Abstract
The chiefly Holarctic Hydrobius species complex (Coleoptera, Hydrophilidae) currently consists of Hydrobius arcticus Kuwert, 1890, and three morphological variants of Hydrobius fuscipes (Linnaeus, 1758): var. fuscipes, var. rottenbergii and var. subrotundus in northern Europe. Here molecular and morphological data are used to test the species boundaries in this species complex. Three gene segments (COI, H3 and ITS2) were sequenced and analyzed with Bayesian methods to infer phylogenetic relationships. The Generalized Mixed Yule Coalescent (GMYC) model and two versions of the Bayesian species delimitation method BPP, with or without an a priori defined guide tree (v2.2 & v3.0), were used to evaluate species limits. External and male genital characters of primarily Fennoscandian specimens were measured and statistically analyzed to test for significant differences in quantitative morphological characters. The four morphotypes formed separate genetic clusters on gene trees and were delimited as separate species by GMYC and by both versions of BPP, despite specimens of Hydrobius fuscipes var. fuscipes and Hydrobius fuscipes var. subrotundus being sympatric. Hydrobius arcticus and Hydrobius fuscipes var. rottenbergii could only be separated genetically with ITS2, and were delimited statistically with GMYC on ITS2 and with BPP on the combined data. In addition, six or seven potentially cryptic species of the Hydrobius fuscipes complex from regions outside northern Europe were delimited genetically. Although some overlap was found, the mean values of six male genital characters were significantly different between the morphotypes (p < 0.001). Morphological characters previously presumed to be diagnostic were less reliable to separate Hydrobius fuscipes var. fuscipes from Hydrobius fuscipes var. subrotundus, but characters in the literature for Hydrobius arcticus and Hydrobius fuscipes var. rottenbergii were diagnostic. Overall, morphological and molecular evidence strongly suggest that Hydrobius arcticus and the three morphological variants of Hydrobius fuscipes are separate species and Hydrobius rottenbergii Gerhardt, 1872, stat. n. and Hydrobius subrotundus Stephens, 1829, stat. n. are elevated to valid species. An identification key to northern European species of Hydrobius is provided.
Highlights
The Holarctic genus Hydrobius Leach, 1815 (Hydrophilidae, Hydrophilinae) has nine species (Short and Fikáček 2011), including H. orientalis Jia and Short, 2009, recently described from a part of China belonging to the Oriental Region
The low variation in the nuclear gene segments may have resulted in overparameterising of the phylogenetic models and explain why some expected clades in the H3 and ITS2 trees are basal paraphyletic groups without a common node (e.g. H. f. rottenbergii specimens with identical haplotypes in ITS2, Fig. S3 in Suppl. material 3)
The ITS2 results differ from the other gene trees by the placement of H. f. rottenbergii, H. arcticus and Clade IV basally in the tree (Fig. S3 in Suppl. material 3)
Summary
The Holarctic genus Hydrobius Leach, 1815 (Hydrophilidae, Hydrophilinae) has nine species (Short and Fikáček 2011), including H. orientalis Jia and Short, 2009, recently described from a part of China belonging to the Oriental Region. The recent study of hydrophilid phylogeny made by Short and Fikáček (2013) indicated that Hydrobius as currently delimited may be paraphyletic. The morphologically variable and strictly Holarctic H. fuscipes (Linnaeus, 1758) is seemingly closely related to the two genera Ametor Semenow, 1900, and Sperchopsis LeConte, 1861, known from North America, the East Palearctic and adjacent parts of the Oriental Region. The circumpolar H. fuscipes group poses some severe problems when it comes to species delimitation, by tradition paid most attention to in West Europe so far, but including three named species in the East Palearctic. In Europe only the two species, Hydrobius fuscipes and H. arcticus Kuwert, 1890, are recognized in current taxonomic works (de Jong 2011; Hansen 1987; Löbl and Smetana 2004)
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