All manner of concrete, steel, and stone rubble from dredging, demolition projects, and other construction is used as artificial reef materials. This material is generally available at very low cost or free from construction companies who are more than happy to get rid of it. Transportation costs determine where this material is used by the Reef Program.
All these are animals that, in a sense, occupy the physical niche of the plants which are missing from the deeper parts of the marine environment. All are filter-feeders, straining plankton from the surrounding waters.