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Capitellidae: Mediomastus australiensis

The distinctly grooved prostomium is a short, blunt cone and the peristomium forms a complete ring (Rouse & Pleijel 2001; in contrast to Rouse & Fauchald 1997 who suggested that the peristomium was reduced to lips around the mouth). Antennae and palps are absent. The nuchal organs comprise a pair of dorso-lateral pits. The longitudinal muscles form distinct bundles. Segmentation is distinct. The first segment lacks parapodia and chaetae and is longer than the second segment which carries the first chaetae. In a variable number of anterior chaetigers, both parapodial rami, bearing short truncate lobes, may be deeply recessed into the body wall; tori replace these lobes in more posterior chaetigers. Dorsal and ventral cirri, and epidermal papillae are absent; ‘branchiae’ are also considered absent although extensions of the posterior body wall, in some species, may have a respiratory function. Pygidial cirri may be present or absent; when present their number varies in different taxa. Lateral organs are present, and dorsal cirrus organs have not been observed and are presumed to be absent. A simple axial eversible pharynx (= proboscis) is present. A gular membrane is situated between chaetigers 4 and 5, and the gut is a straight tube. A circulatory system and heart body are lacking. Aciculae are absent; other chaetae include capillaries and hooded hooks. Capillary chaetae and hooded hooks occur on both rami on a variable number of anterior chaetigers; posteriorly, hooks are present on both rami. The hooks are small and in a single row on each torus.

The above description is based on Hutchings (2000), which in turn is based on Fauchald & Rouse (1997).

Identification tips

Recognising the family
Capitellids are the polychaetes which most resemble terrestrial earthworms, usually with a rounded head and a long cylindrical body with poorly developed parapodia and clearly visible segments. In life, capitellids are often reddish in colour. Capitellidae are superficially similar to the Lumbrineridae, although they are not closely related. Lumbrinerids do not have distinct thoracic and abdominal sections which are evident in capitellids. Lumbrineridae also have complex jaws which are absent in capitellids.

Distinguishing species
For all capitellids mature specimens are necessary for accurate generic identification as only adults will have the adult complement of thoracic segments and chaetal distribution. Some capitellid genera are characterised by presence and position of genital spines. Characters used to distinguish capitellid species include the number of thoracic segments, whether the first segment carries notochaetae or neurochaetae, and the position at which capillary chaetae are replaced by hooded hooks in both notopodia and neuropodia. Future studies should look at the structure of the hooded hooks in detail. Branchiae are also important characters, although some species have retractile branchiae which they may occur only on far posterior segments and are often difficult to see in preserved specimens (branchiae are easily seen if live material can be examined).

Complete or almost complete individuals are necessary for species determination and even for generic determination, all the thorax and some of the abdominal segments must be present. The change from thoracic to abdominal segments is subtle and marked by a change in the orientation of the notopodia and neuropodia. The number of thoracic chaetigers must be determined and the distribution of the type of chaetae determined plus the distribution of the genital chaetae if present. Small individuals can be mounted whole in order to count the number of thoracic chaetigers and the distribution of chaetal types. Future studies should look at the structure of the hooded hooks in detail.


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