Pycnoclavella is one of my favorite tunicates (sea squirts). This photo gives you a feel for how it grows in dense colonies. (But remember, each one is only a few millimeters high!)
I first introduced this species ~6 years ago, so for more information, see the post called "The tiny tunicate, Pycnoclavella" on 17 February 2012.
P.S. I think Pycnoclavella means "dense little clubs." It's an appropriate name!