What are Sedentary Polychaetes?

Order Errantia and Sedentaria

        Polychaetes can be separated into two large orders, Errantia and Sedentaria, based on the development of the anterior appendages and life habits.   Errant Polychaetes are active swimmer or crawler while Sedentary Polychaetes are burrowers or tube dwellers.


The Differences Between Errants and Sedentries

Errantia: Nephytidae

Errant Polychaete

Sedentaria: Terebellidae

Sedentary Polychaete

Errantia: Glyceridae

Errant Polychaete
The jaw of Glyceridae.

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Sedentaria: Pectinariidae

Sedentary Polychaete
Pectinariidae with a sandy tube.

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Errant Polychaetes

Sedentary Polychaetes

Body segments a large number a limit number; may be separated into different regions
Anterior appendages few in number and differentiated into palps, antennae, tentacular cirri, etc may be absent or a few to many similar appendages
Life habit free living and generally rapacious tubicolous or burrowing
Feeding mode all have jaws; scavengers or predators (on plants and animals) usually  filter- or deposit-feeders (i.e. consume sediment)

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Hong Kong Sedentary Polychaetes

        Since the 1970's, about 100 species of sedentary polychaetes have been reported from Hong Kong (Shin 1998).   They belong to 81 genera and 24 families.   Of these, 46 genera were not determined to species level due to the lack of taxonomic expertise and 17 species of sedentary polychaetes were described as new to science from Hong Kong (Shin 1998).   These new species were Scoloplos tumidus (Orbiniidae); Poecilochaetus hystricosus, P. spinulosus, P. tricirratus (Poecilochaetidae); Prionospio saccifera (Spionidae); Polycirrus dodeka, P. multus, P. quadratus, Rhinothelepus occabus, Streblosoma duplicata, Thelepus opinus, T. pulvinus, Eupolymnia umbonis, Lanice auricula, Lomia bandera, Longicarpus nodus and Terebella copia (Terebellidae).

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Sedentary Polychaetes:  Tube Dwellers

        The sedentary polychaetes have its prostomium, proboscis and eyes reduced or absent.   Most of them live in tubes constructed by themselves in the mud or sand of the ocean bottom.   The tubes are straight or U-shaped with two openings.   The glands on the ventral surfaces of the segments secrete the tube-forming materials.   The tubes may be calcareous, membranous, simple mucus-lined burrows, or composed of sand grains and other foreign materials cemented together.

Sandy tube of Pectinariidae

Sandy Tube of Pectinariidae

Click to enlarge

        The sedentary polychaetes are not active swimmers as errant polychaetes, so the tubes are important for protecting them from predators and catching prey.   Any disturances of predator or prey in the surrounding water can be transmitted to the tube.   Through the opening of the tube, the tube dwellers can get the clean and oxygenated water above the mud and sand.

        Many tube dwellers are beautiful with colours such as red, pink, green or iridescent.   Their parapodia are usually small and short, or with rows of hooklike setae for gripping the sides of the tube.  Their anterior part are greatly elaborated for feeding and respiration.    The sabellids and serpulids have prostomial tentacles developed to form a branchial crown of feather-like processes called radioles.   The peristomial tentacles of terebellids are long, filamentous and extensile.   The food is brought by beating of cilia in a groove running along each filament.

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