Caulacanthus okamurae -

The red algae C. okamurae has long been considered synonymous with the morphologically similar Caulacanthus ustulatus and was considered a species with cosmopolitan distribution ^[2]. Molecular genetic research discovered that there are two distinct species ^[3]. C. ustulatus is indigenous to Europe and occurs naturally in southern European countries. The initial distribution of C. okamurae is restricted to Japan, China, Korea and Taiwan ^[2].

It prefers to live in mangroves, brackish water and intertidal areas (i.e. the area in between the high and low tide), where it grows on hard substrates ^[4].

First observation in Belgium

On the 3^rd of November and the 29^th of December 2011, C. okamurae was observed for the first time in Belgium, in the Flemish nature reserve the ‘Bay of Heist’, located on the eastern breakwater of the Port of Zeebrugge. Nowadays, the species is common in this area. So far, only vegetative specimens have been found ^[5]. At the same time, another non-native species was also found, namely Agarophyton vermiculophyllum ^[5].

Spreading in Belgium

In Heist, C. okamurae was observed near the intertidal area, scattered on blocks of the harbour dam where no other species of macroalgae could be found ^[5]. For the time being, this is the only place where it has been found; nothing is known about a possible further spread in Belgium.

Spreading in neighbouring countries  

C. okamurae – initially identified as Caulacanthus ustulatus – was observed in 1986 at Roscoff (Brittany, France), where it is now quite common ^[6]. Molecular studies showed that the French population was more related to the Asian C. okamurae than the Atlantic C. ustulatus. It was concluded that the seaweed was introduced from the Pacific Ocean and was not the result of northward migration of the Atlantic variant ^[7]. The specimens found in France contained little to no reproductive structures. It is assumed that C. okamurae mainly produces offspring using vegetative reproduction ^[8], a common reproductive strategy among successful exotic species ^[4].

Although its distribution in France was limited, C. okamurae reached the Netherlands and Great Britain. In the Netherlands, the red algae were found in 2005 on the artificial island ‘Neeltje Jans’ in the Eastern Scheldt. This find consisted of large specimens that had probably been present for some time. This exotic alga was found both in the low intertidal area and in the higher intertidal area, in contrast to other exotic algae in the Netherlands ^[9]. In Great Britain, the weed has spread rapidly from Devon (2004) to Cornwall (2005) and Kent (2009). Here too, the species is found high up in the intertidal zone ^[10].

Non-native populations of Pacific C. okamurae outside Europe have been found in Prince William Sound (Alaska) ^[11] and Tomales Bay (California) ^[12].

Most likely, C. okamurae was introduced to Europe via the transport of Japanese oysters (Crassostrea gigas). An indication of this was the finding place near an oyster farm in France ^[13]. Further spread across Europe could have occurred by adhesion to ship hulls or by loose fragments of the algae that washed ashore and established itself ^[5].

The exotic species survives and grows in a wide range of temperatures (13-26°C, with optimal growth at 17°C). Even at 6°C, the weed continues to grow, although limited ^[14]. Furthermore, C. okamurae occurs on almost all types of substrates, but it has a preference for living substrates, such as mussels and barnacles ^[4]. Why the seaweed thrives on the Flemish coast has not yet been researched.

The distribution of C. okamurae is probably influenced by two environmental factors. The first is the increase in surface water temperature due to global warming, which is in this exotic Asian species’ favour. A second factor concerns the frequency of storms: a storm creates clearings in the existing algal growth, which C. okamurae can use to establish itself ^[10]. Local factors influencing the distribution of C. okamurae in Belgium are currently unknown.

Unlike other non-indigenous seaweeds, which mainly settle in the subtidal zone and rock pools, C. okamurae is also able to establish itself in the intertidal zone. Here, the species may become dominant and displace the indigenous turf-forming macroalgae (e.g. Gelidium pusillum in the Netherlands) ^{[10, 15]}. Even though C. okamurae displaces indigenous macroinvertebrates such as sea snails and periwinkles, the species also provides a more diverse meiofauna and macroalgae community in the upper tidal zone. This may be explained by the fact that this red alga forms algal turf zones that do not normally occur in this region. This increases habitat complexity and retains sediment and water, which in turn provides food, habitat and safety for the meiofauna. The effects of this species on the environment have not yet been fully studied and, therefore, can be both positive and negative ^[16]. So far, no measures have been taken to combat this alien species.

