Styela clava - Rough sea squirt

At first, S. clava only occurred in shallow parts of the Okhotsk Sea (Siberia), the Japanese Sea, along Japanese and Korean coasts and between the far north of China and the harbour city of Shanghai ^{[2, 3]}.

First observation in Belgium

The first Belgian observation of the Rough sea squirt – also called the Asian tunicate – dates from the 19^th of August 1986. This observation concerned an isolated living individual on a beach groyne along the ‘Albertstrand’ (English translation: ‘Albert beach’) in Knokke-Heist ^[4].

Spreading in Belgium

On the eastern breakwater of Zeebrugge, 217 individuals were found on two blocks of concrete in January 1987 ^[5]. Currently, the species occurs in the Sluice Dock of Ostend and all marinas along the Belgian coast, except in Nieuwpoort ^{[6, 7]}.

Spreading in neighbouring countries

The first observation of a Rough sea squirt was in Plymouth (South England) during the summer of 1953. In 1954, the species got described as a new species, namely Styela mammiculata ^[8-11]. It is suspected that the introduction took place in 1952 ^[3]. The expansion of the species along the English south and west coast went quickly: from Plymouth via the waters of Southampton to the port of Milford in Wales (1959). The Rough sea squirt probably crossed the Channel around 1968. That year the species was first observed at Dieppe in France ^{[11, 12]}.

In the Netherlands, the species became a frequent occupant of almost all salty waters. Here, the Rough sea squirt was first observed in 1974 in Den Helder ^[13]. A few months later, observations were reported from the marina of Texel and the Eastern Scheldt ^[14].

Nowadays, S. clava is found along the entire European Atlantic coast, from Portugal to Norway ^[15]. In 2005, the species was also observed in a French basin near the Mediterranean Sea ^[16] and the Sea of Marmara ^[17].

S. clava can be introduced to new areas in various ways ^[18]. As an adult, it can attach itself to the hulls of ships. Most likely, it was accidentally brought in by military battleships in England when they returned after the Korean War ended in 1951 ^[3].

As juveniles, they can attach themselves to spat or shells of oysters that are introduced to new breeding grounds. In this way, the Rough sea squirt ended up in many ports in Brittany (France) and the Netherlands ^{[11, 18]}. The oysters and the spat for new oyster beds came from Japan and other European oyster beds ^[11].

The larvae of the Rough sea squirt can also cover short distances in the ballast water of ships ^[18]. Local spreading between marinas may be the result of attachment to yachts and sailing ships ^[18]. The natural distribution of S. clava is rather limited. The larvae get carried away with the tides and currents but must be able to settle within 27 hours ^[18]. How exactly the Rough sea squirt got introduced to the Netherlands remains unknown.

In our regions, S. clava got introduced to an area where no similar species of sea squirt occurred. This facilitated the colonization of the Rough sea squirt. Furthermore, there are no natural predators of the species present in the study area ^[11].

Because of its size – on average 14 cm (very large for a sea squirt) – the Rough sea squirt is not easily bothered by nearby animals during feeding. This allows it to filter larvae of other species, such as oysters, from the water column ^[11].

S. clava can only successfully settle on hard surfaces, such as shells, port infrastructure, ship hulls, ropes and buoys ^[19]. The Rough sea squirt prefers to settle near the water surface, although it has been observed at a depth of 40 meters. The species can briefly tolerate low salinities down to 10 PSU. However, to successfully establish, it requires a salinity between 22 and 35 PSU ^{[11, 16, 18, 20]}. By comparison, the North Sea has a salinity of about 35 PSU.

The Rough sea squirt’s distribution can also be limited by temperature. Although the species can withstand a wide temperature range (between -2 and 23°C ^{[11, 15]}), the larvae can only settle when the water reaches temperatures above 16°C for several days. This lower limit restricts its distribution to more northern regions, where such high temperatures are rare, even in summer. For this reason, the species does not occur along the eastern coast of Scotland ^[18].

The Rough sea squirt prefers relatively sheltered areas. This species is, therefore, mainly observed in harbours ^[18].

Once the Rough sea squirt is well established, enormous populations are sometimes formed – up to 1,500 individuals per square meter – leading to strong competition with native species, such as mussels and oysters, that also filter their food from the water column ^[19].

This sea squirt can form a growth layer that is a real plague on ship hulls, oyster beds and cultivated mussels. In particular, growth on mussel and oyster beds leads to serious problems. These problems result in additional production, harvesting and processing costs for the farmers. On Prince Edward Island in Canada, this has led to millions of dollars of economic damage per year ^[21]. The costs are mainly related to the damage caused by the biomass (weight) of the sea squirts, making the ropes with mussels so heavy that they could no longer be lifted out of the water. The extra weight causes the mussels to be very heavy so that they fall off the ropes when lifted from the water ^[22]. So far, Europe has been spared from such situations ^[23].

