Odontella sinensis -

This diatom originates from Chinese waters ^[2]. The Red Sea and the Indian Ocean are also mentioned as areas of origin ^[3].

This alien species belongs to the phytoplankton and lives in the upper layers of the water column, where sufficient light can penetrate for photosynthesis, the process whereby CO₂ gets converted into oxygen and energy (carbohydrates) with the help of sunlight. O. sinensis thrives in the open ocean and coastal and brackish waters ^[4].

First observation in Belgium

The first observation in Belgian coastal waters dates back to November 1904. That month, the diatoms were observed at four different locations. The highest abundance was found in open water about 15 km off the coast of Wenduine. The three other observations were also in the open sea ^[3].

The species may have been here already before 1889 (first sighting in the North Sea), but it was not until 1903 that it had sufficient opportunity to grow and was noted by several observers ^[5-7].

Spreading in Belgium

After the first sighting in 1904, this diatom was found in Nieuwpoort and Zandvliet (1906), and later (1909) near the sandbank Wandelaar (located in the North Sea between Wenduine and Zeebrugge) ^[7]. Between 1921 and 1955 the species was spotted near the sandbank West Hinder (located in the North Sea, approximately between Ostend and Nieuwpoort). In 1953 and 1954, it was sighted in the Port of Ostend ^[8], and again in 1959 ^[7]. Meanwhile, this species is considered an established species in Belgium ^[9]. Despite the lack of recent publications mentioning the occurrence of this species off the Belgian coast, its overall presence has been confirmed ^[10].

Spreading in neighbouring countries

The species established itself along the Atlantic coast, from Normandy (France) to central Norway, including the North Sea and Baltic Sea ^[4]. The first sighting in the North Sea, although under a different name (Biddulphia sinensis), dates back to 1889. This probably makes it one of the first non-native phytoplankton species to be noticed in Europe ^{[3, 6, 11]}. Since then, the species spread throughout the North Sea, the English Channel, the western Baltic Sea, the coast of Norway and finally, in 1909, in the Irish Sea ^[12].

In the Netherlands, the first observation was in November 1905, when the species was found in the southern North Sea ^{[3, 13]}. In 1920, this diatom was one of the most common species near Den Helder (North Holland) and in Lake Ijssel ^[12]. Since then, this diatom was spotted frequently throughout the Netherlands ^[14]. In France, O. sinensis was already observed before 1930 ^[15].

Introduction to Europe occurred via ships’ ballast water ^[3]. Diatoms have resting stages that allow transport in ballast water ^[4]. These special cells can survive adverse conditions, such as the prolonged lack of light in a ballast water tank. Once the light, temperature and nutrient conditions are optimal again, these cells return to their original state ^[16]. O. sinensis might be the first organism to colonise an area by transport with ballast water ^[17].

The diatom O. sinensis reproduces mainly by asexual reproduction and occasionally by sexual reproduction. It has been shown that, under laboratory conditions, this species carries out one to two cell divisions per day if sufficient nutrients are present. These rapid divisions lead to an explosive growth of the population and might result in a bloom. In Europe, the species is most abundant between autumn and spring. Thus, the species can still bloom in November and December. Such a bloom leads to competition with other species of phytoplankton, thereby inhibiting their growth ^[4]. In the English Channel, this exotic species thus developed into one of the most dominant phytoplankton species ^[11].

O. sinensis can tolerate a wide range of environmental conditions. This diatom occurs at water temperatures between 1-27°C, but it prefers temperatures of 2-12°C. ^[4]. It is usually found in salinities of 27 (brackish) to 35 (saline) PSU, although salinities between 2 (almost fresh) and 35 PSU (saline) are tolerated ^[4]. As it is a photosynthetic species that depends on light, its distribution is limited to the upper water layers ^[4].

Before 2003, O. sinensis was relatively rare in Belgian waters. However, the exceptionally hot summers between 2001 and 2005 and the rather cold autumn and winter months during that same period stimulated its growth. The subsequent wet summers and mild winters resulted in a reduced abundance of the species. Climate change might increase the occurrence of O. sinensis blooms in the North Sea in the future ^[6].

During an O. sinensis bloom, the native phytoplankton species can still occur in high numbers, but their population growth is slowed down. This indicates that O. sinensis affects its surroundings ^[4]. There are no known adverse economic effects that can be linked to the occurrence of this diatom ^[18].

