Class POLYCHAETA

Polychaeta

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Class POLYCHAETA

Compiler and date details

2010 - Updated by Pat A. Hutchings and Michelle Yerman

1 July 2003 - Pat A. Hutchings & Rick T. Johnson

Introduction

Polychaetes are a group of predominantly marine, segmented worms. Like other members of the phylum Annelida, they have two presegmental regions, the prostomium and peristomium, a segmented trunk (metastomium) and a postsegmental pygidium. The nuchal organs, a pair of chemosensory structures on the postero-lateral margin of the prostomium, are apparently the only synapomorphy of the Polychaeta that distinguishes them from other Annelida (Rouse & Fauchald 1995). Nuchal organs vary from well-developed posteriorly projecting loops to inconspicuous pits or grooves. The largest nuchal organs are found in the Amphinomidae and the Euphrosinidae, where the ciliated folds on the caruncle represent the nuchal organs (Purschke 1997). Rouse & Fauchald (1995) suggested that Annelida can be a monophyletic taxon only by assuming that chaetae arose independently on the Echiura and the 'Annelida'. Subsequently, Rouse & Fauchald (1997) found that the Annelida is monophyletic, though weakly supported, and consists of the Clitellata and Polychaeta.

Polychaetes usually have a well-defined head with sensory and/or feeding appendages, followed by numerous body segments that may be differentiated into thoracic and abdominal regions. Typically, each segment bears a pair of parapodia and associated chaetae. However, considerable morphological variation is found amongst polychaetes, often reflecting the diversity of life styles of these animals. For example, capitellids that burrow through sediments tend to have few, if any, sensory or feeding appendages and reduced parapodia, whereas nereidids that crawl actively over the substratum have well-developed sensory appendages and parapodia. Also, species of the terebellid genus Hauchiella have a distinctly terebelliform body (enlarged thorax, tapering abdomen), but differ from most other tube-dwelling terebellids (and other polychaetes) in lacking chaetae; they live naked in fine sediments associated with coralline algae of New Zealand's rocky shores; this genus occurs in Australia, but precise habitat data is not available. Members of pelagic families are often very different structurally from closely related benthic groups in having large well-developed eyes, expanded foliaceous parapodial lobes and almost transparent bodies. Many of the features characteristic of polychaetes are lacking or reduced in interstitial, small-bodied forms, making their familial affinities obscure; anterior appendages, parapodia, chaetae and internal features such as circulatory systems and nephridia may be lost or modified compared to larger relatives.

Numerous other 'characteristic' traits are present in members of the Polychaeta. In many taxa, especially active ('errant') predators, antennae and palps are present on the prostomium. The sensory antennae are innervated by single tracts from the forebrain, whereas the paired palps are innervated by double tracts either from the posterior part of the brain or from the nerve ring passing around the foregut. Palps and antennae may appear externally similar, but the former usually emerge from a more ventral position on the prostomium and are often more robust. Palps range in form from paired grooved ciliated feeding appendages in spioniforms to paired sparsely ciliated, ovate or circular (in cross section) sensory appendages in the Phyllodocida, to forming the numerous appendages of the tentacular crown in the Sabellida. The peristomium may bear tentacular palps (paired or multiple) or fleshy cirri (cirri may be absent). Many of the prostomial and peristomial (head) appendages present in 'errant' species are lacking in tube dwelling or burrowing 'sedentary' species.

The anterior region of the digestive tract, the buccal organ, contains the buccal cavity that leads anteriorly to the mouth. In many polychaetes, an eversible non-muscular proboscis forms the anterior part of the buccal organ. In many groups, the buccal organ is in a ventral position and may be either heavily or poorly muscularised. Other groups such as nereidids, nephtyids and glycerids have an axial buccal organ and a well-developed associated musculature (pharynx), whereas in others the buccal organ may be soft and sac-like (Dales 1962). The buccal cavity may have a sclerotinised lining and hardened teeth-like structures or jaws. In other taxa lacking a buccal organ, the anterior gut is a simple tube without any obvious modifications.

Most species have paired, segmentally arranged, fleshy parapodia which differ greatly in size and structure, reflecting their various functions of anchorage, gas exchange, locomotion and protection. Parapodia are usually biramous, consisting of a dorsal ramus (branch), the notopodium, and a ventral one, the neuropodium, each of which may carry bundles of chaetae; the notopodial ramus is often reduced in size. The chaetae are highly specific and greatly diversified. Several different types of chaetae may be present in each species, but in nearly all taxa unjointed, tapering chaetae capillaries) are present. One stout internal chaeta (acicula) may be present in each ramus; they are used to support the parapodia, especially in taxa where the parapodia are well developed. Anteriormost segments may be cephalised during ontogeny and usually involves the modification of associated parapodia and the loss of chaetae; often the parapodial cirrus remains and becomes elongated, forming a tentacular cirrus.

After Beesley et al. (2000), which reference should be consulted for further detail on the class.

Acknowledgements

Chris Glasby is thanked for scanning and OCR of the Day & Hutchings original data. Robin Wilson and Tom Wilson did much of the preliminary editing prior to import of the data into the relational databasing package, Platypus, February 1998.

The following people are thanked for their generous help: Dr Robin Wilson, Museum Victoria; Dr Harry ten Hove, Amsterdam; Dr Mary Petersen, Museum, Copenhagen; Ms Linda Ward, Smithsonian Institute of Natural History, Washington DC; Dr Kristian Fauchald both of the Smithsonian Institute of Natural History, Washington DC; Dr Phyllis Knight Jones, retired, based in Swansea, UK; Professor G. San Martin-Universidad Autonoma de Madrid, Departamento de Biologia, Canto Blanco, Madrid, Spain; and Dr Alex Muir, The Natural History Museum, London.

The library staff of the Australian Museum are thanked for their help in tracking some of the more obscure references.

The work was supported by funding from the Australian Biological Resources Study. We also wish to thank ABRS staff for editorial assistance and nomenclatural advice.

Database Notes

This database is based on the Day & Hutchings (1979) checklist, modified such that Day & Hutchings' subgenera are elevated to genus. Subspecies are here elevated to species status, since the criterion for subspecies (that two subspecies form a hybrid zone where their ranges overlap) has yet to be satisfied for these taxa; subspecies trinomens are recorded as species available names.

Families follow the classification proposed by Rouse & Fauchald (1997) and genus authorities and dates are taken from Fauchald (1977) unless a more recent revision is stated as the source in a family introduction.

This database has been cross-checked with Wilson, Hutchings & Glasby (2003), and family diagnoses have been taken from this CD as well.

Ecological data entered from Day & Hutchings (1979) are standardised in this database as follows:

Day & Hutchings = this database
i = intertidal
e = estuarine
p = planktonic/pelagic
t = sublittoral
o = continental shelf
d, vd and a = continental slope
fw = freshwater