Hypselodoris saintvincentius from South Australia

Dear Stuart,
Thanks for these interesting photos. This animal is in fact Hypselodoris saintvincentius, which looks very like Hypselodoris infucata, but is apparently a southern relative of that widespread Indo-West Pacific species. Hypselodoris obscurafrom southeastern Australia, is another very similarly coloured species.

I suspect why you find it clustered in groups on rocks, is that the rocks are the only place that their sponge food can attach and grow. I may be imagining it but the rock on which your animals are clusterd seems to have a greyish-blue sponge growing on it like the one in my photo at the top of the page. I am not sure just how soft the grain dust sediment is but if the slugs can't crawl on it then they may be essentially prisoners on each rock. They possibly arrived as planktonic larvae and are stuck there until the food runs out. perhaps next time you are there you could check out the rocks to see if there is a layer of sponge on them, and perhaps put a couple of slugs temporarily on the grain dust sediment and see if they can crawl on it. If you ever come across them laying eggs, I would love to see a photo of their egg mass.
Best wishes,
Bill Rudman