Diplosoma listerianum -

Diplosoma listerianum has a global area of distribution. Its exact area of origin is currently unknown ^[2], which is why scientists describe this species as a cryptogenic species ^[3].

This sea squirt grows on stones, pontoons and other hard substrates, but can be found on seaweeds, seagrass and other sea squirts. The species is mainly found in sheltered areas down to depths of 80 meters ^[4].

First observation in Belgium

In Belgium, Diplosoma listerianum was first reported from the marina of Zeebrugge on the 12^th of September 2002 ^[5].

In a 1993 overview of the sea squirts of western Europe, the species got already reported as ‘occurring near the coast of the Netherlands and Belgium’ ^[6]. However, it is unclear whether this report concerns the Belgian, Dutch, or both sea areas. The first observation in Belgium remains, therefore, uncertain. The year 2002 is considered the official date of introduction.

Spreading in Belgium

After the appearance of the species in 2002, it became abundant in the marina of Zeebrugge. Here, it was regularly observed until the end of 2008 ^[7]. However, during search events on and around the pontoons in August 2011, the sea squirt could no longer be found ^[5]. Since 2019, there have again been occasional reports of the species in the Zeebrugge marina ^[8].

In 2020, Diplosoma listerianum was found on the foundations of offshore wind turbines in the Belwind wind farm, about 50 km offshore. The tunicate was observed in the sublittoral zone, attached to mussels ^[9,10]. During an analysis of the subtidal macrofauna at the same location in 2010, the species had not yet been observed there ^[9]. In 2023, Diplosoma listerianum was also reported for the first time in Ostend, once in the Sluice Dock and once in the Vuurtorendok ^[8].

Spreading in neighbouring countries

Diplosoma listerianum occurs worldwide. Its distribution along the Atlantic coast stretches from South Africa to Norway ^[11]. A West European species list, compiled in 1933, specifies the known range of the species as ‘found along the entire west coast of Europe, from Norway to the Mediterranean Sea’ ^[12].

The first official observation of Diplosoma listerianum in the Netherlands dates back to 1977, in Zeeland (near Burghsluis), Flauwers, and the harbour of Zierikzee ^[12]. Until 1988, Diplosoma listerianum was only observed in Schouwen-Duiveland in the Eastern Scheldt ^[13]. Later, in 1990, this species was also present in the ‘Canal through Walcheren’ near Vlissingen, in ‘Nieuw- en Sint Joosland’ and in Veere, and along the Eastern Scheldt. Nowadays, this sea squirt is widespread in Lake Grevelingen, the Eastern Scheldt, and the harbour of Goes ^[14]. The first individual coming from the Wadden Sea was confirmed in 2014, in the port of Oudeschild, on Texel Island ^[15].

In France, Diplosoma listerianum occurs near Roscoff in Brittany ^[16]. In the United Kingdom, the species occurs along the south and west coasts but not yet along the east coast ^[17].

The introduction of this sea squirt occurred via attachment to ship hulls and transport of aquaculture organisms, such as oysters ^[18].

To date, Diplosoma listerianum only got observed in Belgium in the marina of Zeebrugge. The species can, therefore, hardly be considered successful in Belgium.

Nevertheless, the species can adhere to all types of substrates such as stones, piers or other sea squirts ^[19], making ports – with their many artificial surfaces – an ideal environment.

Diplosoma listerianum occurs worldwide ^[11]. This suggests that temperature does not limit its range. The species can reproduce in a wide temperature range (between 10 and 25°C) ^[20].

Once Diplosoma listerianum establishes itself, it quickly becomes the most common species of the biofouling community. Other organisms could be displaced or overgrown ^[17], although there is no clear evidence of any species being displaced by Diplosoma listerianum in Europe. By overgrowing equipment or organisms (and possibly displacing other species), this sea squirt might cause economic damage to the aquaculture sector (e.g. mussel farms) ^[21]. They add additional weight to installations and structures and interfere with the water and nutrient exchange of others organisms ^[22-26]. In the cultivation of shellfish, about 30% of the costs go to the removal and control of the growth of sea squirts. This is done by exposing them to air ^[27], wrapping them in plastic foil ^{[28, 29]}, or by applying diluted bleach ^[30], vinegar ^[31], acetic acid or calcium hydroxide ^[21,32].

The presence of this species on ship hulls increases the hydrodynamic drag of the ships ^[18]. Preventing attachment – by cleaning and treatment with antifouling paint – is expensive ^[33].

