"Croosh" wrote in message oups.com...
Unless a coral can benefit from its algae, it would be called
parasite/host relationship, not symbiosis, and many photosynthetic
corals don't have skeleton to benefit from calcium carbonates...

Actually, soft corals like the hard corals have a skeleton.
However, it is not hard and stony. Their skeleton also contains
calcium carbonate, but only in small clumps called spicules.
We aquarists do not call this skeleton, but biologists do :-)