Description by Wayne Ellis:

Comments: The elongate body is coloured  orange and mottled black with a black spotted tail crest. The mantle is smaller than the foot. A single pair of pink knobs (proturberances) occur on the mantle margin and emit a luminous fluid when the animal is disturbed.  The gills are high set on the animals back and can not be withdrawn. The lamellate rhinophores can be contracted. 

P. imperialis has the ability to use it's tail to swim, presumably as a form of protection. This habit leads to it ending up in prawn trawling nets. Maximum size is 100 mm and it occurs throughout the tropical western Pacific, extending in appropriate conditions to Tasmania and New Zealand in the south. Usually found in 0-15m of water.  P. imperalis feeds of bryozoans.

Plocamophorous (Thompson & Brown 1984, Willan & Coleman, 1984) and Plocamopherus are both names used to describe this species with the latter in current usage. 

Photographic equipment: Nikonos lV-A, 35mm lens with 2:1 extension tubes and Aqua-Sea Strobe. Kodak Kodochrome 64 film. Image digitally enhanced.

References: 
Rudman. W. B., 1999. The Sea Slug Forum.
Willan & Coleman., 1984. Nudibranchs of Australasia.
Thompson & Brown., 1976. Biology of Opisthobranch Molluscs. Vol. 1
Thompson & Brown., 1984. Biology of Opisthobranch Molluscs. Vol. 2