Hydrodroma megalonyx, Gerecke, 2020

Gerecke, Reinhard, 2020, The early derivative water mites (Acari: Hydrachnidia, superfamilies Eylaoidea, Hydrachnoidea and Hydryphantoidea) of Madagascar, Zootaxa 4727 (1), pp. 1-77 : 18-20

Hydrodroma megalonyx sp. nov.

Figs 9 f – k , 10

Type series: Holotype ♀, MNHN Ac 1258, MD 120a, 21.10.2001 Maromandia ( Majunga ), R Andranamalaza , 70 m, on wood, slide mounted . Paratypes: Same site and date as holotype, (1/0/0) slide mounted , Ac 1259; (1/0/0) in liquid , Ac 1423; MD 026, 11./ 13.08.2001 Ionilahy (Fianarantsoa) , River Ionilahy , 200 m, on sand bottom (1/0/0) , Ac 1261, on gravel (0/1/0) , Ac 1260, both slide mounted; MD 60a, 05.09.2001 Tsimelahy (Tulear) , R Antarantsa downstr. ‘piscine naturelle’, 200 m (1/0/0) slide mounted , Ac 1262; MD 69, 09.09.2001 Andohahela (Tulear) , Isaka, W stream at the S National Park border (W RIP 118), 200 m (0/1/0) slide mounted , Ac 1263; MD 070, 09.09.2001 Andohahela (Tulear) , Isaka, E stream at the S National Park border (W RIP 118), 200 m (1/0/0) slide mounted , Ac 1264; MD 074a, 11.09.2001 Andohahela (Tulear) , Isaka, stream crossing RIP 118 at km 32, 360 m, riffle sector (2/0/0) slide mounted, Ac 1265 – 1266

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Material examined for comparison: Hydrodroma reinhardi Pešić, 2002 , paratypes 1 – 4 in coll. Gerecke ( Montenegro, 2/2/0).

Diagnosis: Integumental papillae rounded or slightly extended, in some cases surrounded by a granulate area of additional small elevations ( Fig. 9 h – k ). Medial margin Cx-I smooth or with very inconspicuous projections near setae bases ( Fig. 9 f – g ). Cx-IV with a relatively long posteromedial apodeme, directed medially ( Fig. 10 b ). Swimming setae numbers (anterior/posterior) II-L-5, 0/0 – 1; III-L-4, 0/5 – 7; III-L-5, 0/4 – 8; IV-L-4, 3 – 7/5 – 8; IV-L-5, 0 [rarely 1 – 2–see remarks]/4 – 6. Leg claws strongly enlarged–length (all legs) 40 – 75 – 18 – 31 % penultimate segment. Leg segments stout (e.g., L/H ratio I-L-5, 3.3 – 3.9; II-L-5, 4.0 – 4.6; III-L-5, 4.0 – 5.1; IV-L-5, 4.7 – 5.6–see Fig. 10 d–e ).

Description: Colour deep red. Integumental papillae as long as wide, closely arranged and adpressed to each other (in top view producing a honey-comb-like pattern) or (probably in juveniles) surrounded by narrow, granulated interspaces. Lateral eyes lens diameter anterior 40 – 60, posterior 30 – 50, separation 40 – 80 ( Fig. 10 a ). Cx-I (anterolateral) and Cx-II-/III (both antero- and posterolateral) with groups of stronger, densely-arranged setae; finer setae along anterior and posterior margins of Cx-I (mostly in a single line), posterior margin of Cx-II (medially 1 – 3 lines), and posterior margin of Cx-III and –IV (each with 10 – 15 setae in 1 – 3 lines).

Measurements: Males: Idiosoma L/W 725 – 1075/625 – 875; Cx-I+II, 210 – 300/140 – 200; Cx-III+IV, 230 – 300/200 – 280; genital plate 180 – 215/75 – 110, genital field with 40 – 70 pairs of acetabula (4 – 7 per transect) and 40 – 75 pairs of medial setae. Distal leg segments, given as L/H (ratio): I-L-4, 135 – 180/55 – 70 (2.3 – 2.8); I-L-5, 170 – 225/48 – 58 (3.4 – 3.9); II-L-4, 185 – 225/55 – 68 (3.1 – 3.4); II-L-5, 220 – 265/50 – 60 (4.0 – 4.6); III-L-4, 180 – 233/55 – 65 (3.1 – 3.6); III-L-5, 220 – 270/48 – 58 (4.0 – 5.1); IV-L-4, 235 – 285/55 – 68 (3.9 – 4.5); IV-L-5, 260 – 325/50 – 60 (5.2 – 5.6). Mouthparts: Gnathosoma L 175 – 190; chelicera L 248 – 298, L/H 4.5 – 5.3, basal segment/claw L ratio 3.5 – 4.7; palp total L 303 – 364; L/H (ratio) P-1, 33 – 40/50 – 60 (0.5 – 0.8); P-2, 60 – 66/48 – 53 (1.2 – 1.4); P-3, 33 – 38/45 – 53 (0.6 – 0.8); P-4, 133 – 148/35 – 43 (3.2 – 4.2); P-5, 43 – 60/15 – 18 (2.5 – 3.8); L ratio P-2/P-4, 0.4 – 0.5, P-4/P-5, 2.4 – 2.2.

