Tricellaria inopinata -

T. inopinata is native to the west coast of North America ^[3]. This species attaches itself to various objects such as pieces of wood, the walls of docks, hulls or ropes, and other organisms such as algae and mussel shells ^[4].

First observation in Belgium

In the Netherlands, T. inopinata was observed for the first time in August 2000. In response to this, many coasts and marinas – from northern Spain to the Netherlands – were searched for this alien species. This search resulted in the first observations in Belgium (October 2000), namely in the marinas of Blankenberge and Ostend. In each case, only one colony was found ^[5].

Spreading in Belgium

After finding the first colonies of T. inopinata in October 2000, no more individuals were found at the same locations in 2001. The species probably did not survive the winter in these locations. In September 2001, some colonies washed ashore on the beach between Ostend and Blankenberge, presumably coming from the English Channel. In November of the same year, colonies were found in the marina of Zeebrugge. These are descendants of colonies introduced to the harbour via attachment to the hulls of recreational yachts ^{[5, 6]}. T. inopinata is currently considered a common species in the marinas of Belgium and is a frequent sight on washed-up items near the tidemark on the beach ^[7].

Spreading in neighbouring countries

T. inopinata expanded its range from the American west coast to Japan and New Zealand. Eventually, it ended up in the Mediterranean Sea. The species was first described based on individuals from Venice in 1985 ^[3].

Along the northeast Atlantic coasts, T. inopinata was first observed in 1996, in Galicia (northern Spain) ^[8]. Soon after, in 1998, the species was present in Poole Harbour (southern coast of England) ^[9]. In 2006, the species was observed in Scotland ^{[10, 11]}.

In the Netherlands, the first colony settled in Goesse Sass (Zeeland), where it already was the most common bryozoan. This suggests that the establishment of the species took place in 1999 ^[5]. A monitoring project was subsequently set up in France, the Netherlands and Belgium. The projects resulted in the first sightings of this alien species in each of these countries. At the moment, T. inopinata is colonising the harbours of Europe at a rapid pace and is considered to be a common species there ^{[4, 7, 12]}.

The species was recently also spotted on the east coast of North America ^[13].

The primary introduction occurred by attachment to ships ^[4] and/or via the import of shellfish ^[14]. In our region, transportation with recreational yachts plays a particularly important role ^[4].  

The minimal requirements in regards to the substrate to which it attaches itself play a major part in the success of T. inopinata. Attachment to ships, floating objects or seaweed ensures that individuals of this species can quickly reach new areas and be spread over large distances ^[4].  

This bryozoan can survive a wide range of temperatures and reproduce between 8.6-16,7°C ^[10]. The species has a wide salinity tolerance, ranging from 20 PSU (brackish water) to 35 PSU (saltwater) ^[9]. The seawater in the North Sea has a salinity of approximately 35 PSU.

With increasing water temperature due to global warming, it is expected that T. inopinata will continue to spread to the north and reach the southern and western coastal areas of Norway. However, it will not occur in areas where the salinity is less than 26 PSU (e.g. central and eastern Baltic Sea) and where the temperature can drop below 2°C during the winter months ^[10].

Both Bugulina stolonifera and T. inopinata are experiencing population growth in our harbours. However, it has been observed that B. stolonifera decreases in numbers each time T. inopinata is introduced. This is because T. inopinata has a larger tolerance for different temperatures and salinities and, therefore, competes for space with other bryozoans. On the other hand, B. stolonifera thrives in lower salinities, so it is predicted that its distributions will remain limited to waters with low salinity and that T. inopinata will dominate in harbours ^[4]

Bryozoans – but also barnacles, tubeworms, sea squirts and many other species – are part of the biofouling community present on hard substrates in harbours, including ship hulls. Removing this fauna is very costly: the boats have to be lifted out of the water and cleaned under high pressure. This is followed by treatment with antifouling paint to prevent the new settlement of plants and animals ^[15].

