Hydryphantes dispar ( Schaub, 1888 )

Gerecke, Reinhard, 2024, Water mites collected from oxbow lakes in the Prater area (Vienna) in 1932 / 1933, with the redescription of Hydryphantes dispar (v. Schaub, 1988), Ecologica Montenegrina 74, pp. 27-38 : 30-38

Hydryphantes dispar ( Schaub, 1888) : a redescription

Material examined.

Neotype ♂, here designated. Austria. Vienna, Prater, Lusthausweiher, Litoral, 27.05.193 3 Vornatscher coll., undissected in Koenike's fluid (Natural History Museum Vienna).

Further material: Same site and collector. Undissected, in Koenike's fluid: 14.09.193 3 1 ♀; 19.07.193 3, 5 deutonymphs; 24.04.193 3, 3 ♂♂, 5 ♀♀. Dissected, slide-mounted in Hoyer's liquid: 08.05.193 3, 1 ♀; 26.06.193 3, 1 ♀; unclear habitat, 05.06.193 3, 2 ♂♂; unclear habitat, year and date, 1 ♂, 1 ♀, 1 deutonymph. Dissected, slide mounted: Russia. Samara province, Stavropol district, near village Mordowo, temporary reservoirs, Tuzovskij coll. 17.7.199 7, 1 ♂, 1 ♀ (nr 7502); 11.9.199 8, 1 ♀ (nr 7697) .

Description

(For detailed measurements, see Tables 2 and 3)

Both sexes: With the character combination typical for hydryphantine mites. Colour in preserved specimens no more visible - surely red as in all species of the genus, and described by Schaub for H. dispar . Integument densely covered by fine, apically flattened papillae (in some of the studied specimens only locally visible, membranous parts covered by fine precipitations). Glandularia fine and weakly sclerotized, dorsal and ventrocaudal muscle attachments unsclerotized. Frontal area: Fig. 1 . Frontal shield with prominent anterolateral projections and a pair of long posterior extensions embracing a deep posteromedial bay, lateral margins deeply concave. Venter: Fig. 2 . Coxal plates with uniformly long and narrow setae, those of distal edges a bit longer than the ones at posterolateral plate borders. Medial apodemes of Cx-I+II separated from each other by membranous interspace. Medial margin of Cx-III+IV mostly formed by Cx-III. Leg setation (see Fig. 3 for I-IV-L-4-6, swimming setae here given as truncated) consisting of four different, rather homogenous types: (1) Stronger, pointed setae various in length, situated on segments 1-5, here scattered over dorsal and ventral margins, more densely arranged at distal margins (ventrally on IV-L-4-5 differentiated in parallel rows of longer and shorter setae); (2) fine, relatively short hair-like setae, arranged all over the surface of segments 6, more densely on I-L- III-6 than on IV-L-6; (3) short and stout peg-like setae, restricted to the distoventral parts of segments 6; (4) long, hair-like swimming setae, in length corresponding to leg segment length. In the material studied, setae of this type are often lost, but their insertion points remain clearly visible. Swimming setae are found on segments II-L-4-5; III-L-3-5, and IV-L-3-5, in higher numbers (summarized number of all legs in brackets) in females (77-96) than in males (65-70). Genital field with minute pre- and postgenitalia; Ac-2 tiny, Ac-3 larger than Ac-1; setation of genital flaps restricted to medial part. Excretory pore longish oval, with a sclerotized knob in anterior part.

Gnathosoma with large mouth opening, ventral margin broadly convex in proximal 2/3, concave in distal 1/3. Chelicera with rather short claw; basal segment/claw L ratio 2.33-2.88; L/H ratio 3.64- 4.08. Palp stout, setation as given in Fig. 4 C-D; L/H proportions of segments: P-1, 0.73-0.88; P-2, 1.09- 1.29; P-3, 0.75-0.86; P-4, 2.38-2.86; P-5, 1.88-2.00; L ratio P-2/P-3, 1.63-1.73; P-2/P-4, 0.76-0.81.

Males: Idiosoma L/W 1350-1450/1050-1230. Frontal shield proportions: medial L/total L 0.59- 0.65; anterior W/central W 1.63 -1,73. Swimming setation: II-L-4, 2-6; II-L-5, 8-10; III-L-3, 1-4; III-L- 4, 10-11; III-L-5, 12-15; IV-L-3, 2-5; IV-L-4, 13-14; IV-L-5, 9-15.