Treatments with altered salinity and temperatures or exposure to air turned out to be successful – and also biologically justified – methods in the fight against this non-native species. These measures lead to the mortality of the Rough sea squirt without the loss of mussels or oysters ^[3]. In addition, the free-swimming larvae appear to be susceptible to medetomidine, a chemical that ensures the larvae don’t settle on the surface to which the substance has been applied and, hence, look for other suitable spots. Unlike other chemicals, medetomidine does not kill the animals and, therefore, appears to be a more sustainable alternative to use in antifouling paint ^[24].

The Rough sea squirt is commonly consumed as seafood in Asian (Korean) cuisine ^[25]. Nevertheless, attention is needed when consuming them. When damaged –  e.g. when scraped off oyster beds –  the sea squirts produce toxins that can cause respiratory problems in humans ^[15].

Besides a food source, the Rough sea squirt is also of interest to the scientific community since it produces certain compounds that proved to be antioxidants and useful in the treatment of cancer ^[26]. For example, an ethanol extract from this species can prevent skin ageing ^[27], and it can be used in anti-wrinkle cosmetics or ointments that treat burns ^[28].

S. clava is a filter feeder. It uses an internal ‘sieve’ to filter out phytoplankton, zooplankton and organic matter from the water column ^[29]. To make this filtering efficient, the sea squirt ensures a constant water flow. The water enters the body through an inflow opening (oral siphon), passes through the sieving device (pharynx) that captures food particles and gets removed via the outflow opening (atrial siphon).

The Rough sea squirt usually occurs along sheltered shores, down to a depth of 40 meters. The species belongs to the biofouling community and is found on ropes and hard structures such as quays, ship hulls, poles, buoys, mussel beds and oyster beds. Densities of up to 1,500 individuals per square meter can be reached. These organisms grow up to 16 centimetres and have an average lifespan of two to three years ^[29]. Their skin has a wrinkled and leathery appearance and usually has a brown colour. The Rough sea squirt generally has an elongated shape, with a clear narrowing towards the base ^[13].

The species is hermaphroditic, but the male and female gonads mature at different times so that these sea squirts cannot self-fertilize. To maximize fertilization across individuals, they spawn at about the same time. They synchronize fertilization by taking into account the temperature and day length; they spawn when the water has a temperature of 15°C, and the sun has been shining for at least twelve hours ^[30]. Fertilization occurs externally (in the water), and the eggs and larvae float freely in the water column for one to three days. This is called the planktonic stage. After this stage, they attach themselves to a hard surface, where they undergo a metamorphosis into their adult form ^[29].

[1]          World Register of Marine Species (WorMS) (2020). Styela clava Herdman, 1881. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=103929] (2020-11-17).

[2]          Millar, R.H. (1960). The identity of the ascidians Styela mammiculata Carlisle and S. clava Herdman. J. Mar. Biol. Ass. U.K. 39(3): 509-511. [http://www.vliz.be/imis/imis.php?module=ref&refid=121293]

[3]          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]

[4]          d'Udekem d'Acoz, C. (1986). Etude sur la faune de Knokke-Heist: III. Présence de Styela clava Herdman, 1882 en Belgique. De Strandvlo 6(4): 83. [http://www.vliz.be/vmdcdata/imis2/imis.php?module=ref&refid=18249]

[5]          Dumoulin, E. (1987). Nieuwe waarnemingen van de Knotszakpijp Styela clava langs de Belgische Oostkust. De Strandvlo 7(2): 61-62. [http://www.vliz.be/vmdcdata/imis2/imis.php?module=ref&refid=18286]

[6]          Eneman, E. (1995). Knotszakpijp of Japanse zakpijp Styela clava (Herdman, 1882) in de Spuikom van Oostende. De Strandvlo 15(3): 113. [http://www.vliz.be/vmdcdata/imis2/imis.php?module=ref&refid=19115]

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

[8]          Carlisle, D.B. (1954). Styela mammiculata, a new species of ascidian from the Plymouth area. J. Mar. Biol. Ass. U.K. 33: 329-334. [http://www.vliz.be/nl/catalogus?module=ref&refid=206171]

[9]          Coughlan, J. (1969). The leathery sea squirt – a new ascidian from Milford haven. Nature in Wales 11: 192-193. [http://www.vliz.be/imis/imis.php?module=ref&refid=138242]

[10]        Buizer, D.A.G. (1980). Explosive development of Styela clava Herdman, 1882, in The Netherlands after its introduction (Tunicata, Ascidiacea). Bull. Zool. Mus. Univ. Amsterdam 7: 181-185. [http://www.vliz.be/imis/imis.php?module=ref&refid=128806]

[11]        Lützen, J. (1999). Styela clava Herdman (Urochordata, Ascidiacea), a successful immigrant to North West Europe: ecology, propagation and chronology of spread. Helgol. Meeresunters. 52(3-4): 383-391. [http://www.vliz.be/imis/imis.php?module=ref&refid=206256]

[12]        Minchin, D.; Duggan, C.B. (1988). The distribution of the exotic ascidian, Styela clava Herdman, in Cork Harbour. Ir. Nat. J. 22(9): 388-393. [http://www.vliz.be/imis/imis.php?module=ref&refid=128813]

