Nudibranchus exiguus (Alder & Hancock, 1849)

10 April 2013.
Inverkip Pier, Firth of Clyde on the west coast.
5 mm animal photographed in the lab. Water temperature 7° C.
Sony NEX-5 in Nauticam housing, 18-55mm lens with SubSee +10 dioptre and 2 x Inon D-2000 flashes.