Where the mouth leads straight into the oesophagus without any ealboration a buccal organ is said to be absent. Among polychaetes this situation is found in those taxa that are filter feeders in one way or another and in , which has no buccal apapratus at all. Since all filter feeding taxa are derived groups of polychaetes it would appear that they have lost a buccal organ. Taxa that fall into this category include all members of Sabellida (Canalipalpata) and Chaetopteridae which belongs in Spionida (Canalipalpata).
The group contains a variety of members, some of which are quite similar to taxa such as Spionidae and are probably selective deposit feeders. However, the most well known chaetopterid is Chaetopterus where the buccal region is a simple tube with no vestige of a proboscis. Chaetopterus lives in U-shaped tube that resembles parchment (actually layers of shed cuticle) about 25 cm long, buried in mud with with each end emerging from the mud as a small 'chimney'. Chaetopterus can be observed alive by transferring it to a glass tube. Feeding is a complicated process involving major modifications to the parapodia in the front part of the body. The notopodia of segment 12 are long and wing-like (aliform) with a ciliated and mucus-secreting epithelium. The parapodia on 14-16 are piston-like and fit against the cylindrical walls of tube. They beat synchronously at up to 60 beats/minute, producing a current through the tube from the anterior end. The aliform notopodia are splayed and a sheet of mucus like a net is secreted between them. It is continuously secreted to form a mucus bag, at 1 mm/sec. Almost all incoming water is filtered through this bag; detritus and plankton are retained. The end of the bag is grasped by a ciliated 'food cup' middorsally on segment 13. This cup (seen here in dorsal view, though the bag is not visible) rolls up the end of the bag continually.When the ball of mucus and food (a bolus) reaches a certain size it is cut loose from the notopodia. The food cup then places the bolus on the ciliated mid-dorsal groove which carries it forward to mouth. The food ball averaged 3 mm diameter in an 18 cm (6 to 8 inch) specimen. Large objects brought into tube are detected by cilia on the head and shunted to either side; the aliform notopodia are then raised to let the large objects pass, thus avoiding damage to the bag.
Sabellidae (e.g. Sabellastarte australis) and Serpulidae (e.g. Galeolaria caespitosa), Sabellariidae (Idanthyrsus pennatus) and Siboglinidae (Riftia pachyptila) (formerly the phylum Pogonophora). Sabellidae and Serpulidae are typically true filter-feeders using only ciliated prostomial plumes. Sabellariidae are also filter-feeders though using a different method. Siboglinidae are extremely unusual on thier mode of nutrition and will be dealt with in the next section.
Sabellastarte australis is large sabellid from the east Australian coast and is a good example for viewing the tube and the crown of tentacles. The crown of radioles is developed from prostomium and represents multiple transformed palps. The crown often forms two half circles, opposed to form a funnel when expanded beyond tube. Feeding is initiated by the beating of cilia on the pinnules of the radioles produces a current of water that flows through radioles into funnel and then flows upwards and out. Particles are trapped on the pinnules and are driven by cilia into a ciliated groove running the length of each radiole. At the base of the radioles there is a complex sorting mechanism. The largest particles are rejected and fine material is carried by ciliated tracts into the mouth. Medium-sized particles may be used for tube construction. The thorax has dorsal capillary setae (notosetae) and ventral uncini (neurosetae). The abdomen has the reverse situation, with dorsal uncini and ventral capillary setae. A large Sabella introduced via ballast water from the Mediterranean Sea is now a major problem in Port Phillip Bay.
Serpulids have a basically similar mode of feeding to that of sabellids.The group is distinguished from the Sabellidae in having the right dorsal filament modified as an operculum; having a thoracic membrane (dorsolateral frills) and a calcareous tube. The setae are arranged as in sabellids. The thorax has 7 setigerous segments, with dorsal capillary notosetae, and ventral uncini. The abdomen has a variable number of segments. There are ventral capillary neurosetae and dorsal uncini. The animal commonly lies on its back. Galeolaria is an extremely abundant Australian worm, the white calcareous tubes of which forms a distinct midlittoral band on rocky shores throughout New South Wales and southern Queensland. A Galeolaria caespitosa 'clump' is home to a considerable diversity of fauna. Some serpulids, by virtue of their dense populations and calcareous tubes, are pests. They may block lock-gates and fowl dock piles. Hydroides norvegica has a delicate tube but seriously fowls the undersides of boats.
Sabellariidae represent an interesting group of polychaetes whose systematic placement has varied in the last 40 years from Spionida (Dales 1962), to Terebellida (Fauchald 1977), to Sabellida (Rouse and Fauchald 1997). The current placement of the group in Sabellida is based on the presence, shared with Serpulidae and Sabellidae, of dorsal uncini in the abdominal region and the anterior pair of nephridia. Sabellariids however do not have a radiolar crown and in fact have a pair of grooved palps similar to those seen in Spionida. However, the palps are not involved in feeding and they instead use 'feeding tentacles' that appear to have no equivalent in other polychaete groups. The anterior end of sabellariids are also unusual with several segments surrounding the head. The stout golden chaetae (paleae) of the first segment project forward that plug the opening of the tube. Sabellariids live in tubes comprised of sand, shell fragments etc that are glued strongly together. The free-swimming larvae preferentially settle on the tubes of other sabellariids and, in Europe and the USA, this results in the formation of large reefs that can reach several kilometres in length. In south-eastern Australia, Idanthyrsus pennatus reaches up to 5 cm in length, is a prominent sabellarid found in clumps on rocks in the lower intertidal region. Idanthyrsus can form extensive subtidal reefs if conditions are favourable. Feeding is extension of their ciliated feeding tentacles into the surrounding water which gather and sort any particles that are in the vicinity.
Copyright © University of Sydney. Last updated September, 2005. Site construction and maintenance: SOBSTDU. Email us here with your comments and feedback.