Batwing Slug

Although brightly coloured - purple & yellow, you would think they stands out and easy to spot. However, they are super tiny, around 5mm in size.
In both pictures, there is an even tinier Eubranchus Nudibranch by its side, (towards the right side of the pictures) which I didn't see during the dive.

This spectacular Sagaminopteron ornatum, sometimes known as the "Batwing Slug" because of its greatly enlarged wing-like parapodia. It was first found in Sagami Bay, Japan which gives rise to its name Sagaminopteron.

One of the most fascinating features of the gastropterids is their swimming behaviour. The genus name Gastropteron can be loosely translated as "winged stomach", which is quite apt. Like most opisthobranchs, they are normally benthic crawlers, but when disturbed they can vigorously flap their greatly enlarged parapodia and remain suspended in the water column for some minutes.