Dasysiphonia japonica -

D. japonica is native to the Pacific Ocean (Japan and Korea). It lives in the lower intertidal and sublittoral zones (the area below the low water line). The species usually grows on stones or oysters in places that are protected from waves. The siphoned Japan weed can also grow on rocks or attach to other species of seaweed ^[3].

First observation in Belgium

In December 2014, the species was found on the hull of the research vessel ‘RV Belgica’, based in Zeebrugge ^[4]. At the time, it was uncertain whether the weed was native to Belgium because the ship had made international sea voyages (e.g. to Southern Europe).

In October 2015, D. japonica was found in situ on the pontoons in the marina of Zeebrugge together with some other species, including Aglaothamnion hookeri, Melanothamnus harveyi, Pterothamnion plumula, Antithamnionella spirographidis and Undaria pinnatifida. So far, this is the only known site in Belgium where the species is present.

Spreading in Belgium

In Belgium, D. japonica remained undetected for a long time. Since 1944, the siphoned Japan weed has been observed in the neighbouring countries of Belgium. Therefore, the species has likely been present in Belgium for some time. In 2009, D. japonica was not yet reported as a non-indigenous species in Belgium ^[5].

Spreading in neighbouring countries

In 1984, the siphoned Japan weed probably arrived in France with the import of oysters ^[6]. From there, it spread rapidly along the eastern Atlantic to Sweden ^[7], Norway ^[8-10], Scotland ^{[11, 12]}, Great Britain ^[13], France ^{[13, 14]}, Spain ^{[13, 15]} and the lagoons in the Mediterranean Sea (e.g. Étang de Thau) ^[16]. In 2009, the species was reported for the first time on the west coast of the Atlantic Ocean ^[17]. There, the species spread quickly over a wide geographical area ^{[18, 19]}.

In the Netherlands, the siphoned Japan weed was found for the first time in an empty oyster pit in 1994 ^[20]. D. japonica is very common in the Eastern Scheldt and Grevelingen Lake ^[2]. During winter and spring, it serves as food for the sap-sucking slug Elysia viridis ^[20]. Since 2014, this exotic species has been observed on floating pontoons in the marina near Den Helder (The Wadden Sea) ^[21].

The siphoned Japan weed was probably introduced with the oyster imports from Japan or Korea ^[22]. Biofouling on ship hulls is a possible secondary dispersal mechanism within Europe. In Norway, the first individuals of D. japonica were found in ports in which oil tankers, mostly coming from Dutch ports, arrived ^[10]. The ability to survive in the dark may be a critical factor contributing to the long-range dispersal of red algal propagules in ballast water ^[9].

The high growth rates of D. japonica explain its invasive success in different regions ^[23]. According to laboratory experiments, this rapid growth is due to the efficient absorption of nitrates ^{[23, 24]}. Because this species absorbs nitrate more efficiently than local seaweed species, it has an advantage. Furthermore, it is often less subject to grazing  ^[25].

Indigenous herbivores prefer native seaweed species to exotic ones. A herbivore's food preference plays a role in determining the growth and survival of an alien species. The chemical properties of D. japonica and its low nutritional value makes it a less favourable food source for herbivores than the native species of seaweed ^{[24, 26]}. On the contrary, another study showed that one local herbivore grazed more on D. japonica in comparison to local species, while another local herbivore exerted lower grazing pressure ^[23].

D. japonica is thought to have spread in Europe primarily by vegetative reproduction, producing small monosiphonous (one cell row) side branches or pseudolaterals. These fragments attach to a hard substrate and form new individuals. Pseudolaterals are formed during all seasons. However, according to experimental studies, they’re most likely to survive in late summer, when the temperatures are optimal for growing (between 12-20°C) ^[27]. Reproductive gametophytes of D. japonica have not yet been found in European populations ^[8]. According to aquarium studies, asexual reproduction by tetraspores results in as much reproductive success as producing the pseudolateral fragments ^[27].

The siphoned Japan weed can survive in a wide salinity and temperature range. The species can survive temperatures between 0-30°C, with adult plants having a slightly higher tolerance than the reproductive cells (carpospores). Optimal growth conditions for carpospores are a water temperature of 19-25 °C and a salinity of 30 PSU. Growth rates at 20 PSU and 30 PSU do not differ significantly, but the development of tetrasporangial stichidia is reduced at 20 PSU. At 15 PSU no stichidia are produced and growth is minimal, whereas salinity below 10 PSU is lethal ^[9]. Dispersal in ‘the Kattegat’ and Baltic Sea is unlikely, due to the low salinities and cold winter temperatures ^[9]. Spores can survive for at least 40 days in the darkness under various temperature conditions.

D. japonica lives in a habitat with hard substrates, but also spreads to areas with other types of substrates. The species can also grow on other algae and the soil. It has been observed at a depth of 40 metres, indicating that its growth is not dependent on much light ^[8]. Usually,  the siphoned Japan weed grows at a depth of 6-22 metres, in the lower intertidal and sublittoral zones, where it is most abundant ^[28].

The invasive success of D. japonica may be the result of the combination and interaction of intrinsic properties (efficient nitrate uptake, rapid growth, etc.) and extrinsic factors (species richness of local communities’ occurrence, preference of grazers, etc.) ^[23].

The siphoned Japan weed possesses several properties that are advantageous for establishing itself in different types of habitats. These characteristics are at the root of its rapid distribution across Europe ^[29]. The most important factors are its wide tolerance limits regarding temperature and salinity, high specific growth rates and effective reproduction by fragmentation.

The ability of D. japonica to quickly establish a dense population ^[24] can be very problematic for indigenous communities. Due to the dominance of D. japonica, it can occupy up to 80% of the available space in some circumstances ^[18] or form a virtual monoculture ^[3]. These high abundances negatively impact biodiversity and the richness of the local community ^{[23, 25, 29, 30]}.

In the long term, the introduction of this species may cause changes in nutrient flows within a local plant community. Studies have shown that, over time, a redistribution of space occurs, and the growth rate and nutrient uptake of the non-native species are reduced. ^[25].

Although the ecological impact is not directly visible, it may pose an economic threat due to its ability to attach to shellfish. This may be a potential problem for local shellfish farms ^[30].

D. japonica is a red seaweed that grows up to 30 cm high with a pinkish to deep red colour and a discus-shaped adhesive disc ^{[2, 31]}. This seaweed is soft, floppy and does not stay upright once it is taken out of the water.

The thallus (plant form) is filamentous, with several main axes per individual consisting of polysiphonous segments (one central cell surrounded by four pericentral cells). A cortex is formed in the oldest parts of the thallus, that covers the pericentral cells (only in these older parts). In young axes, the cortex is limited to filaments that grow downwards between the pericentral cells. The main axes are up to 1 mm in diameter. On each segment of an axis a monophonic (a single cell per row) side branch, that branches four to five times, is formed.

So far, only tetrasporangia have been found as reproductive structures. These tetrasporangia are grouped in so-called stichidia: unbranched axes of limited growth, of which each cell bears a ring of six to seven fertile pericentral cells. Each of these pericentral cells have a tetrasporangium on the top and (two or) three so-called apical ‘cover cells’ on the outside (these remain in place when the spores are released) ^[2].

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VLIZ Alien Species Consortium (2020). Dasysiphonia japonica – Siphoned Japan weed. Non-native species of the Belgian part of the North Sea and adjacent estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 7 pp.