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Paraonidae: Levinsenia gracilis

Paraonids are small, thread-like polychaetes with up to 200 segments. They range in length from 2-3 mm up to 40 mm and are usually uniform yellow to brown in colour in life. The body comprises an abranchiate anterior and posterior regions and a slightly wider branchiate mid-section where parapodia are best developed and which may be flattened dorso-ventrally. The prostomium is well developed, more-or-less bluntly conical, and carries a single postero-dorsal median antenna (for example, Aricidea sensu lato, Cirrophorus) or the antenna may be absent (Levinsenia, Paraonis, Paraonides, Paradoneis); a pair of eyes is often present. The peristomium is poorly developed, comprising only the ventral area around the mouth in adults. The anal lobe is slanted obliquely dorsally and usually bears two to three pygidial cirri. Paraonids typically have a non-muscular axial eversible buccal organ. The pharynx opens to a ventral mouth, which is surrounded by lips, of which the lateral ones are ciliated.

Parapodia are biramous, although both rami are poorly developed and have no aciculae. The postchaetal notopodial lobe is long and cirriform to foliose and the postchaetal neuropodial lobe is often smaller, more rounded, but absent in the posterior body. The cirriform to strap-like branchiae emerge from the dorsal surface above the parapodia on the mid-anterior body. Transverse rows of cilia occur on the dorsal surface of branchial segments and on the lateral margins of the branchiae. Capillary chaetae are present throughout the body together with specialised, mainly, acicular chaetae in posterior parapodia, which purportedly function as anchor points during locomotion. Capillaries in anterior parapodia are flanged and finely serrated whereas those of posterior parapodia are thin, straight and rounded in cross-section. Thicker specialised chaetae include the lyrate and bayonet types, pseudocompound chaetae, hooks with a subterminal or terminal spinelet, hooded hooks and spines with or without distal teeth. Specialised chaetae may occur either in the notopodia (Paradoneis and Cirrophorus) or in the neuropodia (Aricidea, Levinsenia, Paraonis).

Identification tips

Paraonids are not easy to identify, and we suspect that they have often been misidentified in collections. The ease with which median antennae becomes detached is problematic, but the scar is usually visible. Whole animals need to be mounted on a slide in order to clarify chaetal structure. Paraonids are probably under-represented in collections; because of their small size they may be more common in benthic communities than present collections would suggest.

Recognising the family
Paraonids are elongate, thread-like worms with a bluntly conical prostomium. Preserved specimens often have a coiled body. RW to write more ...

Distinguishing species
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