Species description: A non-native invasive colonial ascidian that forms flat sheets (around 3mm thick) with an irregular outline. Zooids are arranged in twisting/meandering double rows, and they usually occupy most of the surface of the colony. This ascidian can be in two contrasting colours, with each inhalant orifice in orange, white or yellow with a darker colony background, or can also be uniform in colour. The embryos are brooded in this species but they are not conspicuous (as they are in Botrylloides violaceus).

Habitat: Often found in marinas and near aquaculture sites on a variety of hard substrata, also found growing on seaweeds. Seems to prefer sheltered areas and can be found from lower shore to around 10 m.

Distribution: First recorded from Northern Ireland in 2024 on oyster trestles in Strangford Lough. So far there are no other records from the island of Ireland. It has been recorded from the south of Britain from around 2004.

Similar Species: Could be confused with other species of Botrylloides, the native Botrylloides leachi and the non-native Botrylloides violaceus.

Key Identification Features:

• Colony can be contrasting colours or uniform in colour.
• Meandering rows of zooids.
• Does not harbour conspicuous embryos.

Distribution Map from NBN: Botrylloides diegensis at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Botrylloides diegensis at iNaturalist World Species Observations database.

WoRMS: Botrylloides diegensis at World Register of Marine Species. Accepted name: Botrylloides diegensis Ritter & Forsyth, 1917. AphiaID: 252278.

Classification: Biota; Animalia; Chordata; Tunicata; Ascidiacea; Stolidobranchia; Styelidae; Botrylloides