Water mites of the genus Hydrodroma Koch, 1837 (Acari, Hydrachnidia: Hydrodromidae) from Argentina, with description of two new species

Water mite species of the genus Hydrodroma Koch, 1837 are common in running and standing waters in the Neotropical region. In this study four Hydrodroma species are reported from Argentina. Redefinitions are given for H. peregrina (Koenike, 1905), H. robusta K. Viets, 1954 stat. nov., and H. longiseta K. Viets, 1954, stat. nov. Two species, i.e., Hydrodroma argentinensis sp. nov. and H. ventana sp. nov. are described as new for science.

In this paper we report and describe four Hydrodroma species from Argentina, two of these as new for science.

Material and Methods
Water mites were collected by hand netting, sorted live in the field, and immediately preserved in Koenike-fluid by the junior author. In the lab, selected specimens were dissected, and slide mounted in Faure's medium. Holotypes and some paratypes will be deposited in the Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires (MBR), some paratypes will be deposited in Naturalis Biodiversity Center in Leiden (RMNH).
All measurements are in µm. The photographs of ejaculatory complexes were made using a camera on a Samsung Galaxy smartphone. The following abbreviations are used: Ac = acetabulum; Cx-I = first coxa; dL = dorsal length; H = height; I-L-4-6 = fourth-sixth segments of first leg; L = length; NHRS = Swedish Museum of Natural History, Stockholm; NP = National Park; P-1-P-5 = palp segment 1-5; W = width.

Systematics
Family Hydrodromidae K. Viets, 1936Genus Hydrodroma Koch, 1837 Diagnosis (based on Di Sabatino et al. 2010) -Idiosoma soft, without sclerotized dorsalia and ventralia, not laterally compressed wider than high. Cx-I+II medially separated by a fine membranous line; a row of long fine setae along the medial margins of Cx-I, and posterior margins of Cx-II, -III, and -IV. Genital flaps with more or less equally rounded lateral and concave medial margins, bearing fine setae and numerous Ac arranged in several rows along medial edge. Excretory pore sclerotized. Leg claws without claw blade, with a dorsal clawlet. P-2 with pectinate mediodistal setae. P-4 with a long, pointed extension reaching tip of the slender and elongate P-5.
any differences with Koenike's type material. We were able to locate the type material of H. peregrina in the Koenike's collection in the Museum für Naturkunde der Humboldt-Universität Berlin. The type material was in very poor condition but it was possible to recognize the presence of the characteristic long, medially directed posteromedial projections of Cx-I. In regard to the shape of coxal field and palps our specimen agrees well with specimens of H. peregrina s. str. as illustrated by Lundblad (1941).
In describing specimens from Mexico, Cook (1980) mentioned that his specimens differ from each other in proportional size of the genital field, separating the large and the small genital field forms, respectively. Our specimen as well as the specimen from Brazil illustrated by Lundblad (1941) have genital field proportions more or less intermediate between those of the two Mexican forms (see Cook 1980).
Hydrodroma peregrina robusta K. Viets, 1954, a subspecies described by K. Viets (1954a) from northeastern Brazil differs from the nominal species in the absence of swimming setae on II-leg and the characteristic slender shape and setation of the palp (P-4 L/H ratio 6.0); P-2 with four long and slender, a hair-like mediodistal pectinate setae, see Figures 16a-b in K. Viets 1954a). In regard to these differences, there is no reason to support the assignment of this taxon as subspecies of H. peregrina and we propose to the taxon to the level of a separate species H. robusta stat. nov.
Remarks -Our specimen is provisionally assigned to Hydrodroma longiseta K. Viets, 1954. This taxon was originally described by K. Viets (1954a) as a subspecies of H. despiciens. Hydrodroma longiseta resembles H. despiciens in shape of the coxal field, with short and medially directed posteromedial projections of Cx-I, but differs in the absence of swimming setae on II-L and in the shape of the palp with a long and slender, hair-like dorsodistal pectinate seta on the medial surface of P-2 (see Figures 14a-c in K. Viets 1954a). In regard to these differences, there is no reason to support the assignment of this taxon as subspecies of H. despiciens. In the shape of the coxal field and the palp our specimen from Delta de Paraná matches the original description. Anyhow, the variability of more specimens including material from the locus typicus needs to be examined to clarify the taxonomy of this species.
Female (paratype from Manso River, n = 1; in square brackets some measurements of the specimen from small lake ± 15 km N of Villa La  Etymology -Named after the country where the new species was collected. Discussion -In regard to the shape of the coxal field with relatively short, medially directed posteromedial projections of Cx-I, the new species resembles H. despiciens, a species considered previously to have a nearly cosmopolitan distribution (see Gerecke 2017 for a discussion). In former times, Hydrodroma populations from Brazil (Lundblad 1941), Argentina (Cook 1980) and Chile (Cook 1988) have been attributed to H. despiciens s. str., while six additional subspecies of the latter were introduced for other South American populations (see below). Recently, H. despiciens was redescribed by Gerecke (2017) allowing a reliable identification and enhancing the taxonomic status of populations previously assigned to that species. Studies on extra-European populations, e.g., from Australia Smit 2007a, b, 2011) have already shown that the concept of a cosmopolitan distribution of H. despiciens must be rejected.
In addition to the relatively smaller claw (mean L in males 27-29, in females 29-30 µm; L ratio claw/segment 5, 8-12%; data taken from Gerecke 2017), H. despiciens, can be separated from the new species from Argentina by the presence of one swimming seta on II-L-5 and an increased number of swimming setae on III/IV-L (more than 10 on both posterior III-L-4 and anterior IV-L-4). Moreover, H. despiciens differs in the absence of swimming setae from anterior IV-L-5. However, swimming setae numbers of single segments may overlap and the numbers of all segments must be considered together.
Based on the original descriptions, we give here a brief overview of the South American subspecies of H. despiciens with their main features (if not mentioned otherwise, data taken from original descriptions or calculated from figures given therein). Cook (1988) stated that it is more likely that some of these subspecies deserve to be ranked as distinct species.

