Could you please tell me what this is. I took the photo at Merimbula, southern New South Wales, [Australia] in 15 m of water in September 2003.
It was about 3 cm long. Any help would be gratefully accepted.
This is a young Pteraeolidia ianthina. Have a look at the species' Fact Sheet where I explain how they change from white to brown as they begin to nurture single-celled plants in their bodies.