Hydrodroma capensis (K. Viets, 1914 )

Gerecke, Reinhard, 2017, The water mites of the genus Hydrodroma (Acari, Hydrachnidia, Hydrodromidae) in Europe and Africa, Ecologica Montenegrina 13, pp. 1-24 : 11-14

Hydrodroma capensis (K. Viets, 1914)

( Figs 5 A-E)

Type series: Lectotype, here designated: ♀, SMF, South Africa, Coll. K. Viets, 43904, " Hydrodroma despiciens capensis (Viets) TYPE"; " Kapland , Vley b. Lakeside, 28.7.1903. D. Südpolar-Expedit. 1687". Status: Idiosoma undissected, partly damaged and folded, containing several eggs; legs in situ; gnathosoma separate, palps and chelicerae detached, one palp lateral, the other lacking, one chelicera lateral, one visible dorsoventrally . Paralectotype: ♀, SMF, 43903, label as for lectotype, "1686". Status: Idiosoma dissected (dorsal integument separated), no eggs conserved; left I-L-2-6 and IV-L-4-6 detached, other legs in situ; gnathosoma separate, gnathosomal base destroyed, palps and chelicerae detached, under a separate cover slide, in partly dried mounting medium with precipitated droplets .

Material examined: SMF, coll. Viets, S Africa 43882, Cape Town, Firgrove, 8.1.57, 2 ♂♂ ; 43883, Firgrove 1. River Estuary , 13.1.57 Harrison 8034, 1 ♂, 1 ♀ .

Rejected identifications (all labelled " Hydrodroma capensis "):

(A) populations from Congo similar in integument papillosity and II-L-5 setation, but distinctly major in size and with higher Ac number: NHMB, Rwanda, 28.11.1985 Coll. Kongo 8, 1330-1336, 1338, 6 ♂♂, 2 ♀♀ [1337: Limnesia sp. 1 ♀ with wrong label " Hydrodroma capensis ", same site and date; SMF, Congo , 51154-55, Lake Kivu, 27.2.1972, 1 ♂, 1 ♀; SMNH, coll. Lundblad, Congo, 43337-38, Voitshumbi, 2 ♂♂; 44002-03, Congo, "315", 1 ♂, 2 ♀♀ Madagascar, 51141, 1 ♂.

(B) populations differing also in integument structure and II-L-5 setation: SMF, coll. Viets, Madagascar 51126, 51140-41, 1 ♂, 2 ♀♀ ; S Africa , 51142-45, 2 ♂♂, 2 ♀♀ ; Central Sahara , 43876, 1 ♂ ; Zimbabwe, 51153, 1 ♀ ; SMNH, coll. Lundblad, Congo, 44003, Congo, 3 ♂♂ ; 44000-01, 44003-04, 9 ♀♀; Madagascar, 43329, 1 ♀ .

Diagnosis: Integument papillae uniform, longish, apically rounded, slightly curved in posterior direction, at light microscope no surrounding elevations visible ( Figs 5 C-D). Coxal setation: Cx-I medial setae on only slightly elevated projections, in a weakly curved line, in anterior part directed ventrally, in posterior part medially (and thus visible as obtuse angled projections in ventral view); groups of 3-4 distal tip setae at Cx-I and Cx-II, a group of densely arranged posterolateral setae at Cx-II ( Fig. 5 A ). Coxal plate Cx-III+IV with strong, elongated anterior and posterior apodemes. Genital plates slightly enlarged, with 40-60 pairs of Ac, maximum number per transversal transect 5-6 and 25-30 pairs of medial setae. Legs with relatively short claws (mean L in males 25-35, in females 30-33, ratio claw L/segment 5 different in legs: I-L: 13-15 %, II-IV-L: 9-12 %); segments relatively stout (e.g., L/H I-L-4, 2.8-3.2; I-L-5, 4.2-4.6; II-L-4, 3.7-4.4; III-L-4, 3.8- 4.3; III-L-5, 5.2-5.7; IV-L-4, 4.8-6.2; IV-L-5, 6.6-7.7); swimming setation (anterior/posterior): II-L-5 (0/2); III-L-4 (0/9-13); III-L-5 (0/8-10); IV-L-4 (9-11/7-13); IV-L-5 (0/7-8). Chelicera with curved claw ( Fig. 5 E ).

