Species description: This colonial ascidian forms thin sheets (2-5 mm) with a firm texture, that are usually easily peelable. The surface is smooth and leathery, and there are some thicker leathery patches on larger colonies. When it is growing on overhanging substrata (e.g. boat, buoys or trestles) it can form long, hanging formations that reach up to 1 m in length. These can break away and attach elsewhere, allowing this ascidian to spread. It is uniform in colour and can be cream to darker orange, with the internal water channels showing as dark veins. Usually, a white patch surrounds the inhalant openings, and when out of the water individual zooids are marked by a minute white spot.

Habitat: Usually first appears on artificial substrata in marinas or aquaculture sites but spreads easily to nearby seaweeds, rocks or boulders. It can grow on virtually any hard substrate and even on other organisms.

Distribution: First reported in Northern Ireland in 2015 in Strangford Lough and is also present in Carlingford Lough. Distribution is increasing, with records from various marinas and aquaculture sites around Britain and Ireland, as well as the areas surrounding these sites.

Similar Species: There are other didemnids, both native and non-native that are being recorded around Britain and Ireland which could be confused with D. vexillum. The firm texture, uniform colour and dark channels should help distinguish this species.

Key Identification Features:

• Colonial ascidian that forms peelable sheets or long, hanging formations.
• Uniform colour with internal water channels forming dark veins.
• Out of water a minute white spot marks individual zooids.

iNaturalist: Didemnum vexillum at iNaturalist World Species Observations database.

WoRMS: Didemnum vexillum at World Register of Marine Species. Accepted name: Didemnum vexillum Kott, 2002. AphiaID: 250126.

Classification: Biota; Animalia; Chordata; Tunicata; Ascidiacea; Aplousobranchia; Didemnidae; Didemnum