Hydrodroma despiciens ( Müller, 1776 )

Gerecke, Reinhard, 2017, The water mites of the genus Hydrodroma (Acari, Hydrachnidia, Hydrodromidae) in Europe and Africa, Ecologica Montenegrina 13, pp. 1-24 : 4-7

Hydrodroma despiciens ( Müller, 1776)

( Figs 2 A-F)

Type series: Neotype, here designated: ♂, SMNH, Denmark, Coll. Lundblad, NHRS- GULI 000046050, Själland, Gribsö, 19.04. 1919 in Koenike fluid. Paraneotypes : Same site and date, 1 ♀ in Koenike fluid; 1 ♂, 1 ♀ slide mounted in Hoyers medium.

Material examined: NHMB 1339, unclear origin, "I E 2.d XXX/29" 1 ♂, 1 ♀; SMNH, Denmark, Coll. Lundblad, NHRS-GULI 000046050, Själland, Funkedamm, 25.04.1919 1 ♂, 2 ♀♀, slide mounted Gerecke 2017; GULI 000046049, Själland, torvgrav v. Arresö, 23.05.1919, 5 ♀♀, slide mounted Gerecke 2017 (together with 1 ♂, 3 ♀♀ H. pilosa in the same tube, undissected in Koenike fluid). SMF, Germany, 43905, Holstein , Trammer See , 9.8.1918, Viets leg. 2519; "Puppe; Type ", 1 dn; 51136, Vehne-Bach an Querung Küstenkanal bei Feddeloh II, Oldenburg-Land, 20.7.1946, 225, 1 ♂; 51137, ditto, 226, 1 ♂, 1 dn; 51138, Oberneuland bei Bremen, Karl Viets leg. 10.6.1907, 342, 1 ♀; Spain, 43890, Madrid, Guadarrama, 20.05.1919, 39093, 1 ♀; 43891, Madrid, Mormoto, Manzanares, 25.5.1919, 1 ♀; 43902, Ibiza, Rio S. Eulalia, Margalef coll. 7102, 1 ♂ [bad state of conservation] .

Rejected identifications (labelled as " Hydrodroma despiciens "):

A) Hydrodroma pilosa (see there): Specimens from NHMB ( Algeria, Turkey, Switzerland); SMF ( Germany, Spain, Sweden) and SMNH ( Denmark).

B) Hydrodroma cf. americana Marshall, 1926 [characteristic in elevated posterior swimming setae number on II-L-5, see data for H. americana in Wiles 1986] FMNH, U.S.A., Wisconsin, Mirror Lake, Delton. Mis, 8-28-19 Young, numerous specimens - 1 ♂ slide mounted Gerecke; Wisconsin, Green Lake, Delton. Mis, near Concrete bridge, July 23-24 1921; "1-1, 1; 7-11, 8, 7-2,21 (& on slide)" " Aug 1921 (Juday, Baker)" "Bartmouch Bay 2 Juday", numerous specimens - 1 ♂ slide mounted Gerecke.

C) Hydrodroma torrenticola (see there): SMF Spain, 43891, ♀; N Germany, 51136-37, 2 ♂♂ ; S Germany, 51139, ♂ .

D) Uncertain, possibly undescribed species: FMNH, Liberia, DC 80, ♀ [generally lower swimming setae numbers on all leg segments, larger leg claws]. SMF, Kamerun, 43888, ♀ ; Madagascar, 51146, ♀ ; SMNH, Uganda, 43339-40, 2 ♀♀ [higher Ac numbers (85-105 pairs, 7-9 per transect)].