Description (based on specimens from South Africa):

Both sexes: Colour undescribed. Lateral eyes far distanced (90-140 µm), anterior lens rounded (diameter 40- 50), posterior lens oval (length 40-50, width 30-45). L ratio P-2/P-4, 0.3-0.4, P-4/P-5, 1.9-2.7. Shape of mouthparts, coxae and excretory pore, and setation of palps as typical in all species of the genus, sexual differences very weakly developed. No sexual differences in total setae numbers: Cx-I, 14-21; Cx-II, 17-21; Cx-III, 13-19; Cx-IV, 16-22; swimming setae II-L-5, 2; III-L-4, 9-12; III-L-5, 7-10; IV-L-4, 16-22; IV-L-5, 7-8.

Measurements: Males: Idiosoma L/W 1000-1500/800-1000; L/W Cx-I+II, 235-260/150-155, Cx-III+IV, 250-260/240-270; genital plate 190-200/75, Ac pairs n 38-50, genital setae pairs n 30; ejaculatory complex: Fig. 5 D, L /proximal chamber W 180-200/55; gnathosoma L 180-185; chelicera L 285-290, basal segment/claw ratio 3.5-3.8, L/H ratio 4.8; palp total L 416-420, L/H ratio (relative L %) P-1, 0.73-0.83 (11); P-2, 1.17 (16-17); P-3, 0.80-0.86 (11); P-4, 3.9-4.0 (42-43); P-5, 3.9-4.0 (16-18). L/H (ratio in parentheses) I-L-4, 155-160/55 (2.8-2.9); I-L-5, 195-200/45-48 (4.2-4.3); II-L-4, 223-230/60 (3.7-3.8); II-L-5, 263-270/48- 50 (5.3-5.7); III-L-4, 210-215/55 (3.8-3.9); III-L-5, 253-260/45-50 (5.2-5.6); IV-L-4, 265-335/55 (4.8-6.1); IV-L-5, 295-345/45 (6.6-7.7).

Females: Idiosoma L/W 1100-1500/900-1200; L/W Cx-I+II, 260/170, Cx-III+IV, 270-280/260-280; genital plate 190-240/80-100, Ac pairs n 50-60, genital setae pairs n 24-30; egg diameter 85-140; gnathosoma L 220-230; chelicera L 320-330, basal segment/claw ratio 4.1-4.3, L/H ratio 5.1-5.3; palp total L 440-445, L/H ratio (relative L %) P-1, 0.71-0.85 (11-13); P-2, 1.15-1.25 (17); P-3, 0.82-1.00 (11-14); P-4, 3.55-3.58 (39- 44); P-5, 3.56-4.11 (18-21). L/H (ratio in parentheses) I-L-4, 185-190/63-65 (2.9-3.0); I-L-5, 225-230/50 (4.5-4.6); II-L-4, 265-280/60-65 (4.1-4.4); II-L-5, 315-320/53-55 (5.7-6.1); III-L-4, 255-260/60-65 (4.0-4.3); III-L-5, 305-315/55-58 (5.4-5.7); IV-L-4, 340-350/55-63 (5.6-6.2); IV-L-5, 350-360/48-50 (7.0-7.4).

Discussion: The significance of K.Viets' comparison between H. capensis and H. despiciens (in the following set in quotation marks) is devaluated by the fact that the author did not differentiate between H. despiciens and H. pilosa . In fact, most of his considerations suggest that he compared character states of H. capensis and H. pilosa (for the latter species, see below).

(1) papillae "less elevated, less pointed-conical, more dense": Integument papillae of H. capensis and H. despiciens are similar in shape, long and apically rounded, in contrast to the pointed papillae of H. pilosa . As a difference to both other species, papillae of H. capensis are in fact more densely arranged, H. capensis differs from H. despiciens in the absence of small, hexagonally arranged secondary papillae.

(2) "gnathosomal rostrum shorter and stouter, oral disk relatively larger": It is doubtful if this character is generally suitable for species discrimination in the genus. The investigated specimens differ from typical H. pilosa in fact as described by K. Viets, but, in contrast, in the investigated H. despiciens the gnathosomal rostrum is still more stouter and the mouth disk relatively larger. From this point of view H. capensis has an intermediate position between H. despiciens and H. pilosa , probably with an overlap with both species in intraspecific variability.

