytilicola orientalis ^[35],  Wakame Undaria pinnatifida ^[36] and de oyster parasite B. ostreae ^[37].

The Japanese oyster affects the ecosystem of Belgian coastal waters and must be controlled according to the EU Marine Strategy Framework Directive (MSFD) ^[9]. For the situation in the Sluice Dock of Ostend specifically, it was suggested to close the sluice during the spawning period of the Japanese oyster (July-August) to avoid the influx of larvae from the harbour ^[12]. Furthermore, hard structures, e.g. old oyster bags, wooden posts and stones, should be removed as much as possible ^[9].

Due to the local high densities, the oysters need to get removed in some places. For the Sluice Dock of Ostend, it has been proposed to remove the Japanese oysters with dredging activities. However, these policies require a continuous effort and cause an incessant disturbance of the environment ^[9]. According to some scientists, no physical control is possible without damaging other components of the ecosystem ^[38].

Others propose to commercialise sterile Japanese oysters, which cannot reproduce outside the cultivated areas ^[39]. However, reality shows that this method is not very successful. ^[9]. Other possibilities include the introduction of a natural enemy, such as the parasitic copepod M. orientalis. This parasite got once introduced in the United States and France to control the Japanese oyster ^[40]. The oyster herpes virus OsHV1 (harmless to humans) is also a major enemy of the Japanese oyster and has been raging in European oyster cultures since 2008 The virus caused high mortality in juvenile Japanese oysters, led to a collapse in production, and lowered the economic viability of many oyster farms. The virus can cause up to 100 per cent mortality in larvae and young oysters. However, it is highly inadvisable to introduce this virus as it also affects the native European flat oyster O. edulis ^[41]. Another pathogen, Vibrio aestuarianus, appears to be very effective against Japanese oysters but also has drawbacks ^[42-44].

To date (2014), there is no standard monitoring programme for the Japanese oyster along the Belgian coast or in the Belgian part of the North Sea ^[9]. Within the framework of the national programme of measures for the Belgian marine waters ^[45], aimed at monitoring the evolution of the health of the marine environment, the Flanders Research Institute for Agriculture, Fisheries and Food (ILVO) and the Royal Belgian Institute of Natural Sciences (RBINS) regularly monitor the presence of alien species introduced by humans ^[9].

Because the Japanese oyster is found both inland (in harbour areas, estuaries and tidal flats) and at sea (on buoys and offshore wind farms), this species falls under Flemish and federal legislation. Therefore, it is advisable to coordinate monitoring efforts and control measures and programs at both levels ^[9].

The two shells of the Japanese oyster are very different: the lower or left valve is spherical, while the upper or right valve is flat and covered with flaky lamellae ^[46]. This characteristic form led to the commercial name ‘creuse’, as opposed to the native European flat oyster O. edulis. The colour of the shell varies from off-grey to violet. The Latin name ‘gigas’ means ‘giant’. The species can grow up to 30 centimetres ^[7]. Recently, the biggest individual (so far) was reported on our coast. The oyster was found dead on the beach and measured 38 centimetres, setting a new world record ^[47].

The Japanese oyster is a filter feeder ^[10]. Therefore, it maintains a constant in- and outflow of water from which they can filter food particles. Scientists calculated that 1m² of oyster bed filters up to 677 litres of seawater per hour ^[48].

The Japanese oyster can have separate sexes, change sex or occasionally be hermaphroditic. These complex reproductive strategies depend on genetic and environmental factors, of which the underlying mechanisms are still unknown ^[48]. The animals release their eggs into the water (spawning) at temperatures above 16-18°C (July and August). One oyster can produce up to 100 million eggs. Unlike the European flat oyster O. edulis – where fertilization occurs inside the female’s shell – fertilization of the Japanese oyster takes place in the seawater. Therefore, the egg and sperm cells are simultaneously discharged into the water column. While the larvae float around for 15 to 30 days with the prevailing currents, they start developing their shells. Due to the weight of their shell, they sink to the bottom after a couple of weeks and settle on hard structures ^[7].

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