Weird filter feeder?

[anti]

What is this attached to the rock with my soft coral?
It seems to change size sometimes, and appears to open up to filter feed...

 

[hunt]

cool lookin, if you disturb it, does it retract? I would guess some kind of tunicate

 

[spanko]

Yup, colonial tunicate.

 

[anti]

Ok great.
What can u tell me about them or link me to?
Thanks!