Females: Idiosoma L/W 1700-2130/1230-1450, in general larger in dimensions than males (see Tables 2-3). Frontal shield less compact than in males, proportions: medial L/total L 0.47-0.50; anterior W/central W 1.80 -1.90. Swimming setation: II-L-4, 1-4; II-L-5, 11-16; III-L-3, 2-3; III-L-4, 10-13; III- L-5, 11-17; IV-L-3, 5-8; IV-L-4, 15-20; IV-L-5, 12-23.

Table 2

Ac-1 45 38 36 45 54 49 Ac-2 36 31 29 45 36 40 Ac-3 56 49 54 67 63 67 Frontal shield mL 229 229 166 180

Frontal shield lL 386 355 350 359

Frontal shield mL/lL 0,59 0,65 0,47 0,50

Frontal shield W1 427 373 413 427

Frontal shield W2 247 229 229 225

Frontal shield W1/ W2 1,73 1,63 1,80 1,90

Frontal shield W3 323 301 314 314

Frontal shield posterior bay L 189 148 180 189

gnathosoma L 278 247 229 292 274 328 chelicera basal segment L 265 251 310 310

chelicera claw L 99 108 108 130

chelicera basal segment H 99 99 108 108

chelicera total L 364 359 418 440

chelicera basal segment/claw 2,68 2,33 2,88 2,38

chelicera L/H 3,68 3,64 3,88 4,08

P-1 L 63 61 72 67

P-1 H 85 76 99 76

P-1 L/H 0,74 0,79 0,73 0,88

P-2 L 117 112 139 112

P-2 H 99 90 108 103

P-2 L/H 1,18 1,25 1,29 1,09

P-3 L 67 67 85 76

P-3 H 90 81 99 94

P-3 L/H 0,75 0,83 0,86 0,81

P-4 L 153 139 180

P-4 H 58 58 63

P-4 L/H 2,62 2,38 2,86

P-5 L 34 38 36

P-5 H 18 20 18

P-5 L/H 1,88 1,89 2,00

P-2/-3 L 1,73 1,67 1,63

P-2/-4 L 0,76 0,81 0,78

Palp total L 433 418 512

Remarks

With the original description by Robert Ritter von Schaub in 1888 (under the genus name Hydrodroma ), Hydryphantes dispar became immediately one of the best known water mites species ever. The author particularly focused on its anatomy, presenting a documentation of all inner organs to the best level possible at his time, but did not give much care to taxonomical rules. The introduction of the species name is found in a subordinate clause only at the end of the introduction of the paper, indicating the Prater in Vienna as the type locality but without designating any type material.

Since then, the species was not much subject to discussion – by the combination of a rather stout palp and the frontal shield with slender, long posterolateral extensions, Hydryphantes dispar appeared an easily identifiable species. This is reflected also by the fact that no junior synonyms of it were introduced – only two subspecies were published, H. d. mucronatus K. Viets, 1910 ( Germany) and H. d. muzzanensis Walter, 1922 ( Switzerland). Lundblad (1962) proposed to synonymize both with the stem species.

Table 3 IV-L-1 1,3 1,4 1,3 1,7 IV-L-2 L 180 157 193 193 IV-L-2 H 112 94 117 112 IV-L-2 1,6 1,7 1,7 1,7 IV-L-3 L 265 247 292 287 IV-L-3 H 94 85 94 94 IV-L-3 2,8 2,9 3,1 3,0 IV-L-4 L 382 350 418 422 IV-L-4 H 90 76 90 90 IV-L-4 4,3 4,6 4,7 4,7 IV-L-5 L 359 350 409 404 IV-L-5 H 72 67 76 81 IV-L-5 5,0 5,2 5,4 5,0 IV-L-6 L 310 283 323 328 IV-L-6 H 49 49 52 63 IV-L-6 6,3 5,7 6,3 5,2

The reason to better define the species and to designate a neotype arose from recent studies done in Norway (Stur et al., unpubl. molecular data) and in The Netherlands (Boonstra pers. comm., morphological and ecological data), showing that at least two clades are hidden behind the “true” H. dispar .

Larvae of Hydryphantes dispar were reared and described for the first time by Sparing (1959). She studied a population from a pond near Erlangen (Bavaria) and reported parasitism of the larvae on adults of Hydrellia griseola Fallén, 1913 ( Diptera , Ephydridae ), but did not refer to the morphology of the adults of the population in question. No material from this investigation is available, and it remains unclear if the investigated individuals really represent H. dispar sensu stricto.

