Bursatella leachii

These are definitely the largest sea slugs we’ve seen when scuba diving around Koh Phangan; they are probably the biggest in Thailand. They are gregarious and active beasties that are fascinating to watch.
We will suddenly see large numbers of them when we’re diving in Chaloklum Bay, probably due to a glut of the cyanophytes and diatom mats and films that they feed on.
Apologies for the poor colour and quality of the video, but at least it shows you how active these beasts are!