August 7, 2007
From: Mark Atwell
I have never seen a nudibranch quite like this. It has a visual texture like wool or a sponge's skeleton. It also seems to share some details - like camouflage - with the type of rock/coral it is on.
Locality: Bethlehem, near Minahasa Lagoon, Around 10 metres., Sulawesi, Indonesia, Celebes Sea, 01 May 2007, Ultra-fine muddy bottom . Length: About 3 cm.. Photographer: Mark Atwell.
Mark Atwell, 2007 (Aug 7) Another record of Doris sp 7. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/20344
This is an interesting find. We have one earlier record of this species which I have called Doris? sp. 7 but I am sure it is not a species of Doris. From the shape of the small upright gill circlet, perhaps it is a species of Hallaxa.
You are also right to liken the mantle texture to that of a sponge and to liken it to the 'rock/coral it is on'. Everything in your photo is a trick. The rock/coral it is on is in fact a very cryptic sponge colony - Dysidea arenaria which incorporates sponge grains into its outer skin precisely to look like a sandy rock. And of course the nudibranch is doing its best to look like the sponge. Note that both have the same conical projections - which in the sponges disguise the exhalent water siphons when not in use. One that is in use is visible alongside the nudibranch in the close-up photo.
As Scott Johnson noted, this is a remarkable example of camouflage by both the sponge and the nudibranch.