Polysiphonia senticulosa -

P. senticulosa is native to the Pacific Northwest and -east. The species is often confused with the indigenous P. stricta or the exotic red seaweed P. morrowii. Some suggest that P. morrowii and P. senticulosa are the same species ^[2].

First observation in Belgium

The first observation of P. senticulosa was on the 26^th of March 2001, near an oyster farm in the Sluice Dock of Ostend ^[3].

Spreading in Belgium

The year after the first observation (2001), P. senticulosa was observed in large numbers on various substrates on the bottom of the Sluice Dock of Ostend, mainly in places with low wave action ^[3]. After 2002, P. senticulosa was no longer observed. However, since 2006 it has become abundant again ^[4]. The reports in 2007 of P. morrowii in the Sluice Dock of Ostend ^[5] concern the same species ^[6]. In Belgium, there have been no other sightings outside the sluice-dock.

Spreading in neighbouring countries

In Europe, P. senticulosa was first observed in the Netherlands in 1993. This exotic species was found in Gorishoek along the northern banks of the Eastern Scheldt ^[2]. In 1999, P. senticulosa was found everywhere in the Eastern Scheldt, in some places even being very abundant (e.g. in the Yerseke Bank) ^[7].

In the Mediterranean – namely in France (Thau Lagoon) ^[8] and Italy (Venice) ^[9] – red algae were reported under the name P. morrowii. Possibly these concern the species P. senticulosa.

It is impossible to say how P. senticulosa ended up in our regions. A suggested hypothesis is that this alien species ended up in our region attached to oysters cultivated in the North Pacific.

Remarkable is that the individuals from the Sluice Dock of Ostend probably do not originate from the Netherlands, the only other place in Northern Europe where the species is currently found. After all, no oysters are imported to Belgium from the Netherlands. Presumably, P. senticulosa got introduced to Belgium via the import of cultivated oysters from Canada’s west coast (British Columbia), this species’ area of origin ^{[3, 7]}.

P. senticulosa is a winter species. During summer, it dies off, although the turf mats remain present ^[10]. During autumn, it starts growing again from spores formed the year before (or from the turf mats). These spores remained dormant over the summer. Other species thrive in the summer and are less abundant during the winter. Due to little competition, P. senticulosa thrives in the winter months ^[11].

P. senticulosa attaches to hard substrates, such as rocks and oysters, and other weeds ^[12]. It prefers sheltered areas with gentle currents and wave action ^[3]. An increase in the number of (artificial) hard substrates, such as quay walls, pontoons and oyster beds, might favour the spread of this invader. At the moment, little is known about the other factors that determine the success of this alien species in the Low Countries ^[2].

P. senticulosa appear to be a winter species. In summer, when the water temperature rises, it is absent, although the turf mats can still be found (see above). During winter, when most other species struggle, this exotic species grows from its resting stages (or from the turf mats) and starts reproducing ^[13].

In the Netherlands, some indigenous algal communities got replaced by non-native algal flora, including the Japanese wireweed (Sargassum muticum) and P. senticulosa ^[14]. The latter is a small species and often grows on other weeds. Consequently, the question arises of whether this species competes with native weeds. Although the species is invasive in the Netherlands, there is nothing in the literature about its impact ^[15]. So far, no measures were taken to control the species.

P. senticulosa is a dark red to almost black coloured algae, which grows up to 30 cm in length. In our regions, it is a winter species ^[11], which occurs from October to June. During the summer, P. senticulosa forms resting cells that mature in the autumn ^[14]. It usually grows on hard substrates (natural or artificial) or lives attached to other organisms, such as oysters and seaweeds ^[12]. P. senticulosa is found below the low water line, in an environment sheltered from currents and waves, and in tidal pools ^[16].

P. senticulosa is very similar to the indigenous algae P. stricta. The former can be recognized by the spines on its short side branches. These structures are absent in P. stricta ^[3].

[1]          World Register of Marine Species (WoRMS) (2020). Polysiphonia senticulosa Harvey, 1862. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=144665] (2020-11-17).

