PolychaetaA Multitude of Many Bristled Worms

The polychaete worms represent the greatest diversity within the annelids, with more than 15,000 species divided among approximately eighty families, and estimates of over 5,000 species remaining to be discovered. Poly-chaetes display a wide range of morphologies and are found in nearly all marine habitats, including deep-sea hydrothermal vents and methane ice seeps. Fewer polychaetes are known from freshwater areas, and fewer than ten terrestrial species have been described. The previously distinct phyla containing the pogonophoran and vestimentiferan worms (important members of hydrothermal vent communities) are now placed within the poly-chaete family Siboglinidae.

Polychaetes typically undergo sexual reproduction and produce trochophore larvae, but asexual reproduction is also exhibited. During development, addition of segments occurs at a growth zone in front of the postsegmental region of the worms (pygidium) surrounding the anus. The presegmental region of the worm (prostomium) contains the cerebral ganglia and sensory structures; the mouth is positioned immediately posterior, surrounded by the first body segment (peristomium). Segments often display exterior projections (para-podia) with bundles of chaetae, although many species exhibit highly modified segmentation.

Divergence in body form among polychaetes

A. Scolelepis squamata, a polychaete found on sandy beaches where the worm suspension feeds by extending two palps from burrows. B. Two earthworms exchanging sperm during reproduction.

is based largely on the type of habitat and feeding biology of the species. The Nereididae (with more than 450 species) are common mobile predators of marine sand and mud habitats, feeding on invertebrates with the use of chi-tinized jaws. In contrast, members of the Capitellidae (with more than 140 species), Glyceridae (more than 75 species), Lum-brineridae (more than 200 species), and related families exhibit streamlined bodies (reduced parapodia) that aid in their burrowing way of life. These benthic worms are ecologically important deposit feeders and are often the dominant species in continental shelf and slope communities. Capitellids and other infaunal species are documented as pollution indicator species, revealing organically enriched areas.

Many additional families contain members that are sedentary. The Sabellidae (with more than 300 species) and Serpulidae (more than 525 species) produce tubes by cementing sand grains together with mucus or secreting calcium carbonate, respectively. The feeding appendages of these worms are modified into a crown of tentacles that is used to filter food from the water; although common inhabitants of coral reef areas, few casual observers would recognize their colorful tentacular crowns as belonging to polychaetes. The Tere-bellidae (with more than 375 species) inhabit tubes or crevices and feed on deposited materials by means of numerous prehensile tentacles. Members of the Spionidae (more than 320 species) possess only two tentacles used for deposit or suspension feeding. Within this family, the genus Polydora and related genera bore into calcareous substrates. The worms can destroy commercially harvested mollusks

(such as clams and scallops) and are therefore economically important pests. These and other polychaetes have affected marine ecosystems as invasive species introduced through ballast water or aquacultural shipments.

In addition to free-living and sedentary polychaetes, at least seven families contain pelagic species. More than 350 polychaetes have formed parasitic or commensal relationships with other phyla. For example, many of the symbiotic Polynoidae are found associated with echinoderms, and certain Syllidae are found associated with sponges, cnidarians, or other invertebrates.

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