The Lesser Janolus
Although they're known as the butterflies of the sea, colourful nudibranchs like this lesser Janolus are typically toxic and use their colour to advertise their unpalatability to predators. Fortunately for the underwater photographer, they pose no threat, and even better, they move very slowly! This gave me time to position my off-camera strobe / fibre snoot combo above and a little behind it, resulting in a pleasing soft light that brings out the colours while maintaining the nudi's translucency.
Olympus EM5, Olympus 60mm Macro, 1/250s @ f11, ISO 200