C. okamurae is a rather inconspicuous, dark brown to red coloured weed that consists of cylindrical shoots with branches up to three centimetres high ^[10]. Together, these branches form a tangle that can cover whole areas of substrate ^{[9, 15]}. A longitudinal section of seaweed shows a noticeable central filament made up of large cells with interconnections known as pit connections ^[9].

C. okamurae grows primarily as an epiphyte, a non-parasitic plant growing on other organisms. In this way, the species attaches itself to other algae. In the Netherlands, C. okamurae is mainly found on the false Irish moss Mastocarpus stellatus ^[9]. The attached C. okamurae is often indistinguishable from the small secondary disk-like holdfasts of M. stellatus. The adhesion is usually not very strong so that the weed can be easily detached ^[9].

The life cycle of C. okamurae is not well known because the reproductive structures are rarely observed in the field ^[17]. In laboratory cultures, the life cycle has already been described for several isolates ^{[14, 18]}.

[1]          World Register of Marine Species (WoRMS) (2020). Caulacanthus okamurae Yamada, 1933. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=496188] (2020-11-17).

[2]          Petrocelli, A.; Wolf, M.; Cecere, E.; Sciuto, K.; Sfriso, A. (2020). Settlement and spreading of the introduced seaweed Caulacanthus okamurae (Rhodophyta) in the Mediterranean Sea. Diversity 12(4): 129. [http://www.vliz.be/en/imis?module=ref&refid=330625]

[3]          Hartnell College Genomics Group; Aguilar, A.; Ahumada, T.J.; Amezcua Moreno, N.; Bohn, J.; Bustamante, D.E.; Calderon, M.S.; Cardoso, E.; Carranza, R.; Castillo, M.; Cazares, E.; Cazares, E.; Companion, J.K.; Cruz, J.; Cuevas, N.; De La Torre, L.; Dietz, D.P.; Fernando, K.M.; Garcia, B.; Gomez, P.; Gonzales-Miramontes, B.; Hernandez, Y.; Huaracha, K.; Hughey, J.R.; Lazaro, G.; Zhai Lorenzo, F.; Medrano, D.; Mendoza, A.; Mendoza, D.; Mohssin, A.; Orozco Medina, J.; Pacheco, A.; Palacios Ruvalcaba, G.; Patel, J.; Patel, J.; Patino, S.; Perez-Alfaro, K.; Ponce, A.N.; Poso, J.G.; Ramirez, G.; Ramirez, H.A.; Resendiz, N.; Reyno, R.; Rodriguez, D.; Russell, I.A.; Saenz-Verdugo, P.; Carmona, A.S.; Sanchez, F.; Sheffer, S.X.; Solorio, C.; Soto Trujillo, A.; Vasaya, G.S.; Velasquez Lopez, V. (2020). The complete mitochondrial and plastid genomes of the invasive marine red alga Caulacanthus okamurae (Caulacanthaceae, Rhodophyta) from Moss Landing, California, USA. Mitochondrial DNA Part B: Resources 5(3): 2067-2069. [http://www.vliz.be/en/imis?module=ref&refid=330626]

[4]          Murray, S.N. (2007). Improving understanding of invasive seaweeds in California’s coastal waters: Moving beyond Caulerpa taxifolia. Final report. Department of Biological Science, California State University: Los Angeles. 48 pp. [http://www.vliz.be/en/imis?module=ref&refid=218238]

[5]          Kerckhof, F.; Verbeke, D.; Bauwens, F. (2012). Nieuws uit de Baai van Heist: de roodwieren Caulacanthus ustulatus (Mertens ex Turner) Kützing, 1843 en Gracilaria vermiculophylla (Ohmi) Papenfuss 1967 nieuw voor de Belgische kust en een merkwaardig habitat van intertidale mossels. De Strandvlo 32(1): 19-23. [http://www.vliz.be/en/imis?module=ref&refid=215164]

