Polychaetes as habitat former and biocostructors

Polychaetes are widespread across marine ecosystem reaching high density in various seabed habitats, where they play a relevant role in the functioning of benthic communities. Many polychaetes are sessile and tube-dwelling forms, often gregarious, which are able to create primary biogenic structures or to modify the abiotic characteristics of the pre-existing substrate. They act as real builders of forest-like structures that offer new substrates for other benthic species, affect habitat conditions and regulate ecosystem functioning. Thus, polychaetes are ascribed to the engineers of animal forests. In this chapter, the main evidences of polychaetes as habitat formers on both rocky and sedimentary bottoms are reported; the time-scale variations of the biogenic structures are considered and the sensitivity and threats of polychaete forests are exhibited for contributing in decision-making regarding protection measures and management of coastal ecosystems.