Females: Idiosoma L/W 825 – 1125/675 – 1000; Cx-I+II, 240 – 310/150 – 210; Cx-III+IV, 240 – 320/220 – 310; genital plate 180 – 220/90 – 95, genital field with 60 – 80 pairs of acetabula (6 – 8 per transect) and 25 – 28 pairs of medial setae. Distal leg segments, given as L/H (ratio): I-L-4, 148 – 205/63 – 75 (2.4 – 2.7); I-L-5, 180 – 245/55 – 65 (3.3 – 3.8); II-L-4, 190 – 240/65 – 75 (2.9 – 3.2); II-L-5, 225 – 283/55 – 65 (4.1 – 4.6); III-L-4, 185 – 245/60 – 73 (3.1 – 3.4); III-L-5, 230 – 310/53 – 63 (4.4 – 5.0); IV-L-4, 245 – 315/63 – 73 (3.9 – 4.3); IV-L-5, 270 – 350/55 – 65 (4.7 – 5.4). Mouthparts: Gnathosoma L 200 – 225; chelicera L 268 – 303, L/H 4.5 – 4.7, basal segment/claw L ratio 3.7 – 4.0; palp: Fig. 10 c , total L 350 – 390; L/H (ratio) P-1, 43 – 50/53 – 63 (0.8); P-2, 68 – 75/53 – 55 (1.3 – 1.4); P-3, 26 – 45/45 – 58 (0.6 – 0.9); P-4, 148 – 163/38 – 44 (3.4 – 4.1); P-5, 43 – 63/16 – 18 (2.6 – 3.9); L ratio P-2/P-4, 0.4 – 0.5, P-4/P-5, 2.5 – 3.5.

Derivatio nominis: megalos (Greek: μεγάλος), big, and onyx (Greek: ὄνυξ), claw.

Remarks: Hydrodroma megalonyx is unique in the combination of thickened leg segments bearing rather large claws (L 18 – 31 % of penultimate segment), with an elevated number of swimming setae (total nr 22 – 37, 4 – 8 on posterior surfaces of III/IV-L-4 – 5). In the relatively large claw size, H. megalonyx is similar to H. perreptans (see above) and H. reinhardi Pešić, 2001 , a species reported from S and E Europe. In H. perreptans , enlargement of claws is less distinct (claw L in general 13 – 19, rarely up to 23 % penultimate segment). This species differs furthermore in the longer integument papillae, more slender leg segments (e.g., L/H III-L-4,> 4.0, IV-L-4,> 4.2, IV-L-5,> 6.5), the typical extremely reduced swimming setation, smaller lateral eye lenses (maximum diameter anterior 35 – 45, posterior 20 – 30), and often a stouter chelicera (L/H 4.6 – 6.1). In H. reinhardi , claws are similar in relative size (20 – 30 % L penultimate segment) and leg segments are generally stouter (e.g., L/H I-L-5 2.9 – 3.3, II-L-5, 3.3 – 3.8, III-L-5, 3.6 – 4.0, IV-L-5, 4.6 – 5.2). Important differences of H. reinhardi are found in the integument (two types of papillae, the larger central ones with blunt tips) and leg setation (lower total number of swimming setae, 15 – 20, no segment with more than 5 setae per surface).

A further species with rather large claws (about 20 % penultimate segment L) described from India, H. tonapii Cook, 1967 , differs from both H. reinhardi and H. megalonyx in having more slender leg segments (L/H I-L-5> 4.0, IV-L-5> 6.0, calculated from figures in the original description).

From the typical standing water species treated in the following, H. megalonyx differs not only in rather stout leg segments (proportions not found in any other Madagascan species), but also in lower swimming setae numbers (never exceeding 15 per segment).

Two males from site MD 74a differ from the remaining specimens by the presence of 1 – 2 anterior setae on IV- L-5. As both specimens agree from all other viewpoints with H. megalonyx , and a typical specimen of the latter was collected at a site nearby, I attribute the phenomenon to individual variability.

Habitat: Probably a rhithrobiont with a preference for lenitic sectors (most records come from pool areas with fine sediment or organic material).

Distribution: Madagascar. In forested areas of the south and northwest, at lower altitudes (70 – 360 m asl.).

Figures

FIGURE 9. Family Hydrodromidae, genus Hydrodroma. a.–e. H. perreptans (MD 074a); a.–b. female and male Cx-I+II; c.–e. integument papillae in various views–note in e. the underlying hexagonal structure visible upper left; f.–k. H. megalonyx (MD 120a); f.–g. male and female Cx-I+II; h.–k. integument papillae in various views.

FIGURE 10. Family Hydrodromidae, genus Hydrodroma, H. megalonyx, female (MD 120a). a. dorsal idiosoma; b. coxal and genital field; c. palp; d. III-L-4–6 posterior; e. IV-L-4–6 posterior.