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Spionidae
Description
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The prostomium is often rounded and truncate, but may be pointed or have a pair of lateral horns. A pair of highly mobile, grooved tentacular feeding palps which arise from the peristomium is always present; they are often lost in preserved specimens. A median antenna may also be present. The prostomium is elongated posteriorly and bears paired nuchal organs. The parapodia are biramous, except for the first segment which may lack notochaetae in some taxa. Dorsal and ventral cirri are absent. Branchiae are often present, either restricted to a few anterior segments or present on most segments; they are located on the dorsum adjacent to the notopodial lobes to which they may be partially or completely fused. Branchiae are usually some-what flattened, and may be simple and laterally ciliated, or may carry pinnae or lamellae. Other structures also referred to as branchiae may occur ventrally (in Lindaspio, not known from Australia) or as accessory structures on notopodia (in Dispio species). The pygidium may have paired elongate anal cirri, or more numerous and smaller cirri and/or lobes. Aciculae are absent. Notochaetae and neurochaetae include simple capillaries, either smooth or limbate, and simple hooks with apical teeth; compound chaetae are absent. Both hooks and capillaries may be sheathed, the latter in some taxa as stout sabre chaetae in ventral neuropodial positions. Other chaetal types present include recurved hooks and modified spines. In many spionids, all segments are similar, but in Polydora and related genera (informally but widely known as polydorids), the fifth segment is modified and carries one or several additional types of chaetae including simple spines, stout brush-tipped and terminally cuspate spines, and more slender accessory chaetae.
This description, taken from Wilson (2000), is based on the studies of Blake & Kudenov (1978) and Fauchald & Rouse (1997).
Identification tips
Recognising the family
Spionidae should be readily distinguished by the presence of one pair of palps, a prostomium that is prolonged posteriorly, and leaf-like notopodia and neuropodia. Often there are gills as well. Admittedly, the palps are often lost, nevertheless, the family has a distinctive "look".
Inexperienced sorters sometimes confuse members of the families Orbiniidae are Paraonidae with spionids, even though neither of those families have palps.
Distinguishing species
The segment on which the branchiae first appear is important, and their relative connection to the notopodia, as well as their structure. In genera with modified segments, the structure of these modified chaetae is important. In some species specialised chaetae do not occur until posterior segments, so complete specimens are needed for species identification.
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