Didemnum vexillum



Conservation status

Not Assessed

Legal status

Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011. (Note: Regulation 50 not yet enacted). Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).

First reported in the wild



Invasive species - risk of High Impact

Irish status


Introduction pathways - 1


Invasive score


NAPRA Ireland risk assessed


Species Biology


Colonial ascidian that forms thick mats on pontoons, ropes and boats in marinas. Difficult to identify to species.


Most studies on the effects of Didemnum to date are from the United States. It can alter habitat by covering over substrates and altering the benthic community (Mercer et al., 2009), increasing the abundance of some infaunal species (Mercer et al., 2009) but decreasing the area available for the larvae of scallops (Morris et al., 2009) or eelgrass habitats (Carman & Grunden, 2010).




Can reproduce vegetatively from fragments and can complete a generation cycle in one week, (Morris & Carman, 2012).

Pathway and vector description

Likely introduced as a fouling organism on the hull of leisure craft from within Europe (Minchin & Sides, 2006). Due to the confusion over taxonomy of Didemnum spp (Lambert, 2009) it is difficult to identify an exact pattern of introduction and range expansion globally.

Mechanism of impact


Broad environment


Habitat description

Largely confined to marinas and bays in Ireland, it attaches to substrates and then proceeds to overgrow them (Minchin & Sides, 2006).

Species group


Native region

Temperate Asia


World distribution(GBIF)

Irish distribution

Established - Widespread. Likely under-recorded and may be more common than current records indicate due to difficulties in identification.

Native distribution

Thought to be Japan but due to initial confusion over taxonomy difficult to be certain (Lambart, 2009)

Temporal change

Date of first record category


Fifty year date category


Records submitted to Data Centre in 2022

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How can you help

Report any sightings to National Biodiversity Data Centre. Obey good biosecurity practices: Check and clean leisure craft when moving from areas containing Didenum spp to uninfected river systems; disinfect, and when possible allow to dry, fishing/angling equipment; disinfect boats when moving from one river catchment to another or upstream within river systems.



Carman, M. R., & Grunden, D. W. (2010). First occurrence of the invasive tunicate Didemnum vexillum in eelgrass habitat. Aquatic Invasions, 5(1), 23-29.

Lambert, G. (2009). Adventures of a sea squirt sleuth: unravelling the identity of Didemnum vexillum, a global ascidian invader. Aquatic Invasions, 4(1), 5-28.

Minchin, D., & Sides, E. (2006). Appearance of a cryptogenic tunicate, a Didemnum sp. fouling marina pontoons and leisure craft in Ireland. Aquatic Invasions, 1(3), 143-147.

Minchin, D. (2007) A checklist of alien and cryptogenic aquatic species in Ireland. Aquatic Invasions, 2(4), 341-366.

Morris Jr, J. A., Carman, M. R., Hoagland, K. E., Green-Beach, E. R., & Karney, R. C. (2009). Impact of the invasive colonial tunicate Didemnum vexillum on the recruitment of the bay scallop (Argopecten irradians irradians) and implications for recruitment of the sea scallop (Placopecten magellanicus) on Georges Bank. Aquatic Invasions, 4(1), 207-211.

Morris Jr, J. A., & Carman, M. R. (2012). Fragment reattachment, reproductive status, and health indicators of the invasive colonial tunicate Didemnum vexillum with implications for dispersal. Biological Invasions, 14(10), 2133-2140.

CABI Datasheet