This huge nudibranch only appears at certain times of the year. This one was about 40cm long.
When I found it, it was laying eggs and remaining fairly stationary. After it has finished, it then started to feed.
It is a filter feeder and has a mesmorising method of gathering the sand/muck it lives on. It extends and flattens it’s head, then uses cilia in it’s mouth opening to sweep up the substrate.