[2]          Maggs, C.A.; Stegenga, H. (1999). Red algal exotics on North Sea coasts. Helgol. Meeresunters. 52: 243-258. [http://www.vliz.be/en/imis?module=ref&refid=110857]

[3]          Kerckhof, F.; Stegena, H. (2003). Nieuwe Polysiphonia-soorten voor België en Noord-Frankrijk, met een gereviseerde determineertabel voor de soorten van het geslacht Polysiphonia in deze regio. Dumortiera 80: 40-45. [http://www.vliz.be/imis/imis.php?module=ref&refid=38761]

[4]          ICES Advisory Committee on the Marine Environment (2007). Report of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO) 21-23 March 2007 Dubrovnik, Croatia. CM Documents - ICES. CM 2007(ACME:05). ICES: Copenhagen. 160 pp. [http://www.vliz.be/en/imis?module=ref&refid=206974]

[5]          Heytens, M.; De Clerck, O.; Coppejans, E. (2007). Studie van macrowiergemeenschappen van de Spuikom van Oostende in functie van de Kaderrichtlijn water. Universiteit Gent - Vakgroep Biologie - Afdeling Algologie: Gent. 65 pp. [http://www.vliz.be/en/imis?module=ref&refid=118621]

[6]          De Clerck, O. (2011). Persoonlijke mededeling

[7]          Gollasch, S.; Kieser, D.; Minchin, D.; Wallentinus, I. (Ed.) (2007). Status of introductions of non-indigenous marine species to the North Atlantic and adjacent waters 1992-2002: Ten-year summary of National Reports considered at meetings of the Working Group on Introductions and Transfers of Marine Organisms. ICES Cooperative Research Report, 284. ICES: Copenhagen. ISBN 87-7482-056-7. 149 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=110296]

[8]          Verlaque, M. (2001). Checklist of the macroalgae of Thau Lagoon (Hérault, France), a hot spot of marine species introduction in Europe = Inventaire des macroalgues de l’étang de Thau (Hé-rault, France), un lieu privilégié d’introduction d’espèces marines en Europe. Oceanol. Acta 24(1): 29-49. [http://www.vliz.be/imis/imis.php?module=ref&refid=207324]

[9]          Curiel, D.; Bellemo, G.; La Rocca, B.; Scattolin, M.; Marzocchi, M. (2002). First report of Polysiphonia morrowii Harvey (Ceramiales, Rhodophyta) in the Mediterranean Sea. Bot. Mar. 45: 66-70. [http://www.vliz.be/imis/imis.php?module=ref&refid=40681]

[10]        van der Loos, L. (2020). Persoonlijke mededeling

[11]        Stegenga, H. (2002). De Nederlandse zeewierflora: van kunstmatig naar exotisch? Het Zeepaard 62(1): 13-24. [http://www.vliz.be/imis/imis.php?module=ref&refid=22955]

[12]        Nelson, W.A.; Maggs, C.A. (1996). Records of adventive marine algae in New Zealand: Antithamnionella ternifolia, Polysiphonia senticulosa (Ceramiales, Rhodophyta), and Striaria attenuata (Dictyosiphonales, Phaeophyta). N.Z. J. Mar. Freshwat. Res. 30(449-453). [http://www.vliz.be/en/imis?module=ref&refid=18581]

[13]        Stegenga, H. (1998). Nieuwe gevestigede soorten van het geslacht Polysiphonia (Rhodophyta Rhodomelaceae) in Zuidwest Nederland. Gorteria 24: 149-156. [http://www.vliz.be/imis/imis.php?module=ref&refid=40461]

[14]        Stegenga, H.; Prud'homme van Reine, W.F. (1998). Changes in the seaweed flora of the Netherlands, in: Scott, G.W. et al. Changes in the marine flora of the North Sea. Centre for European Research into Coastal Issues (CERCI): Scarborough: pp. 77-87. [http://www.vliz.be/imis/imis.php?module=ref&refid=205502]

[15]        Wijsman, J.W.M.; De Mesel, I. (2009). Duurzame schelpdiertransporten. IMARES Wageningen Report. Imares: Wageningen. 111 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=207323]

[16]        Wolff, W.J. (2005). Non-indigenous marine and estuarine species in the Netherlands. Zool. Meded. 79(1): 3-116. [http://www.vliz.be/en/imis?module=ref&refid=101200]

VLIZ Alien Species Consortium (2020). Polysiphonia senticulosa. Non-native species of the Belgian part of the North Sea and adjacent estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 5 pp.