To the senior master of research on water mite larvae in Europe, Petr V. Tuzovskij (1942-2023), we owe a by far more detailed description of larval Hydryphantes dispar ( Tuzovskij 2014) , based on a population from Koltsovo (Stavropol district, Samara province, Russia). He not only gave a diagnosis of this stage, but included also important information about the morphology of the female from which the larvae were reared. In a subsequent paper (Tuzovskij 2017), he elaborated diagnostic features and a determination key for the larvae of altogether 14 Hydryphantes species recorded from Russia. From Tuzovskij's measurements of 5 females from Koltsovo results that the Russian specimens tend to be slightly larger than the specimens from the type locality (data are within the variability range, or exceed slightly the maxima, given here). A data not included in Tuzovskij's description is the taxonomically important number of swimming setae. It is a fortunate circumstance that a mite collection donated to me by Tuzovskij includes three specimens from a locality nearby his study site in Stavropol district. The number of swimming setae of these specimens fits the variability range found in the population from the type locality. This observation gives strong support to the hypothesis that Tuzovskij's description of the larval stage does refer to H. dispar sensu stricto.

Thon (1899) stated to have found a nymphal stage of H. dispar fixed with its mouthparts in the mantle cavity of the snail Viviparus contectus (Millet, 1813) (sub nom. Paludina contecta ). This singular observation, if at all correct, is not congruent with the life cycle typical for the species – Sparing (1959) showed that deutonymphs develop as free-living predators, as in all other Hydryphantes species studied under this aspect.

A water mite larva parasitzing a deutonymph of another water mite species

Obviously, water mites do not display an extreme host specificity. As far as documented, larvae of a given water mite species do often display a host preference for representatives of a selected insect order or family, but with a considerable plasticity over time and space ( Martin 2008). Occasionally the host range may be enlarged to various orders as in the early derivative Hydrovolzia placophora (Monti, 1910) (Hemiptera, Coleoptera, Diptera ); also the only known case of water mite larvae parasitic on nonhexapod invertebrates is reported for this species (Acari, Trombidiformes - see Tuzovskij et al. 2001). The material presented here includes a larval mite (probably of Limnesia fulgida ) firmly attached to the membranous pleura of a deutonymph of Hydryphantes dispar , adjacent to the lateral end of the suture Cx-III/-IV ( Fig. 5 ). Such a special case probably results from an exceptional situation during the mite larva's search for a host. In both cases - the Hydrovolzia placophora larva on a terrestrial trombidiform mite, and the cf. Limnesia larva attached to the deutonymph of a water mite - no engorgement could be observed, and thus no real parasitism is testified. The observation merits attention as a curiosity on the background of understanding water mite host search behaviour.

Acknowledgements

The faunistic part of this study was conducted in 2015 as a part of a Synthesys project at Naturhistorisches Museum Wien (with kind support of the responsible curator: Christoph Hörweg). The impulse for extending the topic and completing this paper originated from new findings on the systematics of Hydryphantes dispar within the framework of the project “Artsprosjekt_17- 22_Calcareous mites and midges” at NTNU University Museum Trondheim, Norway. The discovery of the Vornatscher material was made possible by the Museum of Natural History Basel (with kind support of the then responsible curator Ambros Hänggi). A comparison with Eastern European material was possible due to a gift of Russian specimens of Hydryphantes dispar by Petr Tuzovskij (1942-2023). Christoph Allgaier helped with producing a stacking version of Fig. 5 . Based on a former draft, Harry Smit, Andrzej Zawal and Rink Wiggers gave useful proposals for improving this paper.

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Figures

Figure 1. Hydryphantes dispar, ♂ from Prater in Vienna, Vornatscher coll. 1933, frontal shield. Bar = 100 µm.

Figure 2. Hydryphantes dispar, ♂ from Prater in Vienna, Vornatscher coll. 1933, coxal and genital field. Bar = 100 µm.

Figure 3. Hydryphantes dispar, ♂ from Prater in Vienna, Vornatscher coll. 1933, distal leg segments. A. I-L-4-6; B. II-L-4-6; C. III-L-4-6; IV-L-4-6. Swimming setae are given as short truncs cut near base. Bar = 100 µm.

Figure 4. Hydryphantes dispar, ♂ from Prater in Vienna, Vornatscher coll. 1933, mouth parts. A. gnathosoma lateral; B. chelicera; C. palp medial; D. palp lateral. Bar = 100 µm.

Figure 5. Hydryphantes dispar, deutonymph from Prater in Vienna, with parasitic water mite larva attached to the pleural surface.