© The Norwegian Academy of Science and Letters 2005 • Zoologica Scripta, 34, 3, May 2005, pp313– 328 313 Worsaae, K., Nygren, A., Rouse, G. W., Giribet, G., Persson, J., Sundberg, P. & Pleijel, F. (2005). Phylogenetic position of Nerillidae and Aberranta (Polychaeta, Annelida), analysed by direct optimization of combined molecular and morphological data. — Zoologica Scripta, 34, 313- 328. The phylogenetic position of the most speciose meiofaunal polychaete family, Nerillidae, has remained contentious. Recent hypotheses have generally focused on the fact that Nerillidae shares with Aciculata (a major polychaete subgroup) features such as compound chaetae, ven- tral buccal organ and short ventrolateral palps. Here we present the first phylogenetic analysis of Aciculata, together with Nerillidae, combining morphological and molecular data. We also include Aberrantidae, previously referred to or placed near to spiomorph polychaetes, but recently referred to Aciculata, possibly close to Nerillidae. The data sets of 24 terminals con- tain 53 morphological characters and nearly complete sequences of 18S rRNA. The sequences were analysed simultaneously with the morphological data by direct optimization in the program POY with a variety of parameter settings (costs of gaps: transversions: transitions). The various settings resulted in markedly different phylogenetic hypotheses, but on the basis of congruence (ILD) the results of two parameter settings were chosen. In all analyses, the three included nerillid species constituted a monophyletic group. Only two analyses provided fully resolved cladograms. The morphological analysis gave poor resolution and the position of the nerillids was equivocal. The two molecular-based cladograms (minimizing ILD) were also poorly resolved, but one provided a position for nerillids next to Eunice pennata and Nothria conchilega, from the subgroup Eunicida within Aciculata. The two cladograms of the combined analyses (minimizing ILD) were fully resolved and placed nerillids in a terminal position next to Aberranta sp., within a clade of eunicidan species. The study showed that the analytical conditions for the homology assignment of 18S rRNA strongly influenced the phylogenetic results. The various previous proposals on the phylogenetic position of the Nerillidae are reviewed, some of which are in accordance with the results of the present study. Katrine Worsaae, Zoological Museum, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark. E-mail: kworsaae@zmuc.ku.dk Arne Nygren, Jenny Persson, Per Sundberg, Zoological Institution, Göteborg University, Sweden Greg W. Rouse, South Australian Museum, and Earth and Environmental Sciences, University of Adelaide, Australia Gonzalo Giribet, Department of Organismic & Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA Fredrik Pleijel, National Museum of Natural History, Paris, France Blackwell Publishing, Ltd. Phylogenetic position of Nerillidae and Aberranta (Polychaeta, Annelida), analysed by direct optimization of combined molecular and morphological data KATRINE WORSAAE, ARNE NYGREN, GREG W. ROUSE, GONZALO GIRIBET, JENNY PERSSON, PER SUNDBERG & FREDRIK PLEIJEL Accepted: 13 December 2004 doi:10.1111/j.1463-6409.2005.00190.x Introduction Nerillidae Levinsen, 1883 is, with 49 species in 18 genera, the most speciose meiofaunal family in Polychaeta. Nerillids possess seven to nine chaetigerous segments and are all minute, with most species ranging from only 300 μm to 2 mm in length. They are found in marine and brackish habitats, from the intertidal to abyssal depths, and on all con- tinents except Antarctica. The single freshwater species is reported from groundwater and mountain rivers in North America and continental Europe. Nerillids are mainly known from the interstitial environment, although several species are found in muddy habitats or caves with silt. The wide