(3) "cheliceral claw more strongly curved, proximal end of the basal segment more straight": The first observation is confirmed by the investigated material (compare Figs 2 F and 5 E ) – both, H. despiciens and H. pilosa have a straighter cheliceral claw; with regard to the shape of the proximal end of the basal segment, considerable variability is observed in the compared species – this part of the segment is possibly more turned up in juvenile adults.

(4) "palp segments stouter": If measurements are compared with H. despiciens , this statement finds little support, with differences in sexes (L/H P- 2 male <1.2, H. despiciens > 1.3, female no differences; L/H P- 4 female <3.6, H. despiciens > 3.8, male no differences). My measurements suggest much intraspecific variability in proportions of palp segments.

(5) "medial margin of Cx-I only slightly characterized by elevated setal bases": In this regard, H. capensis differs strongly from H. pilosa , a species with very distinct tubercles at Cx-I medial margin (see Fig. 6 A ), but the difference in comparison to H. despiciens is less distinct. The latter is intermediate between H. pilosa and H. capensis from this point of view.

(6) "Legs not in the same way provided with long and slender setae as Müller's species": This statement suggests in particular that Viets compared H. capensis with H. pilosa . Swimming setae numbers are clearly higher in H. pilosa , while in H. capensis and H. despiciens (in parentheses) ranges of total numbers are widely identical: III-L-4, 9-12 (7-13); III-L-5, 6-10 (7-11); IV-L-4, 16-24 (19-26); IV-L-5, 7-9 (7-10). The crucial difference between the two species concerns the setation of II-L-5: Hydrodroma capensis differs from H. despiciens in the presence of an additional, proximal swimming seta on the posterior surface, about 30 µm away from the distal seta.

(7) A rather strange detail is the small idiosoma size mentioned by K. Viets (L/W 870/770). Unfortunately he did not give account of the material on which his species description was based. As the two female syntypes available are distinctly larger in size, the existence of a further, now lost, smaller specimen on which the original description was based, is necessary. As in this study the idiosoma of all specimens of H. despiciens , as well as most H. capensis , is longer than 1000 µm, idiosoma size is surely not a character suitable for species discrimination in these taxonomic surroundings.

In summary, the combination of diagnostic features most suitable for recognizing Hydrodroma capensis is as follows: (1) Integument papillae of one type only, long, apically rounded; (2) cheliceral claws curved ( Fig. 5 E ); (3) II-L-5 with two swimming setae on posterior surface. The following additional character states could be helpful for recognizing the species: (4) Leg segments relatively stout (e.g., L/H I-L-4, 2.8-3.2; III-L-4, 3.7-4.4; III-L-4, 3.8-4.5; IV-L-4, 4.8-6.2); (5) Leg claws generally short (L 25-30 µm), but I-L claws (L ratio claw/I-L-5, 14-15 %) relatively longer than II-IV-L claws (9-12 %).

In measurements and proportions, the specimens from the surroundings of Cape Town (SMF 43882- 83) agree well with the original description and data taken from the syntypes. In contrast, specimens from Congo and Madagascar which agree in simple idiosoma papillosity and II-L-5 with two swimming setae (see above, "rejected identifications (1)"), differ in a generally larger size (e.g. measurements of coxae, genital field and gnathosoma), more slender leg segments, and higher numbers of acetabula and IV-L-4 swimming setae. They are similar to Hydrodroma zhokhovi Tuzovskij, 2014 , described from Lake Tana in Ethiopia. For a definitive attribution, further investigations on the variability of these populations and of H. zhokhovi are necessary. Further specimens published under the name of H. capensis (see above, "rejected determinations (B)") do not show the character combination typical for H. capensis and represent other, probably undescribed species of the genus.

Distribution: For the moment ascertained only from S Africa, its presence in Madagascar is not documented by collection material.

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.

Figure 5. A-E. Hydrodroma capensis, male, SMF 43882. A. anterior coxal plate; B. integument papillae in tangential view; C. integument papillae in top view (magnification as in B.); D. ejaculatory complex, anterior view; E. chelicera; F-G; H. trigonometrica; F. Male from Algeria, integument tangential; G. female holotype, genital field.

Figure 6. Hydrodroma pilosa, Germany, Mindelsee. A. male, anterior coxal plate; B. male genital field; C. male, ejaculatory complex lateral; D. Integument in tangential view; E. integument in top view.