Thalassiosira punctigera -

The native range of this alien species is not known with certainty. In 1886, the species was described using material from Japan. Until 1950, the species was only reported in the North Pacific. Later, observations came from the South Pacific, Southwest Atlantic and North Sea ^[2]. Because T. punctigera was not reported in North Atlantic waters until 1978, the species is suspected to be non-indigenous ^[2]. However, some scientists are not convinced by this argument and question the exotic status of this diatom ^[3].

First observation in Belgium

This diatom was first observed in the Dutch Western Scheldt on May 4, 1993. These samples came from the brackish water between Hansweert (about 20 km from the Belgian border) and the Belgian border ^[4].

T. punctigera was not present in samples from the Sea Scheldt, beyond the Belgian border ^[4]. For the time being, there are no reports from Belgium.

Spreading in Belgium

The distribution of this diatom in our region is limited to the Dutch Western Scheldt. Although there are no reports in the Belgian Sea Scheldt or the Belgian part of the North Sea, it is very likely that the species also occurs off the Belgian coast ^[5].

Identification at the species level is only possible for this genus after treatment with an acid, which makes the species-specific structures of the skeleton more visible. In standard monitoring, species-specific identification is impossible for reasons related to time and cost. Therefore, there is no certainty about the more recent occurrence and distribution of the species in the study area or Belgian waters ^[5].

Spreading in neighbouring countries

The first reports of T. punctigera in Europe date from 1978, when it was observed around Helgoland (Germany) ^[6] and Plymouth (Great Britain) ^[7]. Between 1980 and 1981, it was present in the English Channel ^[6]. In the following years, the population declined, and in December 2005, the species became again temporarily very abundant ^[3].

In 1979, this exotic species was spotted in the Skagerrak, off the Norwegian coast ^[2]. In the Netherlands, it was observed for the first time in 1981. That year, it spread to the German Wadden Sea ^[7]. In Germany, the species was also found in 1993 in the Elbe estuary ^[4].

How T. punctigera got introduced to our regions is not known with certainty. Most likely, the introduction occurred via ballast water or transport of breeding oysters ^[6]. Once introduced, the single-celled seaweed is further dispersed by the sea currents ^[8].

This diatom is across the globe, but there remains uncertainty about its area of origin. Therefore, it is unknown whether the species got introduced to our region immediately from its native area or from a region to which it was endemic ^[6]. However, the non-native status of the species is disputed by some scientists. It is possible that this diatom has always been present but never noticed because of its low abundance ^[3].

Compared to most other diatoms, this invasive species has a wide tolerance to fluctuations in salinity and temperature. As a result, the species is present all year (except during hot summers)  ^{[3, 9]}.

Concentrations of T. punctigera are usually highest in estuaries such as those of the Scheldt, the Meuse or the Rhine, where the water is brackish. In the open sea, the abundance of this exotic species is much lower. This diatom prefers a less saline environment ^{[3, 7, 9]}. This is confirmed by the high abundance of the species in the English Channel (between 1980 and 1981), where the species is relatively rare in normal years. The period 1980-1981 was characterised by high rainfall and thus an increased supply of fresh water to the English Channel ^[3].

T. punctigera floats freely in the water column and spreads via the currents. But the diatom can attach itself to organisms and other substrates to reach new places ^[8].

The species can become abundant under certain conditions, such as during abnormal seasons (sharp drop in temperature or salinity, due to increased river flow) ^{[3, 9]}. However, it is unknown how a high abundance of T. punctigera affects the ecosystem.

T. punctigera could benefit more than smaller diatoms from future increases in CO₂ concentration. This could impact the biochemical activity and even the food web of the oceans since larger algae produce more biomass and thus provide more food for higher trophic levels. Furthermore, they absorb more CO₂ than smaller algae, which would cause CO₂ levels to increase at a slightly slower rate ^[10].

Diatoms are single-celled algae that are studied microscopically. They have an external silica skeleton (frustule) consisting of 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 ^[11].

T. punctigera is a circular diatom of medium size (40-100 μm) ^[3] with a thick cell wall. Diatoms float throughout the year in the water column (except when water temperatures are too high during summer), or attach to other organisms as individual single-celled algae or in small colonies of a few cells ^[3].

[1]          World Register of Marine Species (WoRMS) (2024). Thalassiosira punctigera (Castracane) Hasle, 1983. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=148936] (2024-10-18).

[2]          Hasle, G.R. (1983). Thalassiosira punctigera (Castr.) comb. nov., a widely distributed marine planktonic diatom. Nord. J. Bot. 3(5): 593-608. [http://www.vliz.be/imis/imis.php?module=ref&refid=120556]

[3]          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]

[4]          Muylaert, K.; Sabbe, K. (1996). The diatom genus Thalassiosira (Bacillariophyta) in the estuaries of the Schelde (Belgium-The Netherlands) and the Elbe (Germany). Bot. Mar. 39: 103-115. [http://www.vliz.be/imis/imis.php?module=ref&refid=3082]

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

[6]          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]

[7]          Kat, M. (1982). Effects of fluctuating salinities on development of Thalassiosira angstii, a diatom not observed before in the Dutch coastal area. J. Mar. Biol. Ass. U.K. 62(2): 483-484. [http://www.vliz.be/imis/imis.php?module=ref&refid=206981]

[8]          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]

[9]          Dürselen, C.-D.; Rick, H.-J. (1999). Spatial and temporal distribution of two new phytoplankton diatom species in the German Bight in the period 1988 and 1996. Sarsia 84: 367-377. [http://www.vliz.be/imis/imis.php?module=ref&refid=32221]

[10]        Wu, Y.; Campbell, D.A.; Irwin, A.J.; Suggett, D.J.; Finkel, Z.V. (2014). Ocean acidification enhances the growth rate of larger diatoms. Limnol. Oceanogr. 59(3): 1027-1034. [http://www.vliz.be/nl/catalogus?module=ref&refid=297672]

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

VLIZ Alien Species Consortium (2024). Thalassiosira punctigera. Introduced alien species of the Belgian part of the North Sea and bordering estuaries anno 2024. Flemish Institute for the Sea (VLIZ). 5 pp.