Part of them, yes - the frame to be more precise. Then there is sealed foam in it for floating, as well as agricultural shade cloth.
Shade structures consisted of polyvinyl chloride (PVC) frames, which were filled with spray foam for flotation and then sealed against water intrusion. Frames were sized for each individual coral, such that the frame extended 5 cm on each side from maximum length and width. Light tan agricultural shade cloth (NCSNA Inc Model PS6160TAN) was attached to each frame using zip ties. The cloth was made from UV-resistant high-density polyethylene, which is durable and chemically stable to prevent degradation or toxic leaching. [source]