Species description: A non-native invasive colonial ascidian that is spreading around Britain and Ireland. This species forms flat sheets (around 3mm thick) with an irregular outline, with zooids arranged in twisting/meandering double rows, and they usually occupy most of the surface of the colony. The colour is always uniform but is vary varied and can be bright orange, red, yellow, purple or brown. When present the large pink or purple larvae are usually visible being brooded just below surface of colony.

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 in Britain in 2004 and is now found in most marinas and many aquaculture sites around Britain and Ireland. In some areas it has spread to the surrounding seabed or shore. Any new records should be reported.

Similar Species: Could potentially be confused with the native Botrylloides leachi but in B. leachi the double rows of zooids are more spaced out in a neater ‘zipper-like’ formation, with clear spaces in the colony only occupied by the tunic rather than completely occupied by zooids; or the non-native Botrylloides diegensis which can also occur in a single colour, but larvae in this species are inconspicuous. If large pink/purple larvae can be seen this is diagnostic.

Key Identification Features:

• Colonial ascidian with irregular outline, made up of zooids arranged in twisting/meandering double rows.
• Uniform colour; often very bright. Can be orange, yellow, purple, red or brown.
• When present, relatively large pink/purple larvae are visible being brooded just below surface of colony.

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

iNaturalist: Botrylloides violaceus at iNaturalist World Species Observations database.

WoRMS: Botrylloides violaceus at World Register of Marine Species. Accepted name: Botrylloides violaceus Oka, 1927. AphiaID: 148715.

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