The Trembling Nudibranch is another fairly common yellow, white, bluish/purple nudibranch — sharing its basic colouring and genus with both the Imperial and the Tom Smith.  But unlike the Imperial, which has yellow spots against a white mantle, the Trembling is covered in white spots against a yellow mantle.  Both have a purple/blue marginal line and both intially appear to have blue- tipped rhinophores and gills.  Their behaviour and body shape is significantly different, so any amount of time with them will lead to a positive identification.  The Goniobranchus vibratus actually vibrates — not just its gills as the Imperial and Tom Smith do, but its entire mantle.  It literally seems to quiver as it moves.  It is also a lot wider (sometimes bulked up or flat) than the imperial and gets to be a much larger size than the Tom Smith.  To about 2.75″ or 70 mm.  These individuals were all found in the rocky North Kihei/Maalaea area of Maui in water less than a metre to 8 metres.