Botrylloides violaceus - Colonial sea squirt

The chain tunicate is native to the northwestern Pacific Ocean, near Japan ^[2].

First observation in Belgium

The first observation in our study area dates back to 2000 when this tunicate got reported in the marina Breskens, in the Western Scheldt. It concerned already more than a hundred colonies ^[3].

The first observation in Belgium dates back to the 15th of September 2003. That day, this tunicate was observed attached to the hull of a yacht during a cleaning operation on the quay. Since the ship had sailed along the Dutch and Belgium coast, the area of origin could not be determined. Therefore, does this sighting not count as the first official observation in Belgium ^[4]. On the 12^th of June 2004, the chain tunicate was found on pontoons in the marina of Zeebrugge ^[4].  

Spreading in Belgium

One month after the first official observation in the marina of Zeebrugge (2004), numerous colonies in various colours were found at the same location. Until now, the presence of this species on Belgian territory is limited to Zeebrugge ^[4].

In our study area, the chain tunicate is also found in the Dutch Western Scheldt, from the estuary to Vlissingen and beyond ^[5].

Spreading in neighbouring countries

The first observation in Europe dates back to May 1991 in Venice, Italy. It is suspected that the species got introduced one to two years earlier ^[6].

In 1999, a small red sea squirt colony was photographed on a mussel farm in Zijpe (North Holland). It is possible that this already concerned the chain tunicate ^[7]. In the following year, the chain tunicate was observed in the marina of Breskens, near the estuary of the Western Scheldt ^[3]. More than a thousand colonies got reported from the marina of Yerseke in the Eastern Scheldt in 2002 ^[8]. The species subsequently invaded Lake Grevelingen ^[7], and

was first observed in the Wadden Sea in 2009 ^[9].

In 2004, a study in the south of England revealed that the chain tunicate was already widely spread in this area as well. The presence of numerous colonies got also reported in Brittany, France ^[10].

Since a large number of colonies were found each time, it is suspected that the first introduction of the species occurred earlier than the official report dates. Therefore, it is unclear where and when this sea squirt got introduced to Europe. The confusion of the exotic sea squirt with the native species B. leachii caused a delay in the identification of the chain tunicate in Europe ^{[3, 7, 10]}.

Introduction to Europe most likely occurred by attachment to ship hulls or living organisms such as shellfish ^[3].

The chain tunicate owes its success to its being unselective regarding the substrate to which it attaches itself. This tunicate attaches itself to a variety of materials, ranging from ship hulls to floating objects, but also living organisms, such as bryozoans, shells and algae ^[3]. After the chain tunicate attaches itself to an organism, it can overgrow its host. The tunicate can, to some extent, displace the native star ascidian Botryllus schlosseri ^[5]. Furthermore, it has been demonstrated that colonies of B. violaceus expand more rapidly in the presence of the native B. schlosseri compared to situations where the latter is absent on the substrate. In addition to sexual reproduction, this exotic species can reproduce asexually via budding ^[2], which also contributes to its success.

Attachment to ship hulls can result in a rapid spread between different ports. In the southern North Sea, recreational yachts play a particularly important role in the secondary spread of the species ^[3].

The chain tunicate prefers cold coastal and marine areas, which explains its rapid spread in the north ^[2]. Additionally, this sea squirt can adapt to various environmental conditions. The species is found in areas with temperatures ranging from -1 to 25°C ^[11] and salinities between 19 and 34 PSU ^[9] (salt to brackish water with a salty influence). By comparison, the North Sea has an average salinity of approximately 35 PSU.

Because marinas – with their many artificial hard substrates – offer numerous growth opportunities, this sea squirt can be locally abundant ^[12].

Along with many other organisms, the chain tunicate is part of the biofouling community. Fouling can affect various substrates and cause significant economic damage ^[13]. Cleaning and treatment of ship hulls with antifouling paint is expensive ^[14]. Additionally, many of these paints are harmful to the ecosystem ^[15].

When present in large numbers, the species may modify environmental conditions or compete with other (native) species ^[16-18]. For example, water quality might be affected when the species is abundant. This is due to their faecal matter or mortality within a colony, which can affect the soil quality ^[19]. Off the Dutch coast, it was shown that the chain tunicate – in water with high salinity – can partly displace the star ascidian B. schlosseri by spatial competition. In brackish water, the star ascidian withstands competition with the chain tunicate, since the former species is more resistant to lower salinities ^[5].

Depending on the turbidity and the amount of food in the system, the presence of the chain tunicate can bring about other, sometimes conflicting, effects. For example, the additional filtration in a turbid system will have a positive effect on the algae and plants by improving the light conditions. However, when there is a food shortage, native species will be more affected by the presence of this non-native tunicate due to more intense competition for food, increasing the possibility that the former species will get displaced ^[19]. Other effects cannot be ruled out as the species is often observed in large numbers ^{[3, 9]}.

The chain tunicate belongs to the class of the sea squirts (Ascidiacea). This species forms colonies with a crust-like appearance. The colonies have a thickness of 2 to 3 millimetres and can reach diameters of more than 30 centimetres ^[2]. The colonies can have different colours, including off-white, yellow, orange and purple. A single colony can comprise multiple colours ^[19].

Each colony consists of different individuals, called zooids. Each individual is cylindrical and reaches a length of 2.5 to 3 millimetres ^[2]. Like all sea squirts, the chain tunicate collects its food by using an inner ‘sieve’ (pharynx), which filters phytoplankton and zooplankton along with organic matter from the water column ^[20]. The water that enters the body through the oral siphon flows through the pharynx, which collects the food particles, and is then pushed out through the atrial syphon ^[21].