[6]          Rio, A.; Cabioch, J. (1988). Apparition du Caulacanthus ustulatus (Rhodophyta, Gigartinales) dans la Manche occidentale. Cryptogam., Algol. 9(3): 231-234. [http://www.vliz.be/en/imis?module=ref&refid=218289]

[7]          Rueness, J.; Rueness, E.K. (2000). Caulacanthus ustulatus (Gigartinales, Rhodophyta) from Brittany (France) is an introduction from the Pacific Ocean. Cryptogam., Algol. 21(4): 355-363. [http://www.vliz.be/en/imis?module=ref&refid=218286]

[8]          ICES Advisory Committee on the Marine Environment (2006). Report of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO) 16-17 March 2006 Oostende, Belgium. CM Documents - ICES. CM 2006(ACME:05). ICES: Copenhagen. 330 pp. [http://www.vliz.be/en/imis?module=ref&refid=111237]

[9]          Stegenga, H.; Draisma, S.; Karremans, M. (2006). Caulacanthus ustulatus: een nieuwe invasiesoort op Neeltje Jans. Het Zeepaard 66(3): 79-82. [http://www.vliz.be/en/imis?module=ref&refid=99750]

[10]        Maggs, C.; Mineur, F.; Bishop, J.; McCollin, T. (2010). Non-natives in MCCIP Annual Report Card 2010-11. MCCIP Science Review. Lowestoft. 11 pp. [http://www.vliz.be/en/imis?module=ref&refid=218211]

[11]        Ruiz, G.M.; Huber, T.; Larson, K.; McCann, L.D.; Steves, B.; Fofonoff, P.; Hines, A.H. (2006). Biological invasions in Alaska’s coastal marine ecosystems: Establishing a baseline. Final report submitted to Prince William Sound Regional Citizens’ Advisory Council & U.S. Fish & Wildlife Service. Smithsonian Environmental Research Center. Center, S.E.R.: Edgewater. 112 pp. [http://www.vliz.be/en/imis?module=ref&refid=218248]

[12]        Gabrielson, P.W.; Widdowson, T.B.; Lindstrom, S.C. (2004). Keys to the seaweeds and seagrasses of Orgeon and California: North of Point of Conception. Phycological Contribution, 6. University of British Columbia. Department of Botany: Hillsborough. ISBN 0-9763817-0-2. 181 pp. [http://www.vliz.be/en/imis?module=ref&refid=218553]

[13]        Miller, K.A.; Aquilar-Rosas, L.E.; Pedroche, F.F. (2011). A review of non-native seaweeds from California, USA and Baja California, Mexico = Reseña de algas marinas no nativas de California, EUA y Baja California, México. Hidrobiológica 21(3): 365-379. [http://www.vliz.be/en/imis?module=ref&refid=218235]

[14]        Rueness, J. (1997). A culture study of Caulacanthus ustulatus (Caulacanthaceae, Gigartinales, Rhodophyta) from Europe and Asia. Cryptogam., Algol. 18(2): 175-185. [http://www.vliz.be/en/imis?module=ref&refid=218291]

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

[16]        Smith, J.R.; Vogt, S.C.; Creedon, F.; Lucas, B.J.; Eernisse, D.J. (2014). The non-native turf-forming alga Caulacanthus ustulatus displaces space-occupants but increases diversity. Biological Invasions 16(10): 2195-2208. [http://www.vliz.be/nl/catalogus?module=ref&refid=297211]

[17]        Zucharello, G.C.; West, J.; Rueness, J. (2002). Phylogeography of the cosmopolitan red alga Caulacanthus ustulatus (Caulacanthaceae, Gigartinales). Phycol. Res. 50(2): 163-172. [http://www.vliz.be/en/imis?module=ref&refid=218249]

[18]        West, J.A.; Calumpong, H.P. (1990). New records of marine algae from the Philippines. Micronesica 23(2): 181-190. [http://www.vliz.be/en/imis?module=ref&refid=218303]

VLIZ Alien Species Consortium (2024). Caulacanthus okamurae. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 6 pp.