A sound taxonomic foundation is fundamental for all biological sciences from ecology and conservation biology to proteomics and genomics (Wheeler et al. 2004; Wilson 2004). The circumscription and naming of taxa enable the quantification of meaningful units as well as reproducibility within and between scientific studies, the very cornerstone of science. However, species show variable degrees of intraspecific variation, which may be geographically structured, and species delimitation is not always straightforward (Sites and Marshall 2003, 2004). With a vast and complex literature on different species concepts (e.g., Ruse 1969; Nixon and Wheeler 1990; Mayden 1997; Wheeler and Meier 2000), it is encouraging that a consensus view now seems to be emerging, according to which species are seen as separately evolving metapopulation lineages (de Queiroz 2007). Adhering to this “unified species concept” enables more straightforward tests of the validity of species as well as of infrasubspecific taxa. Morphological variants labeled as “forms”, “varieties”, or “ecomorphs” have been described in numerous taxa, both in the past and more recently (Snyder and Hansen 1940; Askew 1970; McLean and Kanner 2005; Mateos 2008). However, what these labels really refer to often remain unclear, undefined or, untested with quantitative data. The international code of zoological nomenclature (ICZN 1999) establishes that infrasubspecific names of the type “var.” and “form” are valid as subspecific names only, if described before 1961 and the author did not explicitly intend them to be of infrasubspecific rank. Here, we leave the debate on subspecies aside, because it is only relevant for allopatric or parapatric distributions (circular range overlap excepted; Wilson and Brown 1953; Starrett 1958; Wilson 1994). Names that refer to sympatrically occurring phenotypic forms or varieties can be explicitly tested using recent advances in applying molecular data and statistical analyses (Sites and Marshall 2003, 2004; Pons et al. 2006; Fontaneto et al. 2007; Knowles and Carstens 2007; Rosenberg 2007; Rodrigo et al. 2008).
Based on the ideas of the unified species concept, there are multiple relevant lines of evidence of speciation, all of which are found in previous species concepts, but as part of the definition (de Queiroz 2007). Examples include the cessation of geneflow, phenetical distinctiveness, diagnosability, ecological niche differentiation, and reciprocal monophyly and several recent methods have been developed to quantitatively test the evidence in favour of, or against, speciation. The general mixed Yule coalescence method (GMYC) (Pons et al. 2006; Fontaneto et al. 2007) provides a quantitative way of circumscribing species without any prior knowledge using single-locus DNA. The method only delimits reciprocally monophyletic species, hence all recognized species under the GMYC model satisfy at least that nonabsolute, but indicative criterion. Specifically, GMYC combines the coalescent process model for populations with the Yule speciation model for species to find the maximum likelihood threshold solution of an ultrametric gene tree. It separates branches that likely represent separate species from branches that are better modeled as within-species coalescents. Rosenberg (2007) and Rodrigo et al. (2008) developed different tests, but aimed at testing the same null hypothesis: could the observed pattern be derived by chance from a single- panmictic population? In Rosenberg's (2007) test, the pattern observed is two reciprocally monophyletic clades and the sample size of each clade determines the probability of observing the pattern under a single-panmictic population. Rodrigo et al.'s (2008) test instead focus on the branch length ratio of the assumed species ingroup node to the tips and the ingroup node to the immediate ancestral node. This is basically a quantitative measure of the “distinctiveness of clusters” often referred to visually on NJ-trees in DNA barcoding studies (Hogg and Hebert 2004; Koch 2010), but is here tested against the probability of seeing the observed ratio under a single-panmictic population. Rejecting the null under both tests imply reduced or absent geneflow between populations and if sympatrically occurring, evidence of species.
Preferably, the circumscription of separately evolving metapopulation lineages should be based on multiple lines of evidence (de Queiroz 2007), why we use quantitative morphological, nuclear, and mitochondrial data for species delimitation. This integrative taxonomic methodology is a powerful tool in resolving taxonomical problems and will in this study on water mites (Hydrachnidia) be applied to already known species (Unionicola minor (Soar, 1900), Piona stjordalensis (Thor 1897) [=curvipes stjørdalensis], P. imminuta s. lat. (Piersig 1897), P. rotundoides (Thor 1897)) (Biesiadka 1977; Davids and Kouwets 1987; Gerecke 2011), which have in the past been regarded as intraspecific forms to a sympatrically occurring nominate species (U. minor in relation to U. crassipes (Müller, 1776), P. stjordalensis and P. imminuta s. lat. both in relation to Piona coccinea (Koch, 1836), P. rotundoides in relation to P. pusilla (Neuman, 1875)) (Viets 1982, 1987). We also test a form presently without accepted species status, synonymous to the nominate species (P. dispersa Sokolow 1926 in relation to P. variabilis (Koch, 1836)) (Böttger and Ullrich 1974; Gerecke 2011). They can all be found in freshwater habitats in Europe and have a chaotic taxonomical history (Lundblad 1962; Viets 1987; European Water Mite Research 2009). For example, the following taxon names are also involved in the same species complexes, but of debated taxonomic status: U. crassipes f. octopora Maglio, 1924, U. crassipes f. reducta Lundblad 1924; U. laurentiana Crowell and Davids 1979; U. nearctica Crowell and Davids 1979; P. coccinea f. confertipora Walter, 1927, P. coccinea f. hankensis Sokolow, 1931, Piona coccinea f. recurva Lundblad 1920; P. coccinea f. gracilipalpis Lundblad 1924; the colour variant P. coccinea f. caesia Thor, 1925; P. pusilla f. disjuncta Viets, 1930, the smaller variant P. pusilla f. tenera Lundblad 1925, P. pusilla f. disparilis (Koenike, 1895), P. pusilla f. acutipes Viets, 1954, P. pusilla f. rotundiformes Lundblad, 1938, P. africana Viets, 1940, and P. sudamericana Viets, 1910) (Lundblad 1920, 1924, 1962; Viets 1982, 1987). Within the species-rich Hydrachnidia, variable sympatrically occurring intraspecific populations have in the past frequently been called forms (Lundblad 1962; Viets 1982, 1987). Despite the large extent of water mite forms currently still unsolved, for example, the problematic P. nodata group, there are few molecular studies on cryptic water mite species (but see Edwards and Dimock 1997; Bohonak 1999; Edwards et al. 1999; Bohonak et al. 2004; Ernsting et al. 2006, 2008). This is the first time the status of Unionicola minor, Piona stjordalensis, P. imminuta s. lat., P. rotundoides, and P. dispersa are tested using molecular data. We apply statistical phylogenetic, species delimitation, and population genetic methods to explicitly test diagnosability, geneflow, monophyly, and phenetic distinctiveness.