Morphological and molecular characters support that the former clitellate family Naididae Ehrenberg, 1828 is nested within another family, Tubificidae Vejdovský, 1876. To avoid paraphyly of the latter, it has been suggested that the two should be regarded as a single taxon. A recent decision by the International Commission of Zoological Nomenclature [2007; opinion 2167 (Case 3305)] ruled against a proposed reversal of the nomenclatural priority of Naididae over Tubificidae, with the consequence that all former tubificids should now be regarded as members of the Naididae whenever these two names are regarded as synonyms. The paper is a plea to clitellate researchers to conform to this ruling.

This is the first phylogenetic analysis of Clitellata using 18S rDNA in combination with morphological data of a selection of species representing Hirudinida, Acanthobdellida, Branchiobdellida, and eight oligochaetous families. The morphological data set includes 48 somatic (light-microscopical) and 34 spermatozoal (ultrastructural) characters. Eight new sperm models belonging to Lumbriculidae (two), Enchytraeidae (two), Phreodrilidae (one), and Tubificidae (three) are compared with the spermatozoal pattern already described among Clitellata. Somatic characters for each species are extracted from both general literature and the original species description. One new 18S sequence of Lumbriculidae and two of Tubificidae are reported, and are aligned together with corresponding sequences of 36 previously studied clitellate taxa. Two polychaete species are used as outgroups. The phylogenetic trees recovered using parsimony and Bayesian inference as optimization criteria of both individual and combined data sets yield largely consistent results. Our combined-data phylogenetic analysis is congruent with recent molecular studies. Somatic and spermatozoal characters contribute to the 18S rDNA phylogeny under both optimization criteria: in resolving the 18S topology, in adding new nodes, and in increasing the support for many groups. Morphological characters in combination with 18S rDNA suggest the following sister-group relationships:(1) between Acanthobdella and Hirudinida, with Branchiobdellida as their plesiomorphic sister group, and (2) between enchytraeids and Propappus, with both taxa grouping at the base of a large assemblage containing Lumbricidae, Lumbriculidae, Branchiobdellida, Acanthobdella, and Hirudinida. Maximum parsimony and maximum likelihood ancestral character state reconstructions on the combined-data tree indicate a new set of somatic and spermatozoal autapomorphies, and propose new evolutionary trends of somatic and spermatological characters. The observed complexity of the spermatozoal characters patterns among oligochaetous clitellates is discussed. This analysis supports a trend from primarily aquatic forms, with bifid chaetae indefinite in number, towards a more terrestrial mode of life leading to a simplification of the chaetae, thus supporting the hypothesis that the first clitellate was an aquatic form.

Abbreviations used in the figuresaa ac ad ai aP at ci em CP eg eP CC co ef f d fp lm lu me mf nc PC Pe Pg PP

An examination of the cuticle of six aquatic oligochaete species using transmission electron microscopy revealed a larger morphological variation than previously known. Three freshwater species, Aulodrilus pluriseta, Spirosperma ferox (both Tubificidae), and Pristina breviseta (Naididae), and three marine species, Clitellio arenarius, Heterochaeta costata (both Tubificidae), and Paranais litoralis (Naididae), were investigated. The arrangement of the collagen fibers in the cuticle differs among the studied species. Only S. ferox shows an "orthogonal grid," i.e., layers of parallel fibers perpendicular to each other, as earlier described for lumbricids and enchytraeids. Clitellio arenarius and H. costata have fibers arranged in layers, while A. pluriseta and P. litoralis have irregularly distributed fibers. Pristina breviseta lacks cuticular fibers. The matrix surrounding the collagen fibers (when present) continues outside the fiber layer, making up a thin epicuticle, which has a unique banding in each of the studied species. The external surface of the epicuticle is covered with epicuticular projections. Their number, shape, and attachment to the epicuticle vary among the studied species. Furthermore, a distinctive internal substructure of the projections was observed in H. costata, A. pluriseta, S. ferox, and P. breviseta. Microvilli, extensions from the epidermal cells, penetrate the cuticle and terminate at its outer surface. In three species microvilli were observed to pinch off the epicuticular projections. The size, number, and shape of the latter vary; no typical microvilli were observed in S. ferox.

The genus Nais is a group of oligochaetous clitellates, common in eutrophic freshwater habitats. About 30 species are described. Species identification is based primarily on chaetal characters, which are often subtle, inconsistent, and even overlapping between nominal species. We investigated the correlation between genetic variation and chaetal morphology in this genus. Eighty‐one individuals from Europe, North America, and China were included in the study. Seventy‐five of these were preserved as vouchers. They were scrutinized with regard to chaetal morphology, and ten different morphotypes were identified. Three molecular markers, two mitochondrial (the COI gene and 16S rDNA) and one nuclear (the ITS region), were used to establish the genetic lineages in the material. Genetic variation was found to be largely congruent with chaetal character patterns. However, at least nine separately evolving lineages (all supported by mitochondrial as well as nuclear data) correspond to at most six nominal species. Four morphotypes/lineages are recognized as Nais barbata, Nais christinae, Nais elinguis, and Nais stolci, respectively, whereas five, or possibly more, lineages represent a morphological continuum covering the variation of the Nais communis/variabilis complex. Thus, cryptic speciation is revealed. Our results indicate that a taxonomic revision of the genus will be needed in the future.