Diagnosis: Integument papillae of two types: Main papillae longish, apically rounded, each surrounded by six very flat elevations in top view forming a hexagonal pattern ( Figs 2 A-B). Coxal setation: Cx-I medial setae in a curved line on elevated projections, in anterior part directed ventrally, in posterior part medially (and therefore visible as prominent elevations in ventral view, Fig. 2 C , lower part); groups of 4 (3-5) distal tip setae at Cx-I and Cx-II, a group of densely arranged posterolateral setae at Cx-II. Coxal plate Cx-III+IV with strong,elongated anterior and posterior apodemes parallel to the median axis. Genital plates enlarged, with 45-65 pairs of Ac, maximum number per transversal transect 4-7 (males 4-5) and 30-50 pairs of medial setae ( Fig. 2 D ). Legs with relatively short claws (mean L in males 27-29, in females 29-30, ratio claw L/segment 5 L 8-12 %). Leg segments relatively slender (e.g., L/H I-L-4, 3.1-3.6; I-L-5, 5.2-6.0; II-L-4, 4.3- 4.9; III-L-4, 4.2-5.2; III-L-5, 5.9-7.3; IV-L-4, 5.4-6.7; IV-L-5, 6.8-8.9; swimming setation (anterior/posterior): II-L-5 (0/1); III-L-4 (0/7-13); III-L-5 (0/6-11); IV-L-4 (9-14/8-15); IV-L-5 (0/6-10).

Description:

Both sexes: Colour red, rarely orange. On each side, lateral eye lenses far distanced (> 100 µm), anterior lens round (diameter 40-50), posterior lens oval (length 40-50, width 30-45). Shape of mouthparts, coxae and excretory pore, and setation of palps as typical in most species of the genus, sexual differences very weakly developed (in general, smallest males and females similar in measurements, largest females distinctly larger than largest males, but sexes similar in proportions range); chelicera with little curved claw ( Fig. 2 F ). L ratio P-2/P-4, 0.4, P-4/P-5, 2.1-2.4. No sexual differences in total setae numbers: Cx-I, 20-28; Cx-II, 17-25; CxIII, 15-22; Cx-IV, 19-22; swimming setae II-L-5, 1; III-L-4, 7-13; III-L-5, 6-11; IV-L-4, 19-26; IV-L-5, 7- 10.

Measurements [where informative, mean values in angular parentheses]: Males: Idiosoma L/W 1170- 1230/1000-1100; L/W Cx-I+II, 270-320/160-216, Cx-III+IV, 270-305/280-325; genital plate 189-250/100- 112 [230/104], Ac pairs n 45-65, genital setae pairs n 30-50; ejaculatory complex L/proximal chamber W 190-210/58-63, anterior arms W 160; gnathosoma L/W 210-230/140-150; chelicera L 319-337, basal segment/claw ratio 3.2-4.1 [3.6], L/H ratio 4.3-5.0; palp total L 429-456, L/H ratio (relative L %) P-1, 0.77- 0.8 (10-11); P-2, 1.32-1.38 (17); P-3, 0.83-0.95 (10-11); P-4, 4.13-4.20 (41-43); P-5, 4.05-5.00 (19-20). L/H (ratio in parentheses) I-L-4, 171-195/51-60 (3.3-3.4); I-L-5, 238-270/45-50 (5.2-5.7); II-L-4, 247-300/56-63 (4.4-4.8); II-L-5, 305-350/45-55 (6.4-6.9); III-L-4, 233-285/49-55 (4.6-5.2); III-L-5, 292-330/45-53 (6.0- 7.0); IV-L-4, 314-370/54-60 (5.7-6.2); IV-L-5, 332-380/40-50 (7.5-8.3).

