Botrylloides diegensis -

Although the species was described based on specimens from San Diego Bay (California, USA), its limited distribution along the west coast of North America and the absence of other native Botrylloides species suggest that this tunicate is non-native to this region ^[2,3]. It is suggested that the species may originate from the northwestern part of the Pacific Ocean, but this has not yet been confirmed ^[3].

First observation in Belgium

Botrylloides cf. diegensis was likely recorded for the first time in Belgian territory in 2019, in the marina of Zeebrugge ^[4]. It may have been previously overlooked due to confusion with the Violet tunicate Botrylloides violaceus ^[5].
 

Distribution in Belgium

This tunicate is currently found in the marina of Zeebrugge and has possibly been observed in the Vuurtorendok and the Sluice Dock of Ostend since 2023 ^[4]. However, it would be desirable for Belgian specimens of this presumed species to undergo thorough examination (DNA sequencing and anatomy) to confirm the identification ^[5].

However, it should be noted that between October 2023 and February 2024, all the old pontoons in the Zeebrugge marina were replaced with new ones, resulting in the removal of the flora and fauna present on them. It remains to be seen how recolonisation on this new type of pontoon will proceed ^[5].
 

Distribution in neighbouring countries

In Europe, the first specimens of Botrylloides diegensis may have been identified along the French Atlantic coast as early as 1999 ^[6]. It was reported in the Netherlands in 2002 ^[7]. Since 2004, specimens of this tunicate have also been found along the French and English Channel coasts ^[8-10]. In 2011, DNA research revealed many colonies along the Breton (France) and South English coasts. Similar research on Botrylloides material from the Mediterranean region (Spain, France, Italy) also identified Botrylloides diegensis in this area ^[11,12]. In 2015, it was observed that the species was rapidly spreading along the English east coast (Suffolk and Essex). Additionally, an apparently isolated northern site was discovered on the English west coast (Lancashire) ^[9]. In 2017, Botrylloides diegensis was observed off the coast of the Channel Islands Guernsey and Jersey ^[2,13]. On the Irish east coast, a possible initial establishment of this tunicate for Ireland was recorded on the Howth Peninsula (Dublin) in 2021 ^[14].

This tunicate is capable of colonising new regions through transport as hull fouling on ships ^[3].

The species is found on hard substrates such as in marinas and docks in other ports, as well as on oyster reefs, rocks, and ship hulls ^[2]. This tunicate has a considerable salinity tolerance ranging from 18 to 35 psu ^[2,15] and occurs in cold to warm temperate waters ^[2].

After fertilisation, a short-lived free-swimming larval stage occurs (less than a day) ^[16,17]. This short larval stage makes secondary natural dispersal over long distances unlikely.

The ecological and economic impact of Botrylloides diegensis is unknown. The expansion of the species is considered a potential threat to shellfish farming ^[2].

Botrylloides diegensis is a colonial ascidian (Tunicata). The colonies are flat and crust-like, approximately 5 mm thick and sometimes larger than 50 cm². Zooids are embedded in a gelatinous matrix (a common tunic) and arranged in meandering chains, with 10 to 15 zooids grouped into smaller units (elongated clusters) that share a common cloacal aperture. The zooids are cylindrical, about 2-2.5 mm long and 1 mm wide. The inlet or oral opening is smooth and circular. The individual outlet or atrial opening of the zooids (which leads into a common cloacal cavity or cloaca) is widely open, and the edge is elongated into a tongue-like projection of variable size and length. There are typically 16 oral tentacles of three different sizes, symmetrically arranged inside at the base of the inlet siphon: four large tentacles arranged crosswise with alternating eight small and for medium-sized tentacles. The branchial sac usually has 10-12 rows of branchial stigmata arranged in a reticulated vascular system. Each ‘mesh’ of the vascular network contains two to three stigmata, except those next to the dorsal lamina, which contain four to six. In the stomach, there are nine to ten loop-shaped organs visible as stomach folds through the stomach wall ^{[2,7,16,18-20]}.

The colors of Botrylloides diegensis vary, making it difficult to distinguish from other Botrylloides species, often necessitating DNA analysis for accurate identification ^[8,11,21-23]. In our region, Botrylloides diegensis is typically characterised by monochrome orange or yellow colonies contrasting against a brownish or translucent pale background of the matrix. Global research on the species indicates that zooid colors can range from brown, orange, yellow, purple to dirty white, and bi-colored colonies are also common. Additionally, the color of the matrix can vary. Around the inlet of the zooids, a clear or pale-colored orange, yellow, green, or white ring is sometimes visible ^{[7,11,16,19,24,25]}.

