Chaetal arrangement in Orbiniidae (Annelida, Polychaeta) and its significance for systematics

Abstract

The systematic position of Orbiniidae within Polychaeta is still uncertain. In order to provide additional comparative data, we investigated the chaetal arrangement in this family, which is considered valuable for polychaete systematics. Specimens of Scoloplos armiger, Orbinia latreillii, and Pettibonella multiuncinata were examined by SEM and serial sections analysed by computer aided 3D-reconstructions. The obtained data suggest that the chaetal arrangement of Orbiniidae resembles that of other sedentary polychaetes in only a few respects. Transverse rows are only present in the main, anterior part of the chaetal patches of thoracic neuropods. The position of the formative site indicates homology with the transverse rows of several sedentary polychaete taxa. The chaetal patches thus diVer signiWcantly from those known in Apistobranchidae. Independent rows with an own caudal formative site, which run along the caudoventral edge of the chaetal patches, resemble the neuropodial ventral longitudinal rows known in Spionidae and related taxa. The abdominal neuropodia of S. armiger and O. latreillii bear longitudinal rows of chaetae. These are reorientated during ontogenetic chaetiger transformation and become the transverse rows of the thoracic chaetal patches. 3D reconstruction of S. armiger revealed that the notopodial chaetal bundles are organized in rows as well. Notopodia and abdominal neuropodia bear deep reaching supportive chaetae. They are the Wrst chaetae formed during neuropodial development and reside dorsally to the longitudinal row of capillary chaetae. Neither position nor structure indicates homology with the supportive chaetae of other sedentary polychaetes. Spionidae and related taxa are thus the only sedentary polychaetes, which speciWcally resemble Orbiniidae in certain aspects of their chaetal arrangement.

Pigmented eyes, photoreceptor‐like sense organs and central nervous system in the polychaete Scoloplos armiger (Orbiniidae, Annelida) and their phylogenetic importance

“…The placement within in Scolecida was found in cladistic analyses using several morphological characters, and usually molecular studies show Orbiniidae within Aciculata although the position is not well supported. Recent morphological studies by Hoffmann and Hausen (2007) on the structure of the chaetal arrangement and on the nature of the supporting chaetae which are discussed to be aciculae indicate similarities with Spionida supporting earlier views (Eisig, 1914). How do the structures of the sensory elements and the brain contribute to resolving these conflicting placements?…”

“…In addition, the structure of the brain indicates that palp nerve roots are present and that the corresponding appendages were lost. However, such a view is contradicted by the view of Hoffmann and Hausen (2007) who regard Spionida as closely related after studying the structure of the supportive chaetae in Orbiniidae which are not seen as homologous to the aciculae of the parapodia-bearing polychaetes. In any case, the observations on the chaetae and on the photoreceptors do not support a placement with the other appendage-less polychaetes in a basal group Scolecida.…”

“…They are either placed within Scolecida (Rouse and Fauchald, 1997), close to Phyllodocida within Aciculata (Bleidorn, 2005), or within Spionida (Hoffmann and Hausen, 2007). The placement within in Scolecida was found in cladistic analyses using several morphological characters, and usually molecular studies show Orbiniidae within Aciculata although the position is not well supported.…”

“…Interestingly, some members of Orbiniidae possess parapodia with supportive chaetae, which by some authors are regarded as homologous to the aciculae found in Aciculata (see Rouse and Fauchald, 1997;Rouse and Pleijel, 2001;Rouse and Pleijel, 2007). An alternative position has been suggested by Hoffmann and Hausen (2007), who discussed a relationship with Spionida following an ultrastructural analysis of the chaetae and chaetal arrangement in S. armiger and other Orbiniidae, which also refused the supportive chaetae being aciculae. This hypothesis was suggested by Eisig (1914) who discussed Apistobranchidae and Paraonidae as other potential relatives.…”

Systematics, evolution and phylogeny of Annelida – a morphological perspective

“…However, supporting chaetae are also present in other polychaete groups such as Chaetopteridae, Orbiniidae, Apistobranchidae, Psammodrilidae and Myzostomida (Hausen, 2005a). Nevertheless, there is a still ongoing debate as to whether these supportive chaetae are homologous or convergent structures Rouse and Pleijel, 2001;Hausen, 2005a;Hoffmann and Hausen, 2007;Struck, 2011;Struck et al, 2011;Eibye-Jacobsen and Vinther, 2012). As stated by, e.g.…”