Little is known about the ecological impact of T. inopinata on other species. Competition for space will most likely be an important factor  ^[14]. Studies have shown that T. inopinata outcompetes native bryozoans. First of all, T. inopinata can easily overgrow other bryozoan species. Secondly, the reproduction of the species can take place earlier, in comparison to other bryozoans (e.g. Bugulina species), as a result of which the larvae of T. inopinata settle and start growing earlier. This way, there is little space left for the larvae of other bryozoans. In some places, the reproduction of T. inopinata was observed throughout the entire year, allowing it to quickly attain high population densities ^{[10, 16]}.

T. inopinata belongs to the moss animals (Bryozoa). Bryozoans live in colonies. These are composed of a group of connected individuals called zooids. The size of a colony can vary between ten to one million zooids. Every zooid consists of a protective shell or cystid in which the polypide or ‘body’ is contained. Each individual consists of a bundle of nerves, a muscular system, a stomach with a feeding channel and a “crown” of tentacles that surround the mouth (Figure 1). The polypide can partly leave the shell through an opening in the protective cystid to catch food using its tentacles. The tentacles contain fine vibrating hairs (cilia) that create a water current, causing suspended particles to flow to the mouth ^{[7, 17]}.

Figure 1: Simplified diagram of two individuals (zooids). The retractor muscle ensures that the body can be retracted in the shell. (Source: VLIZ)

A colony of T. inopinata looks like a raised and branched bush. They are most easily observed in August or September when the colony is at its peak. The larvae are released in the water column during this peak and have only a couple of hours to settle on a suitable substrate before they die. After releasing the larvae, the biggest part of the old colonies dies. Newly settled colonies grow only a little bit during fall and winter, which makes them difficult to observe in this initial phase ^{[7, 14]}.

To study bryozoans, a microscope is required. T. inopinata attaches to the substrate with root-like structures or rhizoids. Characteristic of this species is the two rows of zooids on each branch (figure 2). Every zooid has three spines on the outer part, of which the closest to the main axis is usually forked, and two to three spines on the inner corner. These bryozoans also have a scutum (widened spine) that is often lobed like the antlers of a moose, and large triangular avicularia (see figure). Avicularia are zooids that are not responsible for feeding but have a role in protecting the colony. They are covered with a lid (operculum) that enables them to perform a ‘bite movement’  ^{[5, 7]}.

Figure 2: Detailed view of a branch of a colony of Tricellaria inopinata. The two rows of zooids are visible. Yellow: three outer spines, the first of which is usually forked. Orange: two spines on the inner corner. Red: broadened spine, lobed like moose antlers. (© Julien Cillis)

[1]          World Register of Marine Species (WoRMS) (2020). Tricellaria inopinata d'Hondt & Occhipinti Ambrogi, 1985. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=111254] (2020-11-17).

[2]          Otten, B.G. (2001). Rectificatie: toch geen Scrupocellaria reptans. Het Zeepaard 61(6): 147-148. [http://www.vliz.be/imis/imis.php?module=ref&refid=21226]

[3]          Occhipinti-Ambrogi, A.; d'Hondt, J.L. (1992). The invasion ecology of Tricellaria inopinata into the lagoon of Venice: morphological notes on larva and ancestrula, in: Hayward, P.J. et al. Biology and Palaeobiology of Bryozoans: Proceedings of the 9th International Bryozoology Conference, School of Biological Sciences, University of Wales, Swansea, 1992. International Symposium Series, 9. International Symposium Series, 9. Olsen & Olsen: Swansea: pp. 139-144. [http://www.vliz.be/imis/imis.php?module=ref&refid=140811]

[4]          De Blauwe, H.; Faasse, M. (2001). Extension of the range of the Bryozoans Tricellaria inopinata and Bugula simplex in the north-east Atlantic ocean (Bryozoa: Cheilostomatida). Ned. Faunist. Meded. 14: 103-112. [http://www.vliz.be/imis/imis.php?module=ref&refid=19401]

