Coscinodiscus wailesii -

This diatom originates from the Indian and Pacific Oceans ^[2]. This exotic species is a planktonic alga living in the upper layers of the water column. It only occurs where sufficient light can penetrate to use in photosynthesis (the process of creating sugars and oxygen using sunlight and CO₂). The species can be found both near the coast and in the open sea, in salt and brackish waters ^[3].

First observation in Belgium

It is uncertain when this planktonic diatom was introduced to the Belgian part of the North Sea. No first sighting is reported in the literature ^[4]. In 1977, the diatom was reported for the first time in Europe (England), after which it was observed in 1979 in samples taken from the southern part of the North Sea (the Netherlands). However, it was not until 1984 that the species became permanently established in the southern part of the North Sea (including the Belgian part), and relatively high numbers were observed ^[5].

Spreading in Belgium

The species is present in the open water of the Belgian part of the North Sea ^[6]. During spring and autumn, it is usually observed in greater numbers ^[5].

Spreading in neighbouring countries

The first sighting of C. wailesii in Europe dates from 1977. That year, the species was present in the English Channel near Plymouth, in the southwest of Britain ^[7]. Initially, the species was misidentified as C. nobilis. Later, it turned out that it was C. wailesii ^[8].

Since then, this diatom spread very quickly. In 1978, the species arrived in the north of the Irish Sea ^[5] and the Bay of Biscay ^[9]. Furthermore, it was sighted in Normandy ^[9] and Dutch coastal waters ^[10]. In 1979, sightings were received from Skagerrak ^[11] and, in 1983, from the Baltic Sea ^[5]. From 1982 onwards, the species was also found in the Western Scheldt, near Breskens ^{[11, 12]}.

The diatom continued to spread, and C. wailesii is now established along the East Atlantic Coast from central France to central Norway ^{[9, 13]}. The highest densities are observed during autumn and spring in the southern part of the North Sea and at the entrance to the Skagerrak.

There are extensive populations along the southwest coast of Norway, the western part of the English Channel, the north of the Irish Sea, the west coast of Ireland and the Shetland Islands ^[5].

It is not entirely clear how C. wailesii came to Europe, but there are some suggestions ^[14]. The species produces resting stage cells that can survive in unfavourable conditions. Once the light, temperature and nutrient conditions are optimal again, these resting diatom cells return to their normal state ^[6]. Resting cells have been found in ships’ ballast water, making transport with ballast water a definite possibility ^[3]. Another possible vector is the transport and import of juvenile Japanese oysters Crassostrea gigas from Japan and North America ^[9]. Resting stage cells can be filtered from the water by oysters. During transportation, the resting cells remain in the oyster’s digestive tract. Finally, the cells are released into a new habitat via the oyster’s faeces ^[3]. After their introduction, diatoms spread locally by drifting with the sea currents ^[9].

C. wailesii reproduces asexually at a fast rate. In the presence of many nutrients in the water column, reproduction happens explosively, and we speak of a bloom. Such a bloom can double in size in just 70 hours.

C. wailesii can grow up to 0.5mm in size, which is quite large for a phytoplankton species. As a result, this diatom is too large to be eaten by the native zooplankton (plankton that consumes other organisms, including phytoplankton) ^[12]. Another consequence of the massive blooms and their large size is that it competes for space and nutrients with other phytoplankton species and macroalgae ^[3].

Finally, this diatom also owes its success to the fact that it accumulates fewer toxic metals in its body – compared to other diatoms – and is, therefore, more tolerant to higher concentrations of metal (e.g. zinc, copper, lead and cadmium) in its environment ^[12].

As a planktonic species, sea currents determine its local distribution. This explains the rapid expansion of C. wailesii in Europe since its introduction to Great Britain in 1977.

Moreover, this exotic species can transform into a resting stage cell that withstands the dark for more than 15 months. This resistant stage allows transportation over long distances in ballast water or the stomach and intestines of oysters [6].

Another important factor is its broad tolerance to different environmental conditions. This diatom survives at temperatures between 8 and 32°C and in salinities of 25 (brackish) to 35 PSU (saline). The species also tolerates variable nutrient concentrations ^[9].

Although this exotic species is not toxic, it produces large quantities of mucus that can impact people and the environment ^[6]. This extensive mucus layer forms when the available nutrients in the water get depleted during a bloom. The weight of the slime causes the diatoms to sink to the colder water layers. The lower temperature means that their metabolism slows down and they need fewer nutrients. The resulting thick mucus layer clogs fishing nets and sticks to other fishing material ^[7].

The thick layer of slime on the seabed is a nuisance to benthic organisms. Its bacterial decomposition creates local oxygen-depleted conditions ^[15]. During a C. wailesii bloom, organisms from the open water column (mainly phytoplankton species and macroalgae) need to compete with this large diatom for space and food ^[3].

On the coast of Japan, C. wailesii causes extra competition for species belonging to the genus Porphyra, the seaweed used to produce Nori. This causes the seaweed to discolour, making it unsuitable for consumption. Since the production of Nori is an important economic activity in certain parts of Japan (such as Harima-Nada in the eastern part of Japan’s Inland Sea), this results in economic damage  ^{[15, 16]}.

Diatoms are single-celled algae that are studied microscopically. They have an external silica skeleton (frustule) made from two overlapping halves (thecae) that fit like a box and lid. Each theca consists of a valve and an accompanying series of girdle bands. The thecae can have variable shapes and ornamentations and are used to distinguish species from each other ^[17]. C. wailesii is one of the larger species of diatoms and measures up to 0.5mm in diameter ^[18].

