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Aplysilla sulfurea
Schulze, 1878
Paula LightfootAplysilla sulfurea maybe...?
Liz Morris Id agree with A.sulfurea, I think I've seen it up to 10cm cushions, and looking at the size of your Dendrodoa this prob wasnt more than that? but I'm no expert!
Tony Gilbert Yes, does certainly look like it, I've not seen this, assume its locally abundant in North Sea?
Are those Polycarpa scuba nearby (Dendrodoa grossularia are more rounded)?