<p style="text-align:left">lets look at the anatomy of the giant clam
http://www.google.com/imgres?q=giant+clam+anatomy&hl=en&safe=off&sa=X&biw=945&bih=511&tbm=isch&prmd=imvns&tbnid=jRFoQHB7XpXKvM:&imgrefurl=http://www.gibell.com/stony-corals/chapter-fourthe-biology-of-tridacnid-clams.html&imgurl=http://www.gibell.com/stony-corals/images/4287_17_117-crocea-clam-anatomy.jpg&w=1446&h=1046&ei=iwFFUKekJYf89QStx4CoDA&zoom=1&iact=hc&vpx=543&vpy=150&dur=4406&hovh=191&hovw=264&tx=164&ty=136&sig=104582612254076755280&page=6&tbnh=148&tbnw=205&start=57&ndsp=12&ved=1t:429,r:6,s:57,i:285">http://www.google.com/imgres?q=giant+clam+anatomy&hl=en&safe=off&sa=X&biw=945&bih=511&tbm=isch&prmd=imvns&tbnid=jRFoQHB7XpXKvM:&imgrefurl=http://www.gibell.com/stony-corals/chapter-fourthe-biology-of-tridacnid-clams.html&imgurl=http://www.gibell.com/stony-corals/images/4287_17_117-crocea-clam-anatomy.jpg&w=1446&h=1046&ei=iwFFUKekJYf89QStx4CoDA&zoom=1&iact=hc&vpx=543&vpy=150&dur=4406&hovh=191&hovw=264&tx=164&ty=136&sig=104582612254076755280&page=6&tbnh=148&tbnw=205&start=57&ndsp=12&ved=1t:429,r:6,s:57,i:285</a>
water is pulled in through the inlet siphon passed over the gills all foods that are in the size range for the clam are removed and digested then the byproduct is released by the anus. granted the mantle harbors Zooxanthellae it will not meat the clams daily caloric needs, only about 70-90% in adults with over 10g of soft tissue. Small clams like the op is purchasing are around 1-3g in soft tissue weight and will not have the mantle size to collect even 40% of its daily needs by light alone, and a carbon source will be needed for the best health and growth .
In nature giant clams are sum times found in the waters break with full expansion of the mantle, even still it dusnot meat there daily needs . As i stated before food is in readily avaliable for filter feeders in the wild as zooplankton, bacterioplankton and phytoplankton are in abundance
In our reef tanks we lack the prolific production of these organisms well 90% of us, sum have the volume to support high levels of plankton but is rare.
now on to corals well start with sps ( small polyp stony corals )
this is another organism that is widely thought to achieve all its daily needs from light alone.
lets look at the polyp on a acropora
[IMG]http://www.google.com/imgres?q=acropora+polyp+diagram&start=179&um=1&hl=en&safe=off&sa=N&biw=945&bih=511&tbm=isch&tbnid=lGQxgNyQw9e7oM:&imgrefurl=http://www.gibell.com/stony-corals/scientific-nomenclature.html&imgurl=http://www.gibell.com/stony-corals/images/4287_12_37-montastrea-cavernosa-skeleton.jpg&w=1516&h=1316&ei=Tg1FUPytDYG29QSbs4GoCw&zoom=1&iact=hc&vpx=298&vpy=173&dur=1657&hovh=209&hovw=241&tx=101&ty=204&sig=104582612254076755280&page=16&tbnh=143&tbnw=165&ndsp=12&ved=1t:429,r:9,s:179,i:315">http://www.google.com/imgres?q=acropora+polyp+diagram&start=179&um=1&hl=en&safe=off&sa=N&biw=945&bih=511&tbm=isch&tbnid=lGQxgNyQw9e7oM:&imgrefurl=http://www.gibell.com/stony-corals/scientific-nomenclature.html&imgurl=http://www.gibell.com/stony-corals/images/4287_12_37-montastrea-cavernosa-skeleton.jpg&w=1516&h=1316&ei=Tg1FUPytDYG29QSbs4GoCw&zoom=1&iact=hc&vpx=298&vpy=173&dur=1657&hovh=209&hovw=241&tx=101&ty=204&sig=104582612254076755280&page=16&tbnh=143&tbnw=165&ndsp=12&ved=1t:429,r:9,s:179,i:315</a>
Here we have a organism with a mouth and stomach if it didnt need a source of carbon to meat its daily needs it would of evolved with out a mouth or stomach.
I will share a secret with you on why ulns ( ultra low nutrient systems) have such radical polyp extension. SPS feed on bacterioplankton a by product of the zeolite media so the coral has more energy to expand its polyp, and relies less on the Zooxanthellae it harbors.
Keep in mind Zooxanthellae is a brown pigmented algae wen the corals and clams rely more on light because there food source is low they have darker colors as they increase there Zooxanthellae concentrations to try to make more sugar to support the organisms metabolic needs.
There is nothing on this planet that i know of that is 100% photosynthetic