Ascidiella aspersa (European sea squirt) is a marine organism classified as a tunicate. The classification comes from a transparent, thick and rough (Curtis, 2005) cartilaginous tunic that covers the body of the adult organism (de Kluijver, 2004). The surface of the tunicate is rough and finely papillated (de Kluijver, 2004), greyish-black to brown (Curtis, 2005) with two siphons, a terminal brachial siphon, and an atrial siphon located up to one-third the length of the body away from the terminal siphon (de Kluijver, 2004). The brachial siphon is 6-8 lobed, while the atrial siphon is 6 lobed (de Kluijver, 2004). The siphons may be frilled (Curtis, 2005). The intestinal gut is located to the left of the brachial sac with numerous rows of straight stigmata (de Kluijver, 2004).
Ascidiella aspersa (European sea squirt) has a lifespan of 18 months, from mid-summer to the following year's winter (Millar, 1952). In early to mid-summer larval settlement and metamorphosis occurs and takes approximately 24 hours at 20°C (NIMPIS, 2010). Growth is continued through the fall and into the winter, although growth in winter is reduced. Upon reaching a size of 30mm, the male sexual organs begin to fill with sperm and at a much bigger size the female eggs begin to fill the oviducts. At a size of 40mm, both sex organs are fully mature (Millar, 1952). One spawning season occurs, usually one year after larval settlement (Millar, 1952). Under laboratory conditions (Bolton, 1996), there was an observed synchronous response to light in the spawning of ascidians.
Ascidiella aspersa (European sea squirt) is a solitary tunicate but often forms unfused colonies in close association with each other (Curtis, 2005). It can tolerate salinities from 18-40 ppt (NIMPIS, 2010). This species establishes in subtidal and low water depth, with maximum settlement depth of 90m (de Kluijver, 2004). It can establish on soft muds but is commonly found associated with hard subtrates, like pier pylons (Currie, 1998), rocks, docks, etc. (Pederson, 2003). A. aspersa is found in low energy habitats, like estuaries, harbours and semi-enclosed embayments that protect them from strong currents and tidal forces (NIMPIS, 2010). Establishment is not inhibited by native fauna for substrate sites (Osman, undated). The establishment of A aspersa is increasing as global and ocean water temperatures increase (Stachowicz, 2002).
Ascidiella aspersa (European sea squirt) is hemaphroditic, containing both male and female sex organs, although the male sex organs develop first (Millar, 1952). In spring to early summer both male and female gametes are released into the water column where fertilisation takes place (NIMPIS, 2010). Chemotaxis interactions occur between sperm and egg, with homospecific chemical signalling of the egg, increasing sperm activity in the water column (Bolton, 1996). Larva go through a brief free-swimming stage and then quickly metamorphosise into a juvenile and begin settlement (NIMPIS, 2010).
Principal source: de Kluijver, M.J., & Ingalsuo S.S., Feb. 4, 2004, Macrobenthos of the North Sea-Tunicata, Zoological Museum, University of Amsterdam;
Currie, D.R., McArthur, M.A., & Cohen, B.F., Sept. 1998, Exotic Marine Pests in the Port of Geelong, Victoria, Marine and Freshwater Resources Institute, Report no. 8.;
Hewitt, C.L., Martin, R.B., Sliwa, C., McEnnulty, F.R., Murphy, N.E., Jones, T. & Cooper, S., editors, 2002, National Introduced Marine Pest Information System (NIMPIS), [online database].
Compiler: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
Review: Expert review underway: Dr. Richard Osman, Senior Scientist Smithsonian Environmental Research Center., Edgewater, Maryland, USA
Publication date: 2010-03-10
Recommended citation: Global Invasive Species Database (2019) Species profile: Ascidiella aspersa. Downloaded from http://www.iucngisd.org/gisd/species.php?sc=1126 on 24-01-2019.