Spionids

Spionidae is a family of marine worms within the Polychaeta. Spionids are selective deposit feeders that use their two grooved palps to locate prey. However, some spionids are capable of interface feeding, i.e. switching between deposit and suspension feeding.

Spionids produce tubes by cementing sand grains and detritus material with mucus produced by their glandular pouches. The Spionidae is one of the most studied polychaete families given their biological and commercial importance. Members of this family have been used in regeneration studies and some are capable of boring into calcareous substrate which has destructive implications for commercially important shellfish.

Classification of Spionidae into subfamilies has been relatively little used as, apart from the Polydora-group of genera, the groupings are not obvious. The Polydora-group are also seen as tribe Polydorini (polydorins) following Benham (1896) who created Polydoridae solely for the genus Polydora, based on the chaetiger 5 spines and a lack of branchiae on chaetigers 1-5. Mesnil (1896) analysed the relationship of existing spionid genera, but did not assign subgroup names to them. Mesnil (1897) removed Disoma (now Trochochaeta) from Spionidae to Disomidae (now Trochochaetidae).
Söderström (1920) created subfamilies Spioninae, Nerininae, and Laonicinae. Spioninae Söderström included Spio, Microspio, Pygospio, and Polydora (Boccardia was not mentioned, but it would also include Boccardia and the other Polydora-group genera). According to Blake (1996:82) these genera had thin-membraned eggs, long-headed sperm, and egg capsules, whereas all other spionids had thick-membraned eggs and short-headed sperm. Nerininae Söderström included Nerine (now Scolelepis or Malacoceros), Colobranchus (now Malacoceros), Scolecolepis (now Scolelepis), and Aonides. Thus the current genera are Scolelepis, Malacoceros, and Aonides. Laonicinae Söderström included Laonice, Prionospio, and Spiophanes.
As there are now about 40 valid Spionidae genera, Söderström's classification would require a new analysis to apply it further. Blake & Arnofsky (1999) could confirm only a clade for subfamily Spioninae in their morphological analysis. As yet (May 2021) there appears to be no molecular analysis which examines the relationship of the main spionid genera, or genera in apparently closely related families.