Taxonomic name: Styela plicata Lesuer, 1823
Synonyms: Ascidea plicata, Ascidia plicata Lesueur, 1823, Styela barnhart Ritter & Forsyth, 1917, Styela gyrosa Heller, 1877, Styela pinguis Herdman, 1899, Tethyum plicatum Hartmeyer, 1909
Common names: leathery tunicate, pleated sea squirt, sea squirt, solitary ascidian
Organism type: tunicate
Styela plicata (sea squirt) is a pandemic, temperate to subtropical tunicate. As a pest species, Styela plicata outcompete native encrusters and excludes them from hard substrates. It is a known fouler of sea vessels and other hard substrates, travelling the oceans in this fashion. Few places classify Styela plicata as an invasive species, but some effective management options are available to control this tunicate.
Styela plicata (sea squirt) is an ovular, greyish to tannish white benthic tunicate. This solitary sessile invertebrate is cloaked in an unstalked tunic that is large, tough, (Fuller, 2007), warty and ridged (Howey, 1998). Perry & Larson (2004) report that the lumpy surface of the tunic gives it the appearance of cobblestone pavement. Internal structures are protected by this tunic, which is composed largely of cellulose compounds and contains a circulatory system of "blood" transport vesicles. Dividing the tunic is a membrane which allows fluid to flow up one side and down the other. S. plicata has an incurrent siphon that intakes water into the pharyngeal basket where food particles are filtered out; the waste is then excreted through the excurrent siphon (Howey, 1998). The two short siphons have red or purple stripes on the inside of the siphons and four lobes (Fuller, 2007). When physically disturbed, S. plicata expels water, which explains why it is called a sea squirt. S. plicata is a very eurythermal tunicate; able to tolerate changes in seawater between 10°-30°C and salinites between 22%-34% (Thiyagarajan & Qian, 2003). NIMPIS (2002) reports that S. plicata can tolerate some pollution and brackish waters. Adults can reach sizes between 40-70mm, even up to 90mm in some cases (NIMPIS, 2002). As a protandric hermaphrodite, S. plicata has "testes - small and attached along most of the length of each ovary," with two gonads on the left side of the body and five on the right (Lambertet al. 2005).
coastland, estuarine habitats, marine habitats
The different life cycle stages of Styela plicata (sea squirt) have different habitat requirements for survival. The larval and juvenile stages of S. plicata live on marinas and docks, oyster reefs, rocks and coarse woody debris, while the adults prefer marinas, docks and hard rocky substrates (NEMESIS, 2006). S. plicata also live in coral reef habitats (STIR, undated). S. plicata is found from the low intertidal zone to depths of 30m (NIMPIS, 2002).
Styela plicata (sea squirt) competes with other organisms, excluding them from the space it occupies. Its larvae are capable of invading occupied space and growing to a large size in a relatively short period of time, attached to other organisms. S. plicata then sloughs off because of its large size, often taking other marine organisms with it. This sloughing destabilises the marine community. The presence of this tunicate also inhibits the recruitment or growth of other larval species (Sutherland, 1978). S. plicata has also replaced native solitary tunicates Pyura haustor and Ascidia ceratodes (Fuller, 2007). S. plicata is a fouler of ships, boats, docks, power plants and shellfish ponds, attaching to hard substrates and remaining there until removed (NEMESIS, 2006). S. plicata is usually covered with non-ascidian epibionts, which can travel on the tunicate and add non-indigenous species to aquatic ecosystems (Lambert & Lambert, 1998). Wyatt et al (2005) claims that S. plicata acts as a vector for the invasive Bugula neritina in Australia.
Styela plicata (sea squirt) is a host to several different kinds of organisms, including brittle stars, mussels, chitons, sponges, polychaete worms, diatoms, eggs, etc., that live on its tunic (Howey, 1998).
As a defence mechanism, Styela plicata (sea squirt) concentrates deterrant chemical compounds in its gonads so that they may be passed on to larvae, thus protecting them from predation (Pisut & Pawlik, 2002). Alcohol from the body of S. plicata exhibits anti-Hepatitis B properties (STRI, undated). S. plicata harbours the amphiped Leucothoe spinicarpa and an ascidicolous copepod (Thiel, 1998). Cold winters kill S. plicata, limiting its northern distribution to Cape Hatteras, North Carolina. One way this is thought to happen is by dislodgement from substrates during cold (growth inhibiting) periods (Fisher, 1976). Populations of S. plicata fluctuate; they may be abundant one year and absent the next (Lambert & Lambert, 1998).
Native range: East coast of the USA, Northern Gulf of Mexico, and the West Indes (Fuller, 2007).
Known introduced range: Australia, West coast of USA, Mediterranean Sea, Brazil, Uruguay, Argentina, Japan, Hong Kong, Indian Ocean (Hayeset al, 2005; Lambert & Lambert, 1998; Lambertet al, 2005; NBNG, 2004; NIMPIS, 2002; NEMESIS, 2006; STRI, undated; PSIS, undated).
Introduction pathways to new locations
Live food trade: Hayes et al (2005) report that Styela plicata was introduced to Australia accidently with the translocation of fish or shellfish.
Ship ballast water: Styela plicata can be introduced to new locations in ship ballast water (Fuller 2007).
Ship/boat hull fouling: Styela plicata can be introduced to new locations by hull/ship fouling (Fuller 2007).
Local dispersal methods
Other (local): Styela plicata can be spread by fouling recreational boats (Wyatt et al. 2005),
Chemical: Tributylin (TBT) is used in anti-fouling paints, wood preservation, slime control in paper mills and other industrial processes. It affects Styela plicata by blocking the sperm-egg interaction, thus preventing fertilisation (Mansueto et al, 2003). PAN (2006) reports that Butyltrichlorostannane, Cyhexatin, Dibutyltin dichloride, Fenton hydroxide, Tributylin chloride, Triphenyltin acetate and Triphenyltin chloride cause mortality in S. plicata cells aquired post-fertilisation (PAN, 2006). Copper sulphate was proposed as a broadcast spray control method, but scientists deemed it too expensive and non-specific, lethal to non-target species. It is also absorbed by the soil and ineffective at high pH levels.
Physical: Plastic wraps have been applied to timber pylons in intertidal to subtidal zones, which prevent oxygenated water from touching S. plicata, thus suffocating it (NIMPIS, 2002).
Styela plicata (sea squirt) is a suspension filter feeds that preys primarily on phytoplankton, zooplankton and organic materials. Snails, crustaceans, sea stars and fish have been known to prey on S. plicata (NIMPIS, 2002). Specifically, the species Linatella caudata preys upon S. plicata (Morton, 1989).
Styela plicata (sea squirt) is a protandric hermaphrodite. Initially, S. plicata is a male, then later it changes to a female. Fertilisation is external; eggs and sperm are released into the water column in the late afternoon and the larvae, 1.3mm in total length (Yamaguchi, 1975), hatch the next morning and settle that day (NIMPIS, 2002). S. plicata undergoes reproductive cycles yearly in conjunction with annual temperature changes. According to West & Lambert (1975), S. plicata must experience a period of darkness; approximately 8.5 hours long, prior to the release of gametes. Spawning can occur between 11°-28° C (West & Lambert, 1975), with 20°C being optimal (Yamaguchi, 1975). Water filtration is not optimal during the release of gametes (Fiala-Medlioni, 1978).
The eggs of Styela plicata (sea squirt) are surrounded by a complex ovular envelope (Mansuetoet al. 2003) that supplies the larvae with its nutritional requirements (Pisut & Pawlik, 2002). Once hatched, the larvae attempt to find a suitable substrate. S. plicata can have an extended swimming period of over 2 days prior to settlement without a cost to metamorphosis (Thiyagarajan & Qian, 2003). Larval settlement is most successful in the spring and fall (Fisher, 1977). S. plicata has a life span of less than one year that is characterised by rapid growth. Some sea squirts can live between 2-3 years (Lambert & Lambert, 1998). Yamaguchi (1975) reported that S. plicata reached sexual maturity in 2 months during the summer and 5 months during the winter. S. plicata has an extended breeding season, reproducing all year except during winter (NIMPIS, 2002).
Reviewed by: Expert review underway: Dr. Richard Osman, Senior Scientist Smithsonian Environmental Research Center., Edgewater, Maryland, USA
Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
Last Modified: Tuesday, 8 May 2007