I got some nice macro footage of Nembrotha chamberlaini nudibranchs mating at Red Rock and Twin Rocks in Anilao last year.
Nudibranchs have barbed penises to hold them together and the last clips shows the effort and twisting movement required to pull that pair apart.
Shot with my Panasonic GH4 in Nauticam NA-GH4 housing. I think I was using my Olympus M.Zuiko ED 12-50mm f3.5-6.3 EZ lens, with a Subsee diopter for some shots. The 2 most magnified shots are cropped in from 2160 to 1080p.