Significance of chaetal arrangement for maldanid systematics (Annelida, Maldanidae)

Abstract

Maldanids are usually divided into several subfamilies: Euclymeninae, Lumbriclymeninae, Maldaninae, Nicomachinae, Rhodininae, Clymenurinae, Notoproctinae, and Boguinae. The taxonomy of maldanids and the delimination of these taxa are mainly based on head morphology, total number of segments, chaetal structure, shape of the pygidium, and position of the anus. The maldanid ingroup relationships, as well as the monophyly of the proposed subfamilies, have so far not been investigated. Pilgrim (1977) described a shift of the notopodial chaetal rows from a transverse direction in anterior chaetigers to a more longitudinal one in posterior chaetigers in Clymene torquata (Leidy, 1855) and Euclymene oerstedii (Claparède, 1863), both belonging to the Euclymeninae. We investigated several maldanid species to assess the usefulness of this character for maldanid systematics and used 3D-reconstruction techniques to reveal the complete geometry of the chaetal sacs. Our investigation shows that a shift is apparent in Euclymene, Axiothella, Johnstonia (all Euclymeninae) and Clymenura (Clymenurinae), but absent in species like Maldane sarsi (Malmgren, 1865), Metasychis disparidentata (Moore, 1904) (both Maldaninae) and Petaloproctus borealis Ardwisson, 1906 (Nicomachinae). The shift is not typical for sedentary polychaetes and is apomorphic within maldanid polychaetes. It thus argues for a close relationship between Euclymeninae and Clymenurinae. The investigation of further maldanid species of different subfamilies may shed additional light on maldanid systematics.