Porohalacarus alpinus

Porohalacarus alpinus (Thor, 1910) Halacarus alpinus Thor, 1910. 348–351, figs 1 and 2. Porohalacarus alpinus, Thor 1922: 110, 111; Bartsch 1973: 117–119, figs 1–21, 2007: 80–82, fig. 3, 2011: 497–498, fig. 6A–D; Green & MacQuitty 1987: 164, fig. 69A–D. Porohalacarus alpinus alpinus Viets 1927: 465–469, figs 3–8. Porohalacarus alpinus alpinus, Bartsch 1987: 85, 2006: 130–132, figs 5-9a–f. Porohalacarus alpinus brachypeltatus Viets, 1927: 469, figs 9, 10. Porohalacarus alpinus brachypeltatus, Bartsch 1987: 85–86, figs 2 and 3, 2006:132, 5-10a, b. Collecting data. Southern Madagascar, Tulear, Tsimelahy, River Antarantsa, ca 1 km upstream from village, 300 m, 20.4°C, 0.171 mS/cm, interstitial, 04 Sep. 2001 (MD 058). – Northern Madagascar, Antsiranana, Sambava, Maroambihy, left affluent of River Lokoho upstream from the village, 90 m, 26.0°C, 0.010 mS/cm, interstitial, 12 Nov. 2001 (MD149). Short description (Fig. 6A–G). Female: Length 280– 318 µm [10]. Dorsal plates reticulate, in anterior part of PD reticulation honey comb-like, in posterior part presenting a longitudinal pattern. OC with cornea. AD and OC with spots of black eye pigment (Fig. 6A), that on AD narrow, 13 µm wide, spot on OC 15–18 µm in diameter. Dorsal setae very small, their position as illustrated in Fig. 6A, ds-6 on PD removed from posterior margin. Surface of ventral plates finely porose. Genital plate with (four to) five pairs of pgs, each genital sclerite with (four to) five external acetabula. Interval between anterior margin of GP and that of GO somewhat longer than the latter’s length (Fig. 6B). Gnathosoma hardly longer than wide (Fig. 6C). Rostrum shorter than gnathosomal base and hardly extending to middle of P-2. Palps four-segmented, lateral to rostrum, i.e., distance between pair of P-1 more than their width. P-2 with two dorsal setae (Fig. 4D), P-3 with medial spur. Chelicera with claw and wide, flap-like dorsal process. Legs slender, shorter than idiosoma, length of legs I and IV 0.7 times that of idiosoma. Genu I much shorter than telofemur I (Fig. 6E). Tibiae I to IV with 4, 3, 2, 2 ventral setae, in addition tibiae III and IV with short, faintly pectinate mid-segmental medial seta. Tarsus I with single slender ventral seta (Fig. 6E), following tarsi without ventral setae. Paired claws with delicate tines. Male: Length of idiosoma 286 µm, width 185 µm [1]. Dorsal aspect similar to that of females, plates similar in shape, length:width ratio and ornamentation. Length of AD 65 µm, width 93 µm, length of OC 40 µm, width 20 µm, length of PD 188 µm, width 130 µm. GP and AP fused to GA, length of that plate 148 µm, width 121 µm, GA slightly wider than female GP. Length of male GO 50 µm, width 33 µm, distance between GO and anterior margin of GA 58 µm, i.e., 1.1 times longer than GO. GA with 19 pgs on one side, 20 on the other side; one of genital sclerites with one, the other with two sgs. Lateral to posterior part of GO a circular area present, 10–12 µm in diameter, with internal bars forming reticulate ornamentation (Fig. 6F, ret-a). Posterior to GO a pair of dome-like areas, 12–14 µm in diameter, each with five gac (Fig. 6G). Each acetabulum, about 7 µm in diameter, with an inner central papilla, 3 µm in diameter, and surrounded by inward crooked teeth. Length of spermatopositor slightly more than that of GO. Shape of gnathosoma and legs similar to those of females. Juveniles: With two nymphal and a larval instar. Length of deutonymphs 240–263 µm [6], of protonymphs 186– 210 µm [6] and of larvae 140–185 [6] µm. All instars with dark spots of eye pigment. Biology. Fifteen out of 20 females were ovigerous, the other five held no eggs. The size (length x diameter) of the eggs was between 55 x 63 µm and 88 x 75 µm. Remarks. The females in the Madagascan material have five (rarely four) external acetabula on each genital sclerite. In populations from other parts of the world the number of acetabula is known to vary between four and nine (Bartsch 2011). In Europe two varieties are known, P. a. alpinus and P. a. brachypeltatus. Discriminating characters in females are the length of the idiosoma, i.e., 297–371 versus 248–270 µm, and the position of the GO in relation to length of the GP, namely distance from anterior margin of GO to that of GP about the same versus about half the length of GO, respectively (Viets 1927: fig. 6 versus fig 9; Bartsch 2006: fig. 5-9b versus fig. 5-10a). Similar, nymphs of P. a. alpinus have, in contrast to P. a. brachypeltatus, a larger distance between the area with genital acetabula and the anterior margin of GP. A rather inconspicuous difference is that the PD of P. a. alpinus is slightly longer than that of P. a