Undaria pinnatifida - Wakame
SCIENTIFIC NAME
Undaria pinnatifida (Harvey) Suringar, 1873
1990 - 2000
NW Pacific
Aquaculture - importing live animals
Wakame is native to Northeast Asia, where it is present in the Yellow Sea, the Sea of Japan and along the eastern coast of Japan. The area of origin also includes China, Japan, Korea and Southeast Russia, all countries where it is cultivated for consumption
First observation in Belgium
On the 7th of July 1999, the first specimen of this brown algae was found growing on pontoons in the Omookaai (Omoo Quay) in the marina of Zeebrugge [2].
Spreading in Belgium
The species is mainly observed in the Port of Zeebrugge but will likely expand its range. Wakame will probably survive in this port area in the long term, given that it is already present in the entire marina [4] and the presence of fertile plants [2]. On the 7th of November 2010, a specimen was observed between Koksijde and Oostduinkerke. However, it is uncertain whether it was attached (and growing) or washed ashore [5]. In the meantime, the species is likely living in other places.
Spreading in neighbouring countries
Wakame is thought to have been unintentionally introduced to the European Mediterranean together with the spawn of the Japanese oyster Crassostrea gigas. The first European report (in 1971) came from a French saltwater lagoon in the Mediterranean (Étang de Thau) [2].
In 1983, the seaweed got transferred from Étang de Thau to Brittany by the IFREMER (French Institute of Research for the Exploitation of the Sea) to start experimental cultures. However, the wakame weed 'escaped' from the cultures and, contrary to all expectations, was able to reproduce in the wild [6]. The seaweed quickly spread along the north and west coasts of Brittany, establishing itself on oyster-farming installations, pontoons in harbours, and on the natural substrate available in the tidal zone and below the low tide line, down to a depth of about 15 metres [2].
In 1998, observations of this species came from Calais (France), and thus, entry into the southern North Sea was a fact. The distribution of wakame between the various marinas of the English Channel occurred via the up and down sailing of recreational boats [2]. DNA research shows that many of the wakame established in Europe are not the offspring from the populations in Brittany. Therefore, it is likely that new introductions from Northeast Asia took place [7]. In the Netherlands, the species was first observed in 1999 in the Eastern Scheldt [8]. In the following years, the populations increased and wakame became a common species in the Eastern Scheldt. Shortly afterwards, the seaweed was found in Lake Grevelingen.
On the 15th of June 1994, 35 specimens were collected on pontoons in the marina at Hamble, along the south coast of England [9]. During the same year, another isolated population was discovered on the island of Jersey. These introductions were probably caused by the small boats sailing between England and France, with the seaweed clinging to the sides of these vessels [9, 10]. To date, the most northerly place where wakame was found is in Belfast Lough, Northern Ireland [11]. In 2016, the species was present in Ireland [12]. Expansion across Britain and towards Scotland seems very likely [13]. The conditions even appear to be suitable for further colonisation as far as the Celtic Sea and the coast of Norway
From 1990 onwards, the transport of oysters led to the permanent settlement of wakame on Spain’s west coast [10]. Wakame was also observed in the Venetian lagoon (Italy), although it remains uncertain whether the species established itself there permanently [16]. The species has also been introduced to other continents, including Taiwan, New Zealand, Australia, Argentina, Mexico and California [8]
The transport of broodstock of Japanese oysters (Crassostrea gigas) seems to be the main cause for the introduction of wakame to Europe [10]. It is likely that secondary introductions occurred, both by transport of oyster broodstock and attachment to the sides of international transport vessels. The commercial and economic importance of wakame helped the spread of this species. Around 1981, repeated attempts were made in France to breed the species on ropes in the Mediterranean. In 1983, a successful breeding culture was started on the Atlantic coast in Brittany [8], and today wakame is still cultivated in the Bay of Saint-Malo [17].
Once this seaweed has made its appearance, it spreads locally through spores and/or gametes in the water column [2]. The young algae attach themselves to the hulls of recreational boats, which promote dispersal over short distances [6]. Presumably, the species ended up in the marina of Zeebrugge in this way, from Calais or Bretagne or the Solent (England) [2].
There are five reasons why wakame is always one step ahead of indigenous species, which are often subjected to competition [9]:
- Wakame is an opportunistic species that quickly colonises new substrates, disturbed areas and artificial floating objects, such as rubbish and ship hulls;
- In coastal areas, the species forms a dense cover on top of the existing benthic community;
- The seaweed is very resistant to disturbance;
- The species has a broad vertical distribution. It can settle from the low water line to about 15 metres depth;
- Wakame produces millions of spores that are released into the water column and effectively colonise floating objects.
Wakame is part of the perennial biofouling community. This means that it can easily attach itself to the hulls of ships. If they attach themselves to recreational boats that call at various ports in the English Channel and the North Sea, this greatly promotes the spread of this weed. It has been shown that this alga grows faster in fast-flowing water. Therefore, attaching itself to a moving boat is beneficial for their growth [18].
Across shorter distances², dispersal occurs through natural reproduction. Wakame produces millions of spores that float in the water column for an average of five hours [10]. These microscopic spores withstand extreme conditions: they survive on dry land for more than a month. The species thrives in cold and warm temperate areas, as long as the seawater has a salt concentration of more than 20 PSU [8]. By comparison, the North Sea has a salinity of about 35 PSU. At nutrient-poor winter temperatures, the species may discolour [19].
Non-native brown seaweeds, such as wakame and the Japanese wireweed Sargassum muticum, are found in the Belgian part of the North Sea, particularly in ports. In contrast to other areas, the introduced macroalgae do not cause economic damage here [20]. Ecologically, wakame can form dense layers in coastal areas on top of the existing benthic community [9]. It remains to be seen whether the possible expansion of wakame in our area will have large-scale consequences. The species has some natural predators in the harbour of Zeebrugge, such as the Eurasian coot Fulica atra [21]. Moreover, because the ‘leaves’ (laminae) of wakame form a suitable habitat for other biofouling organisms, this alga can suffer from other organisms growing on top of it [2]. This could potentially reduce the chance that this species continues to spread.
Wakame can have a positive influence on its surroundings. The species is an example of an ecosystem engineer. Through their physical presence, ecosystem engineers form three-dimensional structures in which other organisms can hide. This is called the autogenous ecosystem engineer effect [22]. Scientists investigated this phenomenon in Golfo Nuevo in Patagonia (Argentina) [23]. Wakame offers a more complex structure than most of the naturally occurring algae, thus positively affecting the community living in association with this kelp. The more species thrive, the more abundant species get, and the more food is available for other species higher up the food chain [23]. This phenomenon could also occur in Belgium when brown algae replace morphologically fewer complex algae such as green and red algae.
Wakame is an annual seaweed that grows mainly during the cold autumn and winter [21]. In Japan, this alga can grow to a length of two metres. In Brittany, on the other hand, specimens reach sizes of up to three metres [2]. The reproductive cycle of this seaweed is purely sexual. In adult specimens, a sporophyll (a spiral-shaped reproductive structure) gets formed at the base of the stem. The sporophyll produces meiospores [2, 14] which, once released into the water column, grow into gametophytes.
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VLIZ Alien Species Consortium (2020). Undaria pinnatifida – Wakame. Non-native species of the Belgian part of the North Sea and adjacent estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 7 pp.