Species description: This colonial ascidian forms thick sheets (0.5–2 cm), with a firm, rubbery texture. The surface is smooth and slippery. When it is growing on overhanging substrata (e.g. boat, buoys or trestles) it can form long, hanging formations. The colour is orangish in water, becoming darker and eventually turning greyish-black on exposure to air.

Habitat: On artificial substrata in marinas or aquaculture sites.

Distribution: First reported in Northern Ireland in 2023 in Strangford Lough and is also present in Belfast Lough. The taxonomic ambiguity around this species and lack of a scientific name makes it difficult to assess the wider distribution.

Similar Species: There are other didemnids, both native and non-native that are being recorded around Britain and Ireland which could be confused with this species, especially Didemnum vexillum which has a similar, pendulous growth form. The aerophobic reaction, turning to greyish-black appears to be characteristic for this species.

Key Identification Features:

• Colonial ascidian that forms orange-grey coloured thick sheets and pendulous formations.
• Turns greyish-black on exposure to air.

iNaturalist: Trididemnum "black" at iNaturalist World Species Observations database.

WoRMS: Trididemnum "black" at World Register of Marine Species. Accepted name: Trididemnum Della Valle, 1881. AphiaID: 103461.

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