||Areas dominated by mounds or mats of hydroids that are attached to a hard substrate.
Hydroids are colonies of individual cylindrical polyps arranged in a branching structure,
as a furry or brushy covering, or in other forms. Polyps are specialized for support,
reproduction, and defense, and are armed with nematocysts (stinging capsules in specialized
cells). Hydroids are the benthic life stage of planktonic jellyfish medusae, and most
are predators on small life forms. Hydroid mounds or mats may form very dense assemblages.