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 ^[14]. Characteristic of this species is its rectangular appearance with a protrusion at each corner ^[19].

This diatom occurs solitary or in pairs ^[4], it reproduces all year and sometimes still blooms in November/December. In European waters, the species is most abundant from autumn to spring. Its life cycle includes an asexual and a sexual stage. Asexual reproduction consists of cell division. 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 ^[20].

[1]          World Register of Marine Species (WoRMS) (2020). Odontella sinensis (Greville) Grunow, 1884. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=149095] (2020-11-17).

[2]          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]

[3]          Ostenfeld, C.H. (1908). On the immigration of Biddulphia sinensis Grev. and its occurrence in the North Sea during 1903-1907 and on its use for the study of the direction and rate of flow of the currents. Meddelelser fra Kommissionen for Danmarks Fiskeri- og Havundersøgelser: serie Plankton 1(6): 1-44. [http://www.vliz.be/imis/imis.php?module=ref&refid=39929]

[4]          Gollasch, S. (2009). Odontella sinensis (Greville) Grunow, Chinese diatom (Eupoodiscaeae, 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. 288. [http://www.vliz.be/imis/imis.php?module=ref&refid=135024]

[5]          Gomez, F. (2008). Phytoplankton invasions: Comments on the validity of categorizing the non-indigenous dinoflagellates and diatoms in European Seas. Mar. Pollut. Bull. 56(4): 620-628. [http://www.vliz.be/nl/catalogus?module=ref&refid=297596]

[6]          Gomez, F.; Souissi, S. (2010). The diatoms Odontella sinensis, Coscinodiscus wailesii and Thalassiosira punctigera in the European Atlantic: recent introductions or overlooked in the past? Fresenius Envir. Bull. 19(8): 1424-1433. [http://www.vliz.be/nl/catalogus?module=ref&refid=206979]

[7]          Kufferath, H. (1952). Recherches sur le plancton de la mer flamande (mer du Nord méridionale): II. Biddulphiaeae, Proteomyxa, Rhizomastigina, Heliozoa, Amoebina. Bull. K. Belg. Inst. Nat. Wet. 28(10): 1-39. [http://www.vliz.be/imis/imis.php?module=ref&refid=20625]

[8]          Van Meel, L. (1957). Biddulphia sinensis R. Greville. 1866. Contribution à l'écologie d'une diatomée du plankton marin de la Mer du Nord. Bull. Jard. Bot. Etat Brux. 27(4): 695-702. [http://www.vliz.be/imis/imis.php?module=ref&refid=12157]

[9]          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]

[10]        Sabbe, K. (2019). Persoonlijke mededeling

[11]        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]

[12]        Van Goor, A.C.J. (1922). Het phytoplankton, in: Redeke, H.C. Flora en fauna der Zuiderzee: Monografie van een brakwatergebied. C. De Boer Jr.: Den Helder: pp. 92-123. [http://www.vliz.be/imis/imis.php?module=ref&refid=115195]

[13]        van Breemen, P.J. (1906). Bemerkungen über einige Planktonformen. Verh. Rijksinst. Onderz. Zee. 1(5): 1-7. [http://www.vliz.be/imis/imis.php?module=ref&refid=195989]

[14]        Van der Werff, A.; Huls, H. (1957-1976). Diatomeeënflora van Nederland: Deel 1-10. [S.n.]: Den Haag. 574 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=58460]

[15]        Goulletquer, P.; Bachelet, G.; Sauriau, P.G.; Noel, P. (2002). Open Atlantic coast of Europe: a century of introduced species, in: Leppäkoski, E. et al. Invasive aquatic species of Europe: Distribution, impacts and management. Kluwer Academic: Dordrecht: pp. 276-290. [http://www.vliz.be/en/imis?module=ref&refid=40609]

[16]        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]

[17]        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]

[18]        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]

[19]        Tomas, C.R. (Ed.) (1997). Identifying marine phytoplankton. Academic Press: San Diego. ISBN 0-12-693018-X. XV, 858 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=19862]

[20]        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 (2020). Odontella sinensis. Non-native species of the Belgian part of the North Sea and bordering estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 6 pp.