A large sea slug or nudibranch, the sea lemon, Archidoris pseudoargus, is quite variable in colour from yellow to brown, green or even pink. It often also has darker blotches. Found all round Britain, it feeds on sponges and particularly the bread crumb sponge. The fan-like appendage at the right hand side is an interesting combination of gills and anus!
North Sea - St. Abbs Marine Reserve. Scotland