A microscope is required to distinguish the chain tunicate from other species. Characteristic of this species are the 10 to 11 rows of gill slits (pharyngeal stigmata), which can be found on each side of the pharynx ^[2]. The best feature to distinguish this species from other sea squirts is the morphology of the larvae. Larvae are considerably larger than those of the similar-looking sea squirts B. diegensis and B. leachii ^[19].

Reproduction can be asexual by budding or sexual by the production of larvae that swim freely in the water column for 4 to 10 hours before they settle ^[2].

[1]          World Register of Marine Species (WoRMS) (2020). Botrylloides violaceus Oka, 1927. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=148715] (2020-11-17).

[2]          Saito, Y.; Mukai, H.; Watanabe, H. (1981). Studies on Japanese compound styelid ascidians: 2. A new species of the genus Botrylloides and redescription of B. violaceus Oka. Publ. Seto Mar. Biol. Lab. 26(4-6): 357-368. [http://www.vliz.be/nl/catalogus?module=ref&refid=40653]

[3]          Faasse, M.; De Blauwe, H. (2002). De exotische samengestelde zakpijp Botrylloides violaceus Oka, 1927 in Nederland (Ascidiacea: Pleurogona: Styelidae). Het Zeepaard 62(5): 136-141. [http://www.vliz.be/en/imis?module=ref&refid=27962]

[4]          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]

[5]          Gittenberger, A.; Moons, J.J.S. (2011). Settlement and possible competition for space between the invasive violet tunicate Botrylloides violaceus and the native star tunicate Botryllus schlosseri in The Netherlands. Aquat. Invasions 6(4): 435-440. [http://www.vliz.be/en/imis?module=ref&refid=208012]

[6]          Brunetti, R. (2011). Persoonlijke mededeling

[7]          Gittenberger, A. (2007). Recent population expansion of non-native ascidians in The Netherlands. J.Exp. Mar. Biol. Ecol. 342(1): 122-126. [http://www.vliz.be/en/imis?module=ref&refid=206037]

[8]          Faasse, M.; De Blauwe, H. (2002). Naschrift bij het artikel over de zakpijp Botrylloides violaceus. Het Zeepaard 62(5): 150-150. [http://www.vliz.be/en/imis?module=ref&refid=28299]

[9]          Gittenberger, A.; Rensing, M.; Stegenga, H.; Hoeksema, B. (2010). Native and non-native species of hard substrata in the Dutch Wadden Sea. Ned. Faunist. Meded. 33: 21-76. [http://www.vliz.be/en/imis?module=ref&refid=206549]

[10]        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. J. Mar. Biol. Ass. U.K. 86(6): 1329-1337. [http://www.vliz.be/en/imis?module=ref&refid=119308]

[11]        Lindeyer, F.; Gittenberger, A. (2011). Ascidians in the succession of marine fouling communities. Aquat. Invasions 6(4): 421-434. [www.vliz.be/en/imis?module=ref&refid=208009]

[12]        Simkanin, C.; Dower, J.F.; Filip, N.; Jamieson, G.; Therriault, T.W. (2013). Biotic resistance to the infiltration of natural benthic habitats: Examining the role of predation in the distribution of the invasive ascidian Botrylloides violaceus. J. Exp. Mar. Biol. Ecol. 439: 76-83. [http://www.vliz.be/nl/catalogus?module=ref&refid=297203]

[13]        Carlton, J. (1989). Man’s role in changing the face of the ocean: biological invasions and implications for conservation of near-shore environments. Conserv. Biol. 3(3): 265-273. [www.vliz.be/en/imis?module=ref&refid=142020]

[14]        Schultz, M.P.; Bendick, J.A.; Holm, E.R.; Hertel, W.M. (2010). Economic impact of biofouling on a naval surface ship. Biofouling 27(1): 87-98. [http://www.vliz.be/en/imis?module=ref&refid=206434]

[15]        Dafforn, K.A.; Lewis, J.A.; Johnston, E.L. (2011). Antifouling strategies: History and regulation, ecological impacts and mitigation. Mar. Pollut. Bull. 62(3): 453-465. [http://www.vliz.be/en/catalogue?module=ref&refid=296664]

[16]        Wallentinus, I.; Nyberg, C.D. (2007). Introduced marine organisms as habitat modifiers. mar. Pollut. Bull. Spec. Issue 55(7-9): 323-332. [http://www.vliz.be/en/imis?module=ref&refid=120643]

[17]        Galil, B.S. (2007). Loss or gain? Invasive aliens and biodiversity in the Mediterranean Sea. Mar. Pollut. Bull. Spec. Issue 55(7-9): 314-322. [http://www.vliz.be/en/imis?module=ref&refid=120641]

[18]        Carver, C.E.; Mallet, A.L.; Vercraemer, B. (2006). Biological synopsis of the colonial tunicates, Botryllus schlosseri and Botrylloides violaceus. Canadian Manuscript Report of Fisheries and Aquatic Sciences = Rapport Manuscrit Canadien des Sciences Halieutiques et Aquatiques, 2747. Fisheries and Oceans Canada, Bedford Institute of Oceanography. Dartmouth. 42 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=297204]

[19]        Gittenberger, A. (2011). Persoonlijke mededeling

[20]        Ruppert, E.E.; Barnes, R.D. (1994). Invertebrate zoology. 6th edition. Saunders College Publishing: Orlando. ISBN 0-03-026668-8. 1056 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=9414]

[21]        Millar, R.H. (1970). British ascidians, Tunicata: Ascidiacea: keys and notes for the identification of the species. Synopses of the British Fauna, N.S. 1. Academic Press: London, UK. ISBN 12-496650-0. 92 pp. [http://www.vliz.be/en/imis?module=ref&refid=25181]

VLIZ Alien Species Consortium (2020). Botrylloides violaceus – Chain tunicate. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 6 pp.