Hydrodroma angelieri (Acari, Hydrachnidia: Hydrodromidae) a new water mite species from Corsica based on morphological and DNA barcode evidence

In the present study we used morphological data and DNA barcodes to describe a new species, Hydrodroma angelieri sp. nov. from Corsica, France. A high genetic distance of 17.3±0.017% K2P from its molecularly most closely related European congener, H. despiciens (Müller, 1776), supports H. angelieri sp. nov. as a distinct species. Morphologically the new species can be identified on the basis of relatively small leg claws, the presence of only one swimming seta on II-L-5 and 4-6 swimming setae on the anterior surface of IV-L-5. An updated key for the European species of Hydrodroma is provided.


Introduction
The family Hydrodromidae K. Viets, 1936 comprises of two genera, Oxopsis Nordenskiöld, 1905, known from a single specimen collected in Sudan (see Smit 2020) and the widely distributed Hydrodroma Koch, 1837, recorded from all continents except Antarctica. The representatives of the latter genus are often abundant both in lentic and lotic habitats. Important characters in hydrodromids include i) idiosoma integument completely soft, lacking muscle attachment sclerites, ii) the uppermost layer of the integument is characterized by dense papillosity, with papillae that differ in shape and which are species specific, iii) genital flaps bearing numerous small acetabula arranged in several rows along the medial edge; iv) legs rather uniform with interspecific differences in the absolute and relative size of claws (larger in stream dwelling species), and in number and arrangement of long, fine swimming setae; v) morphology of mouth-parts, with P-4 bearing a long and pointed dorsodistal extension reaching the tip of the slender and elongated P-5 (Gerecke 2017).
Currently, 31 species of the family Hydrodromidae are known worldwide (Zhi-Qiang et al. 2011; Pešić et al. 2021a, five of which are present in Europe, i.e. Hydrodroma despiciens (Müller, 1776), Hydrodroma pilosa Besseling, 1940, H. torrenticola (Walter, 1908, H. reinhardi Pešić, 2002, andH. cf. rheophila Cook, 1967. The latter species, originally described from India (Cook 1963), is known in Europe only from the Greek island of Lesbos (Pešić et al. 2010). Most of these species such as H. pilosa, H. reinhardi and H. torrenticola have a Western Palaearctic distribution. Hydrodroma despiciens was considered cosmopolitan (see Di Sabatino et al. 2010). Nevertheless, studies on extra-European populations of Hydrodroma, such as those on Australian populations (Pešić andSmit 2007a, b, 2011) have revealed the presence of several clearly distinct autochthonous species. Recently, Więcek et al. (2020) applied an integrative approach based on the DNA barcode of the mitochondrial cytochrome c oxidase subunit I (COI) gene sequence and morphology to delineate the status of some Hydrodroma species from North America and Europe.
In this paper we used morphological data and COI barcodes to describe one new species of the genus Hydrodroma from Corsica.

Material and methods
Water mites were collected by hand netting, sorted live in the field, and immediately preserved in 96% ethanol for the purpose of the molecular analyses. After DNA extraction, the holotype specimen was dissected and slide mounted in Faure's medium. Holotype and paratype of the new species are deposited in Naturalis Biodiversity Center in Leiden (RMNH).
All measurements are in µm. Morphological nomenclature follows Gerecke et al. (2016). The genital plates and number of acetabula were measured on both sides, and for and therefore their dimensions were given as a range. The following abbreviations are used: Ac = acetabula; Cx-I = first coxae; dL = dorsal length; H = height; I-L-4-6 = fourth-sixth segments of first leg; L = length; P-1-P-5 = palp segment 1-5; RMNH = Naturalis Biodiversity Center, Leiden; W = width.

DNA barcode analyses
Molecular analyses were conducted at the Canadian Centre for DNA Barcoding (Guelph, Ontario, Canada; (CCDB; http://ccdb.ca/)). In CCDB the specimens were sequenced for the barcode region of COI using standard invertebrate DNA extraction (Ivanova et al. 2007), amplification (Ivanova and Grainger 2007a) and sequencing protocols (Ivanova and Grainger 2007b). The DNA extracts were archived in −80 °C freezers at the Centre for Biodiversity Genomics (CBG; biodiversitygenomics.net), while the specimen vouchers were returned to the first author for morphological examination. In CCDB the chromatograms were assembled into consensus sequences for each specimen and uploaded to the barcode of life database (BOLD; https://www.boldsystems.org/). The sample identifiers in BOLD are given for each barcoded specimen.
Sequence comparisons were performed using MUSCLE alignment (Edgar 2004). Intraand interspecific genetic distances were calculated based on the Kimura 2-parameter model ( K2P; Kimura 1980), using MEGAX (Kumar et al. 2018) software. MEGAX software was used to calculate Neighbour-Joining (NJ) trees based on K2P distances (standard for barcoding studies) and pairwise deletion of missing data. The support for tree branches was calculated by the non-parametric bootstrap method (Felsenstein 1985) with 1000 replicates.

Results and discussion
Species delimitation using DNA-barcodes The final alignment for species delimitation using COI sequence data included 11 Hydrodroma specimens listed in Table 2 and one outgroup, Panisopsis thori (Walter, 1907) (BOLD DCBDJ086-21, Voucher code CCDB 38392 H02) from Germany to root the tree. The final alignment consisted of 658 nucleotide positions. The NJ tree is presented in Figure 3.
Description -Female -Integument papillae distally rounded. Cx-I+II medially separated by a fine membranous line, with a row of long fine setae at medial margins of Cx-I, and posterior margins of Cx-II, -III, and -IV; a few most medial setae on Cx-I inserted on projections. Genital flaps with rounded lateral and concave medial margins. Excretory pore sclerotized. Leg claws without claw blade, with a dorsal clawlet. P-2 with pectinate mediodistal setae; P-4 with a long, pointed dorsodistal extension reaching tip of the slender and elongated P-5.
Male -unknown. Etymology -Named after the French acarologist Eugène Angelier in appreciation of his outstanding contribution to the study of water mite diversity of Corsica.
Discussion -Based on COI data the new species from Corsica seems to be most closely related to Hydrodroma despiciens (Müller, 1776) and H. pilosa Besseling, 1940. All three species are characterized by relatively small leg claws (L ratio claw/segment 5 < 14 %). Presence of only one short swimming seta on II-L-5 (II-L-5 posterior with 8-10 swimming setae in H. pilosa), makes the new species more similar to Hydrodroma despiciens, a species  Two other species, H. torrenticolla and H. reinhardi, both reported from Corsica (see Angelier 1959; Santucci 1971and Pešić 2002 differ from the new species among others in the morphology of legs with rather large claws (L ratio claw/segment 5 > 14%; see Figure 2A for comparison). Hydrodroma torrenticola, a species similar to H. angelieri sp. nov. in the presence of swimming setae on anterior IV-L-5, differs also in having bluntly pointed papillae and anterior face of IV-L-5 bearing 3-4 swimming setae; H. reinhardi differs in the absence of swimming setae from anterior IV-L-5 and a generally smaller measurements of idiosoma and gnathosoma (Pešić 2002; Gerecke 2017. Distribution -France (Corsica).