I'm not sure how well established of a fact it is that all (or any) sponges will consume phytoplankton. IMHO the only macro organisms on TBS LR which directly benefit from the feeding of phytoplankton are the porcelain crabs. I have no proof but IME the sponges, bivalves nor the tunicates actually appear to uptake phyto as a food source. When I feed phyto the porcelain crabs obviously accelerate their already manic "fan waving" motions. The bivalves and tunicates typically close in the presence of any visible phyto. The sponges obviously don't do anything visible, but target feeding phyto didn't help any to survive longer than those who were not target fed. Most of the research I've seen seems to lean toward sponges relying on bacteria rather than phytoplankton for food.
IME some of these sponges are very difficult to keep alive and others very difficult to kill. The one's that have survived on my rock are the viscous sponges (the one's Richard refers to as chicken liver sponges, though if you see them reproduce you'll understand why they're called viscous sponges), the orange ball sponges (though most of mine look yellow to me), some of the encrusting sponges and some black shiny sponges that almost look like tunicates. All of the finger type sponges on my rocks have died or are slowly dying after eight months. The only things that have actually grown back to this point are some yellow colonial tunicates which have actually died off and grown back several times already.
The part of the sponge that appears translucent is the skeleton which is typically made of silica though I believe some sponges have calcium based skeletons.
The best information I've found regarding sponges is from Steve Tyree. There is also some info in Bob Fenner and Anthony Calfo's new "Reef Invertebrates" book.
