Infraclass SCOLECIDA

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Infraclass SCOLECIDA

Introduction

The name Scolecida, formulated by Rouse & Fauchald (1997) in their reclassification of polychaetes, is derived from Scoleciformia, a name introduced by Benham (1896). Scoleciformia appears to have been last used as a taxon name by Goodrich (1945). The Scolecida includes many of the taxa initially placed in the Scoleciformia by Benham and subsequent workers and is derived from skolex (Greek), meaning worm. The families included in Scolecida by Rouse & Fauchald (1997) are listed in Beesley et al. (2000) Table 1.3 and Figure 1.47. The total number of described species in the clade would appear to be over 800 worldwide, divided into about 130 genera. The Scolecida has some similarities to Benham's (1896) Scoleciformia, which contained the Arenicolidae, Maldanidae, Opheliidae, Scalibregmatidae, as well as the Flabelligeridae and Sternaspidae. Benham (1896), however, placed the Capitellidae in a separate suborder, the Capitelliformia, and the Orbiniidae (as Ariciidae) into the Nereidiformia. The remaining three families had not been erected in Benham's time.

Only two autapomorphies support the clade Scolecida in Rouse & Fauchald (1997), the presence of parapodia with similar rami and the presence of two or more pairs of pygidial cirri. In many ways, this group represents the simple-bodied forms of polychaetes and it is likely that further analysis will show that it is not a monophyletic grouping. Other features that are shared by members of the Scolecida, that are not apomorphic, include a prostomium that is clearly demarcated from the peristomium by a groove, the first segment is similar to those following and bears similar appendages (that is, there are no tentacular cirri), the buccal organ has dorso-lateral folds, and capillary chaetae are always present. Members of the Scolecida typically lack prostomial appendages, the exception being some paraonids which have a single median antenna. Relationships within the Scolecida show the Cossuridae as sister group to the remaining taxa, which all share the presence of sensory 'lateral' organs along the body. Cossurids were once thought to be cirratulids until Day (1963) considered them as a distinct group and placed them into their own family. The clade formed by the Orbiniidae, Paraonidae and Questidae is supported by the presence of an eversible ventral buccal organ, but similar buccal organs are found in other polychaete groups. Further, some orbiniid and paraonid taxa have a non-muscular axial proboscis, but this is thought to be derived from the ventral buccal organ (Purschke & Tzetlin 1996; Fauchald & Rouse 1997). This grouping was also postulated by Fauchald (1977) who placed the three families in Orbiniida. Questids bear a superficial resemblance to various marine members of clitellate groups such as tubificids. However, they are clearly polychaetes in having nuchal organs and they have none of the apomorphies of the Clitellata. The Paraonidae and Orbiniidae share the apomorphy of dorsal flattened branchiae in the analysis of Rouse & Fauchald (1997), and this grouping has also been postulated by previous workers (for example, George & Hartmann-Schröder 1985).

The other remaining clade of the Scolecida comprises five families, all with an eversible simple axial buccal organ. These five families were grouped as the Capitellida by Dales (1963). The Scalibregmatidae and Opheliidae were found to form a grade with respect to the Arenicolidae, Capitellidae, Maldanidae clade by Rouse & Fauchald (1997). These two families are often grouped together and were placed as the Opheliida by Fauchald (1977). However, the presence of a gular membrane in the anterior body grouped the Opheliidae with the Arenicolidae, Capitellidae and Maldanidae rather than with Scalibregmatidae. Opheliids like Travisia and Euzonus resemble scalibregmatids like Neolipobranchius in body form and having distinct epidermal rugosity. The possible non-monophyly of the Opheliidae or Scalibregmatidae deserves further investigation. The taxa Arenicolidae, Capitellidae, and Maldanidae have been grouped together as a more restricted version of the Capitellida by Fauchald (1977) and all share the presence of long-handled hooks organised as neuropodial tori. Rouse & Fauchald (1997) placed the Maldanidae as the sister group to the Arenicolidae. It is possible the latter taxon is paraphyletic and the relationship of the Maldanidae to arenicolid groups such as Branchiomaldane should be investigated. (After Rouse 2000).