The Grey Sea Slug, Aeolidia papillosa, grows to about 12cms and is common subtidally on all rocky shores around the UK. Like many nudibranchs, it is a specialised feeder and lives entirely on sea anemones, biting chunks out of their columns. Colours range from almost white to grey and to a reddish brown.
The characteristic upturned v-shaped mark on the head area is sometimes lost or partially lost in older specimens.
St Abbs Marine Reserve, Berwickshire. E. Scotland