Invasive Alien Species in Northern Ireland

Styela clava, Leathery sea squirt

Styela clava Herdman 1882
This tunicate is widely distributed from cold water to temperate regions in both the southern and northern hemispheres. It is a notable fouling species of man-made structures, of aquaculture equipment and on vessel hulls. The species continues to spread in Europe. It has not yet been recorded in Northern Ireland but it is expected to appear because it has recently increased its range on southern and eastern coasts of Ireland.
Styela Clava is solitary and club-shaped, with a long narrow stalk approximately a third of the length of the body. It can reach up to 200mm in length and attaches with an expanded membranous plate that adheres to firm substrata ranging from rock, shells, macrophytes and artificial materials. The outer surface is made up of a tough brown leathery and wrinkled tunic with conspicuous tubercles.
Country of origin
It is native to Japan, Korea, the Sea of Othotsk, and Siberia where it is a common fouling organism.
Current distribution
It occurs on the east and west coasts of North America, Southern Australia and was found in New Zealand in 2005. It was thought to have arrived in 1952 to the south coast of Britain. It may have arrived with returning warships following the Korean War. Its range in Europe now extends from Portugal to Denmark and it is common in several port regions and some oyster culture areas.
Location in Ireland
In Ireland it has been present in Cork Harbour since at least 1971. More recently it has been found in Dingle Harbour, at Fenit Port and in Dun Laoghaire Harbour.
Life cycle
The species is a hermaphrodite that reproduces by cross–fertilization. The male and female reproductive organs develop at different times and this prevents them from fertilizing their own eggs. Styela clava is a filter–feeder; it is a hardy species that can tolerate low temperatures to -2°C and live beneath ice. In summer this species can tolerate temperatures of up to 23°C. Styela clava occurs in sheltered inlets and estuaries where salinities seldom drop below 22psu. Spawning takes place at temperatures greater than 15°C. Larvae hatch from released eggs in late summer and early autumn and have a larval period of approximately twelve hours. They are poor swimmers and tend to settle upon near–surface structures where these are present. Young individuals often settle on larger specimens to form clusters. It is thought they survive up to two years in European waters and during this time may spawn twice. They reach a length of 20–50mm entering their first winter. They can rapidly become abundant.
Wildlife and habitat impacts
The species occurs in estuaries, shallow channels and in bays from below mid-tidal level on shores not exposed to sunlight down to depth of c.25m. In Ireland it is not abundant although it may commonly be found attaching to shells, stones, rock and kelps. The species co–exists with oysters and mussels. Should they become abundant they are capable of causing significant competition for food and space and may filter out larval stages of many native species from the plankton.
Human impacts
In the St. Lawrence Estuary, Canada, their abundance resulted in declines in the production of rope-cultured mussels. Within its native range it fouls cultivated species in hanging culture and fish cages. It also fouls ropes and floating structures in sheltered environments. In docks in The Netherlands it has been known to attain densities to c.1000 m-2. As a fouling organism it requires additional labour to remove them from boat hulls and from oysters before marketing them. Fouled ropes of aquaculture structures can become tangled in water currents; this can result in increases in labour as well as equipment loss. It can foul screens and pipework where water abstraction takes place. It is known to cause an asthmatic–like condition in humans exposed to their fluids when detaching them from oysters under poorly ventilated conditions.
Key vectors
It is likely that the species will appear at marina and port locations in Northern Ireland at some time during the next few decades. It may survive up to three days out of water under cool and damp conditions.
Known modes of transmission include:
  1. Hull fouling on ships, barges, and leisure craft
  2. Transfers of half-grown and adult oysters.
  3. Transmission, including local movements, may also take place with:
    1. Releases of ships’ ballast water from nearby port regions
    2. Movements of port equipment
    3. Translocated dredge spoil
    4. Drifting plants to which they have become attached.

      What you can do as an individual
      Should you come across this species contact a biologist in the Fisheries Division of the Department of Agriculture and Rural Development, The Natural Heritage Division of the Environment and Heritage Service of Northern Ireland and/or CEDaR, Botany Department, Sciences Division, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down BT18 0EU. Tel: 028 9039 5256, email[at]
      Advice to specific groups relevant to this species
      Imports of shellfish should be carefully examined and specimens of sea squirts found in consignments removed. Oyster imported with attached specimens can be treated by using brine dips. Intact oysters are able to seal their shells under these conditions and are unaffected by this treatment. Boat–owners should ensure that they do not transport craft that are fouled with this species.
      Management measures
      This tunicate may survive aerial exposure of some days and so become easily transferred with oysters, mussels and scallops. When they are found attached to oysters, brine dips have been found to be a successful means of control. Movement of floating port structures that have accumulated fouling pose a risk of transmission to uncolonized areas. Inspections of imported shellfish are advised.
      Management groups
      In Ireland the species is known to cause only a minor inconvenience but there may be some environmental conditions where they could become abundant. Should this happen there are likely to be impacts on aquaculture and for native species.
      Further information
      This tunicate is eaten in Korea.
      Bourque, D., MacNair, N., LeBlanc, A., Landry, T. and Miron, G. (2005). Preliminary study of the diel variation of ascidian larvae concentrations in Prince Edward Island. Canadian Technical Report Fisheries and Aquatic Sciences no. 2571 23 pp.
      Davis, M.H. and Davis, M,E. (2004b). The role of man–aided dispersal in the spread of the immigrant Styela clava.
      Herdman, (1882). Journal of Marine Science and Environment 1: 18-24.
      Guiry, G.M., Guiry, M.D. (1973). Spread of an introduced ascidian to Ireland. Marine Pollution Bulletin 4: 127.
      Kato, Y., Ohta, M., Munakata, T., Fujiwara, M., Fujii, N., Shigeta, S. and Matsuura, F. (2001). Determination of solution conformation of allergenically active pentasaccharitol obtained from sea squirt antigen. Magnetic Resonance in Chemistry 39(5): 259-266.
      Lützen, J. (1999). Styela clava Herdman (Urochordata, Ascidiacaea) a successful immigrant to north–west Europe: ecology, propogation and chronology of spread. Helgoländer Meeresuntersunterschungen 52: 383-391.
      Minchin D. and Duggan, C.B. (1988). The distribution of the exotic ascidian, Styela clava Herdman, in Cork Harbour. Irish Naturalists Journal 22: 388-393.
      Minchin, D. Davis, M.H. and Davis. M.E. (2006). Spread of the Asian tunicate Styela clava Herdman, 1882 to the east and south–west coasts of Ireland. Aquatic Invasions 1(2): 91-96.
      Parker, L.E., Culloty, S., O’ Riordan, R.M., Kelleher, B., Steele, S. and van der Velde, G. (1999). Preliminary study on the gonad development of the exotic ascidian Styela clava in Cork Harbour, Ireland. Journal of the Marine Biological Association of the United Kingdom 79: 1141-1142.
      Text written by:
      Dan Minchin, MOI, Ballina, Killaloe, Co Clare