Venusta, Sandy Hook Is. Esperance, Australia
(photo Peter Clarkson)

(How it manages to stay up there, well, beats me?
If anyone thinks they know, I'd like in on it . )
I'd like to thank Mr Max Innes, a very experianced zoila diver,
for supplying the below information and to answer my question:

It's the animals foot, which it uses to suck onto the sponge.
When you touch a shell on a sponge it will retract the foot
and let go of the sponge in all cases unless sitting on eggs
and then it holds fast. The grey sponge, the Venusta is sitting in
is a typical venusta venusta sponge and their favourite food.