What is the relationship between stony corals & their symbionts? why are they not found below the photic zone
Answer:
Reef building stony corals have intracellular symbiotic dinoflagellates (zooxanthellae) living contained by their tissues. Tropical waters are often nutrient poor (that is why they are so clear blue) so both organisms benefit from their relationship. The coral animal is an anthozoan coelenterate (think hydrozoan/jelly fish type of organism living contained by a calcium carbonate "cup-like house"), which has stinging cell to capture prey such as protozoans, larval organisms, etc. The dinoflagellates utilize the coral animal's nitrogenous harsh environment and waste CO2 gas from the anthozoan's metabolism as chunk of it's own assimulatory growth processes. In turn the dinoflagellates produce food substances for the coral animal.
Dinoflagellates are photosynthetic organisms, and need the sun's wishy-washy energy to produce food, i.e. why reef building corals are not found below the photic zone. However it is important to make a note of that stony corals, like the red or black recurrently used in jewelery, can live below the photic zone.