[13]        Huwae, P. (1974). Styela clava Herdmann, 1882, nieuw voor Nederland. Het Zeepaard 34(2): 28-29. [http://www.vliz.be/vmdcdata/imis2/imis.php?module=ref&refid=115912]

[14]        Westerwil, H. (1975). Styela clava Herdmann, 1882 nu ook in Zeeland. Het Zeepaard 35(6): 99. [http://www.vliz.be/vmdcdata/imis2/imis.php?module=ref&refid=115999]

[15]        Minchin, D. (2009). Styela clava Herdman, Asian sea-squirt (Styelidae, Ascidiacea). in: DAISIE (Delivering Alien Invasive Species Inventories for Europe). Handbook of alien species in Europe. Invading Nature - Springer Series in Invasion Ecology,3: pp. 298. [http://www.vliz.be/imis/imis.php?module=ref&refid=135058]

[16]        Davis, M.H.; Davis, M.E. (2008). First record of Styela clava (Tunicata, Ascidiacea) in the Mediterranean region. Aquat. Invasions 3(2): 125-132. [http://www.vliz.be/imis/imis.php?module=ref&refid=206258]

[17]        Çinar, M.E. (2016). The alien ascidian Styela clava now invading the Sea of Marmara (Tunicata: Ascidiacea). Zookeys 563: 1-10. [http://www.vliz.be/en/imis?module=ref&refid=298604]

[18]        Davis, M.H.; Davis, M.E.; Lützen, J. (2007). The spread of Styela clava Herdman, 1882 (Tunicata, Ascidiacea) in European waters. Aquat. Invasions 2(4): 378-390. [http://www.vliz.be/imis/imis.php?module=ref&refid=206257]

[19]        Global Invasive Species Database (2018). Styela clava. [http://www.issg.org/database/species/ecology.asp?si=951&fr=1&sts=sss] (2018-07-23).

[20]        Krone, R.; Wanke, C.; Schröder, A. (2007). A new record of Styela clava Herdman, 1882 (Urochordata, Ascidiacea) from the central German Bight. Aquat. Invasions 2(4): 442-444. [http://www.vliz.be/imis/imis.php?module=ref&refid=206259]

[21]        Davis, M.H.; Davis, M.E. (2009). Styela clava (Tunicata, Ascidiacea) - a new threat to the Mediterranean shellfish industry? Aquat. Invasions 4(1): 283-289. [http://www.vliz.be/imis/imis.php?module=ref&refid=140111]

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

[23]        Gittenberger, A. (2009). Invasive tunicates on Zeeland and Prince Edward Island mussels, and management practices in The Netherlands. Aquat. Invasions 4(1): 279-281. [http://www.vliz.be/en/imis?module=ref&refid=197746]

[24]        Willis, K.J.; Woods, C.M. (2011). Managing invasive Styela clava populations: Inhibiting larval recruitment with medetomidine. Aquat. Invasions 6(4): 511-514. [http://www.vliz.be/nl/catalogus?module=ref&refid=297694]

[25]        Lambert, G.; Karney, R.C.; Rhee, W.Y.; Carman, M.R. (2016). Wild and cultured edible tunicates: A review. Manag. Biol. Inv. 7(1): 59-66. [http://www.vliz.be/nl/catalogus?module=ref&refid=299119]

[26]        Jumeri; Kim, S.M. (2011). Antioxidant and anticancer activities of enzymatic hydrolysates of solitary tunicate (Styela clava). Food Science and Biotechnology 20(4): 1075-1085. [http://www.vliz.be/nl/catalogus?module=ref&refid=299121]

[27]        Koh, E.K.; Kim, J.E.; Go, J.; Song, S.H.; Sung, J.E.; Son, H.J.; Jung, Y.J.; Kim, B.H.; Jung, Y.S.; Hwang, D.Y. (2016). Protective effects of the antioxidant extract collected from Styela clava tunics on UV radiationinduced skin aging in hairless mice. International Journal of Molecular Medicine 38(5): 1565-1577. [http://www.vliz.be/en/imis?module=ref&refid=298565]

[28]        Lee, S.-M.; Lee, Y.-R.; Cho, K.-S.; Cho, Y.-N.; Lee, H.A.; Hwang, D.-Y.; Jung, Y.-J.; Son, H.-J. (2015). Stalked sea squirt (Styela clava) tunic waste as a valuable bioresource: Cosmetic and antioxidant activities. Process Biochemistry 50(11): 1977-1984. [http://www.vliz.be/en/imis?module=ref&refid=298615]

[29]        National Introduced Marine Pest Information System (NIMPIS) (2018). Styela clava reproduction and habitat. [http://www.marinepests.gov.au/nimpis] (2018-07-23).

[30]        Wong, N.A.; McClary, D.; Sewell, M.A. (2011). The reproductive ecology of the invasive ascidian, Styela clava, in Auckland Harbour, New Zealand. Mar. Biol. (Berl.) 158(12): 2775-2785. [http://www.vliz.be/en/imis?module=ref&refid=256218]