Botrylloides diegensis can potentially be confused with another non-native species, Botrylloides violaceus. In this latter species, however, the second row of stigmata (from the inflow opening) is incomplete and never reaches the dorso-median line of the gill sac, the larvae are larger, and the number of ampullae on it is more numerous ^[7,25,33,34]. Botrylloides diegensis can also be confused with the native (but of more Mediterranean origin) Two-toned tunicate Botrylloides leachii in Northwest Europe ^{[22,26-30,32]}. Beyond our faunal area, she has often been confused with local sister species in the past ^[31]. Moreover, there is some ambiguity regarding the species boundaries of Botrylloides leachii, highlighting the complexity of the Botrylloides story ^[32].

Compound ascidians reproduce both asexually (via runners (stolons) from the body wall of zooids) and sexually (fertilised eggs develop into larvae). The zooids are hermaphroditic, but depending on the life cycle, eggs may be fertilised internally or externally (by nearby animals' sperm), resulting in a free-swimming larva that typically settles on a solid substrate within a day ^[16,17].

[1]          World Register of Marine Species (WoRMS) (2024). Botrylloides diegensis Ritter & Forsyth, 1917. https://www.marinespecies.org/aphia.php?p=taxdetails&id=252278 (2024-10-18).

[2]          Nemesis. Botrylloides diegensis. https://invasions.si.edu/nemesis/species_summary/-448 (2024-04-04)

[3]          Carlton, J.T. (2009). Deep invasion ecology and the assembly of communities in historical time, in: Rilov, G. et al. Biological invasions in marine ecosystems: ecological, management, and geographic perspectives. Ecological Studies, 204: pp. 13-56. [https://www.vliz.be/en/imis?module=ref&refid=393519] 

[4]          waarnemingen.be. Botrylloides diegensis Ritter & Forsyth, 1917. https://waarnemingen.be/species/159676 (2024-04-04)

[5]          Dumoulin, E. (2024). Persoonlijke mededeling. 

[6]          Bishop, J.D.D.; Wood, C.A.; Lévêque, L.; Yunnie, A.L.E.; Viard, F. (2015). Repeated rapid assessment surveys reveal contrasting trends in occupancy of marinas by non-indigenous species on opposite sides of the western English Channel. Mar. Pollut. Bull. 95(2): 699-706. [https://www.vliz.be/en/imis?module=ref&refid=393431] 

[7]          Faasse, M. (2006). Botrylloides cf. diegensis (Ritter & Forsyth, 1917) en B. violaceus Oka, 1923 in Nederland. Het Zeepaard 66(4): 101-105. [https://www.vliz.be/nl/imis?module=ref&refid=101765] 

[8]          Bishop, J.; Wood, C.; Yunnie, A.; Griffiths, C. (2015). Unheralded arrivals: non-native sessile invertebrates in marinas on the English coast. Aquat. Invasions 10(3): 249-264. [https://www.vliz.be/en/imis?module=ref&refid=393518] 

[9]          Wood, C.A.; Bishop, J.D.D.; Rennocks, L.; Crundwel, R. (2016). RAS 2015: Non-native species rapid assessment surveys in English marinas (E Anglia & W coast). The Bromley Trust: London. 34 pp. [https://www.vliz.be/en/imis?module=ref&refid=393517]

[10]        Goulletquer, P. (2016). Guide des organismes exotiques marins - littoral atlantique et littoral méditerranéen. Références Nature. BELIN: [s.l.]. ISBN 978-2701190204. 304 pp. [https://www.vliz.be/nl/imis?module=ref&refid=316984] 

[11]        Viard, F.; Roby, C.; Turon, X.; Bouchemousse, S.; Bishop, J. (2019). Cryptic diversity and database errors challenge non-indigenous species surveys: An illustration with Botrylloides spp. in the English Channel and Mediterranean Sea. Front. Mar. Sci. 6: 615. [https://www.vliz.be/en/imis?module=ref&refid=393515] 

[12]        MNHN & OFB (Ed.) 2003-2024. Fiche de Botrylloides diegensis Ritter & Forsyth, 1917. Inventaire national du patrimoine naturel (INPN). https://inpn.mnhn.fr/espece/cd_nom/837382 (2024-05-05)

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[14]        O’Shaughnessy, K.; Lyons, D.; Ashelby, C.; Counihan, R.; Pears, S.; Taylor, E.; Davies, R.; Stebbing, P. (2023). Rapid assessment of marine non-native species in Irish marinas. Manag. Biol. Inv. 14(2): 245-267. [https://www.vliz.be/en/imis?module=ref&refid=393412] 

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[16]        Van Name, W.G. (1945). The North and South American ascidians. Bulletin of the American Museum of Natural History, 84. Order of the Trustees/[American Museum of Natural History]: New York. vii, 476 p., 31 plates pp. [https://www.vliz.be/en/imis?module=ref&refid=393513] 

[17]        Barnes, R.D. (1987). Invertebrate zoology. 5th edition. Saunders: Philadelphia. ISBN 0-03-008914-X. IX, 893 pp. [https://www.vliz.be/en/imis?module=ref&refid=393512] 

