Toward a better understanding of the phylogeny of the Asplanchnidae (Rotifera) Elizabeth J. Walsh 1, *, Robert L. Wallace 2 & Russell J. Shiel 3 1 Department of Biological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA 2 Department of Biology, Ripon College, Ripon, WI 54971, USA 3 The University of Adelaide, Adelaide, Australia (*Author for correspondence: E-mail: ewalsh@utep.edu) Key words: cladistics, evolution, nrDNA, mtDNA, morphology, Monogononta, Synchaetidae Abstract We investigated the phylogenetic relationships of Family Asplanchnidae using both morphological and molecular data. The morphological database, comprising 23 characters from 19 taxa (15 Asplanchnidae and 4 outgroups), was compiled from a survey of the literature and our own observations; the molecular data (ITS and V4 region nuclear regions and mitochondrial cox1) was sequenced from specimens that we collected. Our analysis of the morphological data set (maximum parsimony) yielded 12 most-parsimonious trees with a tree length of 27 steps. From this analysis we conclude (1) Asplanchnidae is a monophyletic group as are the three genera comprising it, (2) there is no compelling support for the argument that Asplanchna should be separated into two discrete genera, and (3) there is some support for the proposal that Asplanchnidae and Synchaetidae are sister groups. Our analysis of the molecular data set supports the first two of these conclusions while the sister group of the family varied depending on the gene region analyzed and families and genera included. Current understanding of the phylogeny of Asplanchnidae is hampered by the need for additional informative morphological characters and a lack of molecular data for the genus Harringia and several other members of the Asplanchnidae. Introduction Family Asplanchnidae (Eurotatoria; Monog- ononta; Ploima) comprises 15 species of omnivorous rotifers, assigned to three genera (Ruttner-Kolisko, 1974; Koste, 1978; Jose de Paggi, 2002; Segers, 2002). These genera have specific habitat preferences that vary along a gra- dient from benthic to strictly planktonic. The benthic genus Harringia has a well-developed foot and is usually associated with plants; the semi- pelagic genus Asplanchnopus has a reduced foot and can be found with plants; the planktonic genus Asplanchna lacks a foot and is typically limited to open water. Whereas Harringia has a complete digestive system, the other two genera lack a gut; the etymon of both genera (G. a, lacking + G., splanchnum, the inward parts) refers to this fea- ture. Although much is known about the general biology of Asplanchnidae (e.g., Salt et al., 1978; Gilbert et al., 1979; Gilbert 1980, 1999; Joanido- poulos & Marwan, 1998, 1999; Kappes et al., 2000), we are aware of only two specific hypothe- ses regarding its phylogeny and neither has been explored (Sudzuki, 1964; Kutikova, 1983). These hypotheses were formulated using the principles of evolutionary taxonomy (Ridley, 1986). SudzukiÕs (1964: Fig. 2 & 75–78) genus-level phylogeny separates Asplanchna into Asplanchna and Asplanchnella based on vitellarium morphol- ogy: Asplanchna = globular ovarium (sic) [vitel- larium] (i.e., priodonta, herrickii); Asplanchnella Hydrobiologia (2005) 546:71–80 Ó Springer 2005 A. Herzig, R.D. Gulati, C.D. Jersabek & L. May (eds.) Rotifera X: Rotifer Research: Trends, New Tools and Recent Advances DOI 10.1007/s10750-005-4103-8