Polycera quadrilineata (Müller, 1776)

August 2000.
On a dive at Johnstone's Hole, Dunbar on the east coast.
One of several darker spotted individuals seen on this dive around 7 metres.

for more information about this species go to one of the links below
OPK - Opistobranquis nudibranchs of the british isles sea slug forum