Diplosoma listerianum is a filter feeder. Water, carrying numerous small food particles, flows through the oral siphon. The particles get caught in mucus produced by a gland called the endostyle. Water carrying waste flows out of the sea squirt via the atrial siphon ^[12].

Diplosoma listerianum forms thin, off-white colonies less than two millimetres thick, covering only a few square millimetres of surface area. Occasionally, they cover much larger surfaces. The soft-feeling colonies are translucent with black dots representing the individuals – called zooids – with one individual having a diameter of about two millimetres. The atrial siphons stand out as large holes ^[12]. When the colony is studied in more detail, many small white dots are visible on the otherwise transparent colony. These white dots are often found in larger densities around the large atrial siphons, making it seem as if a white line runs around these openings. Some colonies of another tunicate, Didemnum vexillum, are also mostly transparent. The white dots in these colonies are no pigment spots, like in Diplosoma species, but small skeletal structures (spicules).

The identification of this alien species is not straightforward. Diplosoma listerianum is often confused in Western Europe with another tunicate, Diplosoma spongiforme. In general, Diplosoma spongiforme is less translucent and thicker (5 to 6 mm) than Diplosoma listerianum, although thicker specimens of the latter have also been found. Consequently, this characteristic is not a definitive identification feature. To reliably distinguish between the two species, microscopic examination of their anatomy is required ^[16].

The freely swimming larvae of Diplosoma listerianum enable the species to spread easily ^[34]. The larvae move by alternately swimming upwards towards the light, followed by sinking. They avoid the bottom ^[35]. When the larvae are ready to settle, they attach themselves to a shady and predator-sheltered place, such as a rock’s sidewall ^{[34, 35]}.

The larval stage lasts only a few hours, which is remarkable for a species that is so widespread. Given the limited propagation rate per generation, big genetic differences are observed between different populations ^[36]. Furthermore, Diplosoma listerianum is actually a species complex consisting of different cryptic species. Conversely, genetic similarities have also been found between populations far away from each other (Plymouth, California and Melbourne). These similarities indicate a recent human influence through long-distance transport by shipping or import for aquaculture.

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[2]          De Blauwe, H.; Dumoulin, E. (2009). De zeefauna en -flora uit de jachthaven van Zeebrugge, in het bijzonder de fouling-organismen van drijvende pontons. De Strandvlo 29(2): 41-63. [http://www.vliz.be/imis/imis.php?module=ref&refid=139489]

[3]          Glasby, T.M.; Connell, S.D.; Holloway, M.G.; Hewitt, C.L. (2007). Nonindigenous biota on artificial structures: could habitat creation facilitate biological invasions? Mar. Biol. (Berl.) 151(3): 887-895. [http://www.vliz.be/en/imis?module=ref&refid=196986]

[4]          Soortenbank.nl: Dieren planten en paddenstoelen in Nederland (2018). Geleikorstzakpijp (Diplosoma listerianum). [http://www.soortenbank.nl/soorten.php?soortengroep=duikgids&id=154] (2018-07-19).

[5]          De Blauwe, H. (2010). Persoonlijke mededeling

[6]          Huus, J. (1933). Ascidiacea, in: Grimpe-Wagler. Die Tierwelt der Nord- und Ostsee, 25(12.a3). Akademische Verlagsgesellschaft: Leipzig: pp. 49-115. [http://www.vliz.be/nl/catalogus?module=ref&refid=32957]

[7]          Waarnemingen afkomstig van Waarnemingen.be: een initiatief van Natuurpunt Studie vzw en de Stichting Natuurinformatie (2018). Grijze korstzakpijp - Diplosoma listerianum. [https://waarnemingen.be/soort/view/27529?from=2007-08-14&to=2009-08-14&species=27529&prov=0&akt=0&from=1983-09-01&to=2009-08-14&prov=0] (2018-07-19).

[8]          waarnemingen.be. Diplosoma listerianum (Milne Edwards, 1841). https://waarnemingen.be/species/27529. (2024-10-03) 

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[16]          Lafargue, F. (1983). Inventaire des ascidies Didemnidae de Roscoff (Tuniciers). Cah. Biol. Mar. 24(4): 377--381. [http://www.vliz.be/en/imis?module=ref&refid=63593]

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[18]        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]

[19]       Soortenbank. http://www.soortenbank.nl/soorten.php?soortengroep=duikgids&id=154 (07/09/2009)

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