[18]        Abbott, D.P.; Lambert, C.C.; Lambert, G.; Newberry, A.T. (2007). Ascidiacea. in: Carlton, J.T. (Ed.). The Light and Smith Manual. Intertidal Invertebrates from Central California to Oregon, Completely Revised and Expanded. Fourth Edition. University of California Press,: Berkeley: 949-964. [https://www.vliz.be/nl/imis?module=ref&refid=300117] 

[19]        Ritter, W.E.; Forsyth, R.A. (1917). Ascidians form the littoral zone of southern California. Univ. Calif. Publ. Zool. 16(24): 439-512. [https://www.vliz.be/en/imis?module=ref&refid=393511]  

[20]        Brunetti, R.; Mastrototaro, F. (2017). Ascidiacea of the European waters. Fauna d’Italia, 51. Calderini/Edagricole: Milano. ISBN 9788850655298. 447 pp. [https://www.vliz.be/en/imis?module=ref&refid=393375] 

[21]        Rocha, R.M.; Salonna, M.; Griggio, F.; Ekins, M.; Lambert, G.; Mastrototaro, F.; Fidler, A.; Gissi, C. (2019). The power of combined molecular and morphological analyses for the genus Botrylloides: identification of a potentially global invasive ascidian and description of a new species. Syst. Biodiv. 17(5): 509-526. [https://www.vliz.be/en/imis?module=ref&refid=393408] 

[22]        Temiz, B.; Clarke, R.M.; Page, M.; Lamare, M.; Wilson, M.J. (2024). Identification and characterisation of Botrylloides (Styelidae) species from Aotearoa New Zealand coasts. N.Z. J. Mar. Freshwat. Res. 58(2): 255-273. [https://www.vliz.be/en/imis?module=ref&refid=393407] 

[23]        André, F.; Lamare, V.; Pean, M. in: DORIS, 27/06/2021: Botrylloides spp. (leachii, violaceus, diegensis) (Savigny, 1816). https://doris.ffessm.fr/ref/specie/481 (2024-05-05)

[24]        Lambert, Ch.C.; Lambert, G. (2003). Persistence and differential distribution of nonindigenous ascidians in harbors of the Southern California Bight. Mar. Ecol. Prog. Ser. 259: 145-161. [https://www.vliz.be/en/imis?module=ref&refid=393506] 

[25]        Cohen, A.N. (2011). The exotics guide: Non-native marine species of the North American Pacific coast. Center for Research on Aquatic Bioinvasions/San Francisco Estuary Institute: Richmond, Oakland. [https://www.vliz.be/en/imis?module=ref&refid=393404] 

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[27]        Bowen, S.; Goodwin, C.; Kipling, D.; Picton, B. (2018). Sea squirts and sea sponges of Britain and Ireland. Wild Nature Press. Princeton University Press: Princeton. ISBN 978-0995567382. 200 pp. [https://www.vliz.be/en/imis?module=ref&refid=393399]

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[30]        Arenas, F.; Bishop, J.D.D.; Carlton, J.T.; Dyrynda, P.E.J.; Farnham, W.F.; Gonzalez, D.J.; Jacobs, M.W.; Lambert, C.; Lambert, G.; Nielsen, S.E.; Pederson, J.A.; Porter, J.S.; Ward, S.; Wood, C.A. (2006). Alien species and other notable records from a rapid assessment survey of marinas on the south coast of England. Journal of the Marine Biological Association of the United Kingdom 86(6): 1329-1337. [https://www.vliz.be/nl/imis?module=ref&refid=119308]

[31]        Monniot, C. (1988). Ascidies de Nouvelle-Calédonie. IV. Styelidae (suite). Bull. Mus. Natl. Hist. Nat., Sect. A Zool. Biol. Ecol. Anim. Serie 4, 10(2): 163-196. [https://www.vliz.be/en/imis?module=ref&refid=393381]  

[32]        Reem, E.; Douek, J.; Rinkevich, B. (2018). Ambiguities in the taxonomic assignment and species delineation of botryllid ascidians from the Israeli Mediterranean and other coastlines. Mitochondrial DNA. Part A 29(7): 1073-1080. [https://www.vliz.be/en/imis?module=ref&refid=393378] 

[33]        Saito, Y.; Mukai, H.; Watanabe, H. (1981). Studies on Japanese compound styelid ascidians: II. A new species of the genus Botrylloides and redescription of B. violaceus Oka. Publ. Seto Mar. Biol. Lab. 26(4-6): 357-368. [https://www.vliz.be/nl/imis?module=ref&refid=40653] 

[34]        Saito, Y.; Watanabe, H. (1985). Studies on Japanese compound styelid ascidians IV. Three new species of the genus Botrylloides from the vicinity of Shimoda. Publications of the Seto Marine Biological Laboratory 30(4-6): 227-240. [https://www.vliz.be/nl/imis?module=ref&refid=395737]

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