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Building Reefs
The tiny marine invertebrates that build coral reefs are the hard corals. Scientists sometimes refer to hard coral as stony coral, reef-building coral or hermatypic coral. Each generation of reef-building coral lives on the solid framework that a previous generation built. As each organism measures only a few millimeters across, large coral reefs take millennia to grow. Not all hard corals build reefs, but all reef-building corals belong to the hard coral group. Coral polyps that live in shallower waters rely on symbiotic algae to help produce their calcium-based shells; in deeper waters, these algae cannot gather enough light to photosynthesize, so deep-water hard corals do not build up reefs, although they still form hard calcium carbonate cups around themselves.
Radial Symmetry
Although both hard corals and soft corals exhibit radial symmetry, hard corals have tentacles in multiples of six; soft corals have appendages in multiples of eight. Soft corals -- the ahermatypic corals -- include sea fans, sea whips, staghorn corals and tree corals. Although the whole colony looks asymmetrical, each individual polyp comprises eight tentacles. This fundamental difference in structure is an indication of their evolutionary divergence hundreds of millions of years ago. Although both sorts of corals are members of the phylum Ctenophora, they fall into different taxonomic orders based on their status as soft hexacorals or hard octocorals.
Environment
Although both varieties of coral inhabit the Earth's oceans and seas, the soft corals extend farther into the depths. As they do not need to rely on algae to supply them with nutrients, soft coral colonies thrive below the photic, or sunlit, zone of the water column. The few species of hard corals that do live in deeper waters cannot build the super-structures that give their order its common name, the reef-building corals. Filter-feeding soft corals colonize the sea floor to a depth of as much as 1.25 miles.
Ecological Niche
Reef-building hard corals build homes for more than themselves. These massive constructions -- the largest animal-created structures that any creature except man has built -- provide the basis for thousands of species' habitats. They also provide food for dozens of marine species including parrotfish, butterfly fish and starfish. Leathery soft corals also fill the role of food, but for different species; their soft protein-based colonies are the favorite diet of some species of sea slugs and crustaceans. While soft corals may provide a home to a few smaller creatures, they do not build the giant infrastructure upon which whole ecological webs have evolved as hard corals have done for millennia.