[5]          De Blauwe, H. (2002). Determinatie en verspreiding van Tricellaria inopinata d’Hondt & Occhipinti Ambrogi (Bryozoa, Cheilostomatida), een recente immigrant uit het noorden van de Stille Oceaan. Het Zeepaard 62(3): 73-88. [http://www.vliz.be/imis/imis.php?module=ref&refid=24927]

[6]          De Blauwe, H. (2003). Aanvoer van zuidelijke soorten aan de Belgische kust in september 2001. De Strandvlo 23(1): 5-8. [http://www.vliz.be/imis/imis.php?module=ref&refid=35262]

[7]          De Blauwe, H. (2009). Mosdiertjes van de Zuidelijke bocht van de Noordzee: Determinatiewerk voor België en Nederland. Vlaams Instituut voor de Zee (VLIZ): Oostende. ISBN 978-90-812-9003-6. 445 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=138282]

[8]          Fernandez Pulpeiro, E.-J.; César-Aldariz, J.; Reverter Gil, O. (2001). Sobre la presencia de Tricellaria inopinata d'Hondt & Occhipinti Ambrogi 1985 (Bryozoa, Cheilostomatida) en el litoral gallego (N.O. España). Nova Acta Cient. Compostel. (Biol.) 11: 207-213. [http://www.vliz.be/imis/imis.php?module=ref&refid=140814]

[9]          Dyrynda, P.E.J.; Fairall, V.R.; Occhipinti-Ambrogi, A.; d’Hondt, J.-L. (2000). The distribution, origins and taxonomy of Tricellaria inopinata d’Hondt and Occhipinti Ambrogi, 1985, an invasive bryozoan new to the Atlantic. J. Nat. Hist. 34(10): 1993-2006. [http://www.vliz.be/imis/imis.php?module=ref&refid=139420]

[10]        Cook, E.J.; Stehlikova, J.; Beveridge, C.M.; Burrows, M.T.; De Blauwe, H.; Faasse, M. (2013). Distribution of the invasive bryozoan Tricellaria inopinata in Scotland and a review of its European expansion. Aquat. Invasions 8(3): 281-288. [http://www.vliz.be/en/imis?module=ref&refid=246474]

[11]        Beveridge, C.; Cook, E.J.; Brunner, L.; MacLeod, A.; Black, K.; Brown, C.; Manson, F.J. (2011). Initial response to the invasive carpet sea squirt, Didemnum vexillum, in Scotland. Scottish Natural Heritage Commissioned Report, 413. Heritage, S.N.: Perth. 23 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=301878]

[12]        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]

[13]        Johnson, C.H.; Woollacott, R.M. (2015). Analyses with newly developed microsatellite markers elucidate the spread dynamics of Tricellaria inopinata d'Hondt and Occhipinti-Ambrogi, 1985 - a recently established bryozoan along the New England seashore. Aquat. Invasions 10(2): 135-145. [http://www.vliz.be/nl/catalogus?module=ref&refid=301898]

[14]        De Blauwe, H. (2019). Persoonlijke mededeling

[15]        Hedgepeth, J.W. (Ed.) (1952). Marine fouling and its prevention. Contributions of the Woods Hole Oceanographic Institution, U.S. Naval Institute: Annapolis, Maryland. ISBN: 978-1124111988. 388 pp. [http://www.vliz.be/en/imis?module=ref&refid=140668]

[16]        Johnson, C.H.; Winston, J.E.; Woollacott, R.M. (2012). Western Atlantic introduction and persistence of the marine bryozoan Tricellaria inopinata. Aquat. Invasions 7(3): 295-303. [http://www.vliz.be/nl/catalogus?module=ref&refid=301895]

[17]        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]

VLIZ Alien Species Consortium (2020). Tricellaria inopinata. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 7 pp.