This mating Chelidonura amoena - they always gather in large numbers to mate. I once seen on the upper surface of a wreck (Olympia Maru wreck, Sangat island, Philippines) where large areas were covered with these slugs, alll mating.
Lembeh, Indonesia 2005 and Maumere, Flores, Indonesia 2006
Nikon Coolpix 5000 in sealux housing and subtronic flash
. Copyright Teresa Zubi