Linnaeus’ amphibius and terrestris, both proposed in 1758 on the same page, are now considered conspecific by most researchers, but which name should be properly used is unsettled. Corbet (1978c:105) noted that "amphibius should have priority (presumably following Blasius (1857) as first reviser). Although strictly correct this is contrary to long-established usage and would cause considerable confusion and ambiguity" (also see discussion in Corbet et al., 1970:315). The usage is not so long established since the two forms were considered separate species through the middle 1900s (Ellerman, 1941; Hinton, 1926a; Miller, 1910, 1912a), and in the first work that considered them as conspecific, Blasius (1857) placed terrestris as a subjective synonym of A. amphibius. The reminder of Blasius’ role as first revisor dates from Van den Brink (1967), who employed A. amphibius as the valid name as have other systematists (e.g., Panteleyev, 2000; Zagorodnyuk, 1992c, 2000). In our view, confusion and ambiguity will be lessened only by using the name combination that is acknowledged as "strictly correct," A. amphibius.

The larger issue involves the homogeneity of populations arranged here under one nominal species. The exceptionally robust water voles in the English Isles (amphibius) have been maintained as specifically distinct from populations on the European continent (terrestris) (Hinton, 1926a; Miller, 1910, 1912a). The status of italicus also deserves critical review: e.g., Miller (1912a) had considered it a species, and Taberlet et al. (1998), using phylogeographic analyses of cytochrome b, found that samples from N Italy (italicus) are more genetically divergent than terrestris (here = A. amphibius) is from scherman. Denser geographic sampling and more critical analyses of morphologies and molecules are warranted.

European populations are generally reviewed by Reichstein (1982b) and Mitchell-Jones et al. (1999), and those of Russia and adjacent territories by Gromov and Erbajeva (1995). Faunal studies and checklists have proliferated to augment regional knowledge of the water vole’s morphology, distribution, and biology: Scotland (Stewart et al., 1999); Portugal (Ramalhinho and Mathias, 1988); N Spain (Ventura, 1992, 1993a, b, 1993-1994; Ventura and Gosalbez, 1989, 1992b; Castien and Gosalbez, 1992); Italy (Amori et al., 1999; Cantini, 1991); Switzerland (Hausser, 1995); N Germany (Dolch et al., 1994); Netherlands (Pelzers, 1992); E Baltic region (Miljutin, 1997, 1998; Timm et al., 1998); Sumava Mtns region, SW Bohemia (Andĕra and Červený, 1994); Slovenia (Kryštufek, 1991); Slovakia (Danko, 1994; Mošanský, 1994; Stanko, 1995; Stanko and Mošanský, 1994, 2000; Stanko et al., 2000); Czech Republic (Šmaha, 1996); greater Serbia and Montenegro (Petrov, 1992); N Israel (Qumsiyeh, 1996); Iran, Iraq, and nearby regions (Harrison and Bates, 1991); and China (Zhang et al., 1997).

Taxonomic and distributional studies have broadly covered morphological variability (Kratochvíl, 1980, 1983; Nikolaeva, 1982; Ventura, 1991). Morphometric analyses document variation along altitudinal transects and within geographic regions (Kratochvíl, 1981b, 1983), and taxonomically illuminate variation among samples from the Iberian Peninsula and C Europe (Ventura and Gosálbez, 1989) and that in Traika Depression of Bulgaria (Mitev and Miteva, 1991). Ventura and Sans-Fuentes (1997) documented geographic variation in non-metric trait frequencies in SW Europe. Molar patterns have been studied in several contexts: dental ontogeny and its significance for interpreting relationships of fossil and living species (Kratochvíl, 1980); concordance with subspecies recognized in the Pyrenees and Iberian region (Ventura, 1991); and correlation with geography, age, and diet (Nikolaeva, 1982). Ventura et al. (1993) described abdominal arterial configuration of Spanish water voles and compared it with other muroids. Variation in cranial size and shape in relation to fluctuations in population density and environment explored by Galaktionov (1995) and Kovaleva et al. (1996). Corbet et al. (1970) morphometrically demonstrated that only one kind of Arvicola occurs in the British Isles and considered it to be conspecific with continental populations (terrestris).

Several fossil species have been described from the middle to late Pleistocene (abbotti, antiquus, bactonensis, cantiana, chosaricus, gracilis, greenii, hunasensis, kalmankensis, praeceptor) and have been treated as extinct subspecies (Hutterer and Koenigswald, 1993; Kolfschoten, 1990; see review in Zagorodnyuk, 2000; see Maul et al., 2000, for a different view). Kratochvíl (1980, 1981b) regarded the Pleistocene cantiana to be merely part of the chronocline of A. amphibius. European Pleistocene records are provided by Kowalski (2001).

Although Neolithic samples document the presence of Arvicola on Sicily 3000-4000 years ago, none was uncovered in an exhaustive survey of barn owl pellets, leading Catalisano and Sarà (1995) to conclude its recent extinction.