In the asexual reproduction of diatoms, new thecae are formed within the mother cell. The thecae of the mother cell become the new “lids”, while the newly formed thecae form the new “boxes”. As a result, two new cells of unequal size are formed after cell division. One of the new cells (the one with the original “lid” of the mother cell) is the same size as the mother cell. The other individual consists of the original “box” of the mother cell (which now forms the “lid” of the new cell) and a newly formed “box”. This means that this cell is smaller than the mother cell. As a result, with each division, a part of the population becomes smaller and smaller, until at a certain point they are no longer viable. Diatoms solve this issue by sexual reproduction, resulting in a larger daughter cell that can reach the original size ^[19].

[1]          World Register of Marine Species (WoRMS) (2020). Coscinodiscus wailesii Gran & Angst, 1931. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=156632] (2020-11-17).

[2]          Wallentinus, I. (2007). Introductions and transfers of plants, in: Gollasch, S. et al. 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: pp. 6-77. [http://www.vliz.be/en/imis?module=ref&refid=110299]

[3]          Gollasch, S. (2009). Coscinodiscus wailesii (Gran & Angst) (Coscinodiscaeae, Bacillariophyta), in: DAISIE (Delivering Alien Invasive Species Inventories for Europe). Handbook of alien species in Europe. Invading Nature - Springer Series in Invasion Ecology, 3. Springer: Dordrecht: pp. 278. [http://www.vliz.be/en/imis?module=ref&refid=135000]

[4]          Kerckhof, F.; Haelters, J.; Gollasch, S. (2007). Alien species in the marine and brackish ecosystem: the situation in Belgian waters. Aquat. Invasions 2(3): 243-257. [http://www.vliz.be/en/imis?module=ref&refid=114365]

[5]          Edwards, M.; John, A.W.G.; Johns, D.G.; Reid, P.C. (2001). Case history and persistence of the non-indigenous diatom Coscinodiscus wailesii in the north-east Atlantic. J. Mar. Biol. Ass. U.K. 81(2): 207-211. [http://www.vliz.be/en/imis?module=ref&refid=14333]

[6]          Laing, I.; Gollasch, S. (2002). Coscinodiscus wailesii: a nuisance diatom in European waters, in: Leppäkoski, E. et al. Invasive aquatic species of Europe: distribution, impacts and management. Kluwer Academic: Dordrecht: pp. 53-55. [http://www.vliz.be/en/imis?module=ref&refid=40581]

[7]          Boalch, G.T.; Harbour, D.S. (1977). Unusual diatom off the coast of south-west England and its effect on fishing. Nature (Lond.) 269: 687-688. [http://www.vliz.be/en/imis?module=ref&refid=113662]

[8]          Boalch, G.T. (1987). Changes in the phytoplankton of the western English Channel in recent years. Eur. J. Phycol. 22(3): 225-235. [http://www.vliz.be/en/imis?module=ref&refid=142914]

[9]          Rincé, Y.; Paulmier, G. (1986). Donnée novelles sur la distribution de la diatomée marine Coscinodiscus wailesii Gran & Angst (Bacillariophyceae). Phycologia 25(1): 73-79. [http://www.vliz.be/en/imis?module=ref&refid=141107]

[10]        TRIPOS (1995). Biomonitoring van fytoplankton in de Nederlandse zoute en brakke wateren, 1994. Geannoteerde soortenlijst. Bijlage 1 bij TRIPOS rapport 95003.1. 94 pp. [http://www.vliz.be/en/imis?module=ref&refid=210031]

[11]        Hasle, G.R. (1990). Kiselalger i Oslofjorden og Skagerrak. Arter nye for området: Immigranter eller oversett tidligere? = Diatoms of the Oslo fjord and the Skagerrak. Species new to the area: immigrants or overlooked in the past? Blyttia 48: 33-38. [http://www.vliz.be/en/imis?module=ref&refid=196957]

[12]        Rick, H.-J.; Dürselen, C.-D. (1995). Importance and abundance of the recently established species Coscinodiscus wailesii Gran & Angst in the German Bight. Helgol. Meeresunters. 49(1-4): 355-374. [http://www.vliz.be/en/imis?module=ref&refid=142937]

[13]        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]

[14]        Eno, N.C.; Clark, R.A.; Sanderson, W.G. (Ed.) (1997). Non-native marine species in British waters: a review and directory. Joint Nature Conservation Committee: Peterborough. ISBN 1-86107-442-5. 152 pp. [http://www.vliz.be/nl/imis?module=ref&refid=24400]

[15]        Manabe, T.; Ishio, S. (1991). Bloom of Coscinodiscus wailesii and DO deficit of bottom water in Seto Island Sea. Mar. Pollut. Bull. 23: 181-184. [http://www.vliz.be/en/imis?module=ref&refid=142946]

[16]        Nishikawa, T.; Yamaguchi, M. (2008). Effect of temperature on light-limited growth of the harmful diatom Coscinodiscus wailesii, a causative organism in the bleaching of aquacultured Porphyra thalli. Harmful Algae 7(5): 561-566. [http://www.vliz.be/nl/catalogus?module=ref&refid=302772]

[17]        Van der Werff, A. (1958). Kiezelwieren. Het Zeepaard 18(2): 19-22. [http://www.vliz.be/en/imis?module=ref&refid=114551]

[18]        Gollasch, S.; Haydar, D.; Minchin, D.; Wolff, W.J.; Reise, K. (2009). Introduced aquatic species of the North Sea coasts and adjacent brackish waters, in: Rilov, G. et al. Biological invasions in marine ecosystems: ecological, management, and geographic perspectives. Ecological Studies, 204. Springer-Verlag: Berlin: pp. 507-528. [http://www.vliz.be/en/imis?module=ref&refid=142923]

[19]        Mennema, J. (1958). De voortplanting van de kiezelwieren. Het Zeepaard 18(6-7): 85-88. [http://www.vliz.be/imis/imis.php?module=ref&refid=114658]

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