Date of introduction and origin
Styela clava was probably introduced in 1952, as it
was found in Plymouth, Devon, in 1953 (Carlisle 1954; Houghton
& Millar 1960). This species was introduced from the
north-western Pacific, where it occurs from Japan to Siberia
Method of introduction
It was transported on the hulls of warships following the end
of the Korean War in 1951.
Reasons for success
It is a hardy species, capable of withstanding salinity
changes and temperature fluctuations.
Rate of spread and methods involved
Its spread has been rapid: from Plymouth in 1953 to
Southampton Water in 1959 and Milford Haven in south-west Wales
(Coughlan 1969) and across the Channel to France by 1968. It was
first recorded in Ireland in 1972 (Minchin & Duggan 1988).
Possible methods of dispersal include transport on ships' hulls or
on transferred oysters.
It is distributed on south and west coasts of England as far
north as Cumbria. It is found in abundance in certain parts of the
Solent (S. King pers. comm.), and also in certain parts of Loch
Ryan and other scattered Scottish localities (S.M. Smith pers.
comm.). Elsewhere in Europe it is found in France, The Netherlands,
Denmark and Ireland (Minchin & Duggan 1988).
Factors likely to influence spread and distribution
It is believed only to be able to spawn in waters above
Effects on the environment
Serious competition for food between individuals and with
other species can result if the population becomes big
Effects on commercial interests
It is a fouling pest on ships' hulls and oyster beds.
Control methods used and effectiveness
Biological control through the deliberate introduction of
Carcinus maenas into cages surrounding the sea squirt has
proved to be an unsuccessful control agent. Various combinations of
salinity, temperature and exposure to air have proved successful in
killing Styela clava without causing the host oysters any
None are known, though it harbours many epibionts so may aid
localised increases in biodiversity.
In Lancashire this species was first found in a man-made pool
at Morecambe from where it spread to other high-level pools, under
boulders and stones and down the shore (Coughlan 1985).
Carlisle, D.B. 1954. Styela mammiculata, a new
species of ascidian from the Plymouth area. Journal of the
Marine Biological Association of the United Kingdom, 33:
Coughlan, J. 1969. The leathery sea squirt - a new ascidian
from Milford Haven. Nature in Wales, 11:
Coughlan, J. 1985. Occurrence of the immigrant ascidian
Styela clava Herdman in Heysham Harbour, Lancashire.
Porcupine Newsletter 3: 85-97.
Houghton, D.R., & Millar, R.H. 1960. Spread of Styela
mammiculata Carlisle. Nature, 185:
Millar, R.H. 1960. The identity of the ascidians Styela
mammiculata Carlisle and Styela clava Herdman.
Journal of the Marine Biological Association of the United
Kingdom, 39: 509-511.
Minchin, D., & Duggan, C.B. 1988. The distribution of the
exotic ascidian, Styela clava Herdman, in Cork Harbour.
Irish Naturalists' Journal, 22:
Acknowledgements (Contributions from questionnaire)
D. Jones, Lancaster University.