Manso River Small Lake in NP Lanín
Hydrodroma despiciens brevirostris Lundblad, 1941, was described from a pond near S. Antonia in Villarica, Paraguay. Based on the original description, the female holotype (prep. 23331, NHRS) is characterized by the presence of swimming setae on II-L. Two other females (preps. 2330 and 2332, both NHRS) included by Lundblad (1941) in the type series, however, probably are not conspecific with the holotype. Both paratypes, as mentioned by Lundblad (1941) do not possess swimming setae on II-L and dorsodistal seta on the medial surface P-2 is longer and slender (hair-like) as in H. longiseta (see above for a description). K. Viets (1954a) already suggested that these females should be assigned more correctly to H. longiseta.
Hydrodroma despiciens clavipes Lundblad, 1953 was described from Colombia, and later on considered by Cook (1980) a separate species on the basis of material collected from Mexico. In regard to the absence of swimming setae on II-L and a strongly reduced swimming setation on III/IV-L (one single, short seta each III-L-4/5, and IV-L-4/5) this species seems to be closely related to H. despiciens pauciseta Lundblad, 1944.
Hydrodroma despiciens macronyx K. Viets, 1954 was described from the river Beberibe in the Brazilian state Pernambuco. In addition to the absence of swimming setae on II-L and reduced swimming setae numbers on III/IV-L (III-L-4 with 3-5 swimming setae; IV-L-4 with 3 in ♂, 6-7 in ♀; IV-L-5 with one in ♂, 3 swimming setae in ♀) this subspecies is characterized by large leg claws (42 µm), shortened swimming setae on III/IV-L and slender palpal segments, especially P-4 (L/H ratio P-4 6.3).
Hydrodroma despiciens crassiseta K. Viets, 1954, was described from the Brazilian Amazon region (Igarapé do Guaranazal). Based on the original description, this subspecies differs from other subspecies of H. despiciens by an increased number of swimming setae on II-L (II-L-4 with 4; II-L-5 with 10 swimming setae). Hydrodroma despiciens micronyx K. Viets, 1954, was described from a swamp in Recife, northeastern Brazil. It resembles H. despiciens crassiseta from which it differs in reduced numbers of swimming setae on II-L (II-L-4 with one, II-L-5 with two swimming setae).
Variability -We found considerable variation in the number of swimming setae on the anterior IV-L-5 between populations from rivers and lakes (a small lake in Lanín NP). Most specimens (eight out of 15) from the small lake N of Villa La Angostura did not have swimming setae on anterior IV-L-5, while other specimens bear here at least one (rarely two) swimming seta(e) at least on one leg; three specimens possessed swimming setae on anterior IV-L-5 on both legs. In the population from the Manso River most specimens possessed 1-2 swimming setae on the anterior IV-L-5 but several specimens were without swimming setae on anterior IV-L-5 on one or both legs.
Diagnosis -Main integument papillae flattened, surrounded by six small, little prominent elevations ( Figures 9B-C), in tangential view short and apically rounded ( Figure 9A). Posteromedial projections of Cx-I short. Genital plates with 37-45 pairs of Ac in at most 4 longitudinal rows. Legs with relatively long claws (L 39-42; L ratio claw/segment 5, 13-22 %). Leg setae   Description -Posteromedial projections of Cx-I short, medially separated by a fine membranous line. Genital flaps with rounded lateral and strongly concave medial margins, each with 37-45 Ac and 23-28 hollow setae. P-2 with three pectinate mediodistal setae similar in size.
In comparison with the six known South American subspecies of H. despiciens, the new species is most similar to H. despiciens macronyx K. Viets, 1954a, described from northeast Brazil (Pernambuco). The latter subspecies is distinguished from the new species from Argentina in more slender palp segments, especially P-4 (L 208 µm; L/H ratio P-4 6.3) and shortened swimming setae on III/IV-L.