Females: Idiosoma L/W 1000-1600/1000-1400; L/W Cx-I+II, 275-337/170-250, Cx-III+IV, 280-337/290- 364; genital plate 198-265/100-117 [227/109], Ac pairs n 50-70, genital setae pairs n 30-38; egg diameter 120-180; gnathosoma L/W 225-260/139-170; chelicera L 314-409, basal segment/claw ratio 4.1-6.1 [4.7 - claw relatively shorter than in males!], L/H ratio 3.9-5.5; palp total L 431-468, L/H ratio (relative L %) P-1, 0.73-0.92 (10-12); P-2, 1.24-1.43 (17-19); P-3, 0.63-0.91 (9-11); P-4, 3.84-4.33 (42-44); P-5, 3.86-4.50 (18- 20). L/H (ratio in parentheses) I-L-4, 170-235/54-65 (3.2-3.6); I-L-5, 238-320/40-58 (5.5-6.0); II-L-4, 265- 355/54-75 (4.3-4.9); II-L-5, 319-420/47-63 (5.6-7.6); III-L-4, 251-330/54-68 (4.2-4.9); III-L-5, 292-400/40- 58 (6.3-7.3); IV-L-4, 320-435/54-60 (5.4-6.7); IV-L-5, 337-410/38-60 (6.8-8.9).

Discussion: Hydrodroma despiciens was first described by Müller (1776) from Denmark under the genus name Hydrachna and has been often misunderstood since then. The designation and detailed description of a neotype is the precondition to give taxonomic stability to Hydrodroma despiciens . The original description does not provide information on a potential type locality and I decided to select the neotype from the material collected by O. Lundblad in 1919 in the terra typica, Denmark. As typical for the consideration of this species at his time, as an ubiquist ("eurythermal, cosmopolitan"), Lundblad (1920) restricted locality information in his paper on the material in question to "generally present at all investigated sites". No collecting site list is given in Lundblad (1920), and the locality records had to be copied from his handwritten labels.

From the list of studied material above it becomes obvious that K. and K.O.Viets, as well as Lundblad did not take care of Besseling's taxonomic work and confused frequently H. despiciens and H. pilosa, K. Viets misunderstood occasionally also H. torrenticola .

In their revisional work on Australian Hydrodroma, Pešić & Smit (2007a , b, 2011) demonstrated the absence of H. despiciens from this continent and described populations previously attributed to this species as H. cooki Pešić & Smit, 2007 . From a comparison between measurements taken from populations collected in Argentina attributed to H. despiciens by Cook (1980), and the data presented here and elsewhere for European populations (e.g. Wiles 1985, Tuzovskij 2015) results that also the Latin American records require revision: Specimens from Argentina represent probably an undescribed species different from H. despiciens in distinctly shorter palp and leg segments and lower numbers of swimming setae.

In addition to H. capensis , in the covered area three further species are characterized by IV-L-5 lacking anterior swimming setae, namely H. liberiensis , H. reinhardi and H. zhokzovi (see below).

Biology and distribution: On the base of the set of diagnostic character states listed above, H. despiciens is defined as a stenotopic stagnant water species, characterized by leg claws relatively reduced in size (as compared with the rheophilic H. torrenticola ), but clearly differing from H. pilosa with its generally higher swimming setae numbers.

For a long time, H. despiciens was considered a rare example of a water mite species with a nearly cosmopolite distribution, but its actual geographical distribution is unclear. The revision of available material shows that it is widely distributed in Central and Northern Europe, probably more scattered in the Mediterranean area. It is highly probable that it is completely absent from the African continent – as also Walter confused H. despiciens with H. pilosa (see there for data from Algeria), a record from Burkina Faso (Walter 1935) is highly improbable and not documented by collection material. As the attribution to H. despiciens was found erroneous for revised material from North America, as well as for the records published from Argentina, the presence of H. despiciens in the New World is at all questionable. All American populations, partly attributed to H. despiciens , partly described as 5 subspecies of that species, require taxonomic revision. The same is true for all records from Asia – Walter (1929) and later Wiles (1986) defined H. monticola Piersig, 1896 , first published from Indonesia as a subspecies of H. despiciens , as a separate species, and no record of H. despiciens is ascertained from this continent.

Figures

Figure 2. A-D: Hydrodroma despiciens, neotypes, Denmark. A. male, integument papillae near lateral eye in tangential view; B. male, integument in top view; C. male, anterior coxal plate with medial setal tubercles on Cx-I; D. female, genital field; E. female, integument in tangential view; F. male, chelicera.