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Polychaetes are multi-segmented worms living in all environments in the world's oceans, present from abyssal depths to shallow estuaries and rocky shores, and even free swimming in open water. They are strictly aquatic annelids, but are the most abundant and diverse group of Phylum Annelida. Notably successful in mud and sand habitats, their densities there often exceed those of the sediment-dwelling molluscs and crustaceans alongside them. Polychaetes have soft bodies usually at most only a few centimetres long and pencil-thick, and they move relatively slowly, aided on each segment by the retractable grip of four dense clusters of bristles and hooks called chaetae, thus the name 'polychaete'.
Each of the over 80 families living today have characteristic body shapes and chaetal types. The families include for example centipede-like free-living crawlers like the nereidids and phyllodocids, colonial reef-building static forms with fans of head tentacles like the serpulids and sabellariids, flattened worms protected with shield-like dorsal scales like the polynoids, and predatory swimming worms with giant eyes like the alciopids. There are polychaetes specialised in many other unique ways, including in the ways they reproduce. Notably most Syllidae, Nereididae, and some Eunicidae (Palolo worms) metamorphose to become night-time swimmers to meet their mates, with timing of swarmings synchronised with the phases of the moon.