Quote from Borneman's "Corals" "Many species are aposematic, having bright colouration as a warning to discourage predators. Dull-coloured zoanthids do not usually have the protection of toxic compounds and may gain protection from sponges with which they associate. These sponges not only provide warning colouration, but also contain toxic compounds these zoanthids lack. Thus, the relationship is more commensal than thought."
There you go... and this could also be applied to the other thread with the Zoo ID I had just replied to. Hope that helps.