You have a Styela clava (Herdman, 1881) – Clubbed tunicate or leathery sea squirt
Synonyms: Styela mammiculata Carlisle, 1954; Bostryorchis clava Redikorzev, 1916; Styela barnharti Ritter & Forsyth, 1917.
Common names: Clubbed tunicate, Leathery sea squirt, Asian tunicate, Rough sea squirt (USA, CAN, UK); Østasiatisk søpung (DK); Østasiatisk sekkedyr (NO); Östasiatisk sjöpung (SE); Ostasiatische Seeschiede, Falten Ascidie (GE); Japanse zakpijp (NL); Ascidie plissée (FR); Ascidia plisada (SP).
Taxonomic problem: The authority is sometimes gives as Herdman, 1882, sometimes as Herdman, 1881 or (Herdman, 1881). The WoRMS database (http://www.marinespecies.org/) gives (Herdman, 1881) as the correct authority and Fleming, 1822 as the author for the genus Styela. However, all three versions are still used in the scientific literature.
This is a relatively large (often >10 cm long), stalked, solitary sea aquirt, which is often found associated with man-made structures, seaweeds or shellfish. It has a warty exterior and a long stalk. There are several native species of Styela, none of which have the long stalk, e.g. S. rustica (Linnaeus, 1767), S. coriacea (Alder & Hancock, 1848), S. gelatinosa (Traustedt, 1886), and S. atlantica (Van Name, 1912).
Picture of Styela clava
For further information see:
For Styela coriacea see: http://species-identification.org/species.php?species_group=tunicata&id=43
For Styela gelatinosa see: http://www.marlin.ac.uk/speciesfullreview.php?speciesID=4399 and http://www.habitas.org.uk/marinelife/species.asp?item=ZD1735
Vector: It has been proposed that S. clava was introduced to Europe as fouling on warships returning from the Korea war in 1953. It was probably introduced to the Danish Limfjord with oyster spat, and ballast water has been suggested as the vector in Canada.
Native distribution: NW-Pacific, including Japan, northern China, Korea and Siberia.
Introduced distribution: The invasion in Europe has been summarized by Davis & Davis (2007) and by Davis et al. (2007). It was first found in 1953 near Plymouth, U.K.
Recently Styela clava was found in the Mediterranean (Davis & Davis, 2008).
Styela clava has also been introduced to both sides of the American continent (Lambert & Lambert, 1998; Locke et al., 2007).
Styela clava also has been introduced to Australia, where it was first recorded in 1976 (Holmes, 1976) and New Zealand (Biosecurity New Zealand – see URL above).
This is a fouling species, and because it is very big it also serves as a substrate for other fouling organisms, mostly other species of tunicates. It is a great nuisance to mussel farms because it attaches in dense clumps to equipment, including hanging ropes. Densities of 1000 individuals per m² have been recorded (Minchin & Duggan, 1988). Management in New Zealand focuses on preventing further spread and they have published an assessment report (available at http://www.biosecurity.govt.nz/files/pests/salt-freshwater/styela-clava-population-management.pdf) last accessed 3. March 2015. The state of Washington has an active management program and has published several reports that are available on their web-site (see: http://wdfw.wa.gov/ais/tunicates.html and http://wdfw.wa.gov/publications/00805/ last accessed 3. March 2015)
It has been attempted to market it as a seafood specialty (Karney & Rhee, 2009).