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The Polychaeta or bristle worms, are the largest group of annelids, all aquatic and almost entirely marine. They are mainly free-living; some are commensally and very few are parasitic.
The body form varies widely. There are many well-defined families, but there is no widely recognised or consistent scheme of higher taxa. Traditionally the polychaetes have been divided into the Errantia (free moving) and Sedentaria (living in burrows or tubes). In many errant species the segments making up the trunk are similar and the head is bearing relatively few short appendages. In sedentary species the body is often divided into two or more regions and the prostomial sense organs are reduced or lost in many species; in forms which permanently live in burrows or tubes, food capture is often accomplished by the development of food-collecting tentacles, which can be extended from the tube entrance. Polychaeta typically have well developed parapodia and many chaetae, but in some species both are absent.

[After Hayward and Ryland, 1995]

Reproduction
Polychaete reproduction involves indirect development for most species, just some species show direct development. Fertilised polychaete eggs cleave spirally end develop into embryos with an equatorial girdle of cilia. Hatching into the plankton may occur at this stage, as the characteristic trochophore larva. Planktonic release can be delayed until segmentation has begun, the planktonic larva is then called a metatrochophore. Polychaete larvae may have a long or a short planktonic existence, or even remain benthic. Metamorphosis of swimming larvae to benthic post-larvae with adult forms and habits may involve considerable and rapid changes.

[After Pleijel and Dales, 1991]

Some Polycheata, particularly those of the Family Nereididae, can transform into a large reproductive body called an epitoke. During this transformation, most body segments develop large, broad-bladed parapodia and paddle-like setae, to perform better swimming. The eyes become enlarged and the antennae and other appendages of the head often are reduced. Epitokes can get caught with plankton sampling.
Other polychaetes, those of the Family Syllidae for example, form an epitoke by asexual reproduction (either through fission or budding from the posterior end); this is also called a stolon and this stage can be found in the plankton.

Identification
The present system provides only limited possibilities to identify polychaetes. Some of the species listed below, which occur as adults in the plankton, are keyed out in the Picture Key at Page 452: Polychaeta — although in a rather simple way by multi-character combination.
Larval stages of polycheate species are not covered by the keys. One reason is the small sizes that exclude them by definition from the present system; another reason is the incomplete knowledge of larval ecology and systematics in literature.
An identification guide to planktonic polychaete larva of the German Bight is provided by Plate and Husemann, 1994. More information on larval polychaetes in the area can be found in Hannerz, 1961 and Husemann, 1999.
The few common pelagic species in the area are listed below, but all polychaete species that might be regularly found in the plankton of the North Sea, either as adult or as a post-trochophore larva, are listed separately at the page "Polychaeta — list of pelagic species".

Truly pelagic Polychaeta in the North Sea are:

Tomopteris septentrionalis de Quatrefages, 1865 [Tomopteris septentrionalis]
Tomopteris helgolandica Greeff, 1879 [Tomopteris helgolandica]
Alciopa reynaudii (Audouin & Milne Edwards, 1833) [Alciopa reynaudii]


The following polychaete species are included in the present volume:

Order Phyllodocida
Family Nereididae
Nereis fucata
Nereis irronata
Nereis longissima
Nereis pelagica
Nereis succinea
Nereis virens
Nereis zonata
Perinereis cultrifera
Platynereis dumerilii
Websterinereis glauca
Family Alciopidae
Alciopa reynaudii
Family Tomopteridae
Tomopteris helgolandica
Tomopteris septentrionalis
Order Spionida
Family Spionidae
Polydora hermaphroditica
Order Terebellida
Family Pectinariidae
Pectinaria auricoma
Pectinaria belgica
Pectinaria koreni
Family Terebelliidae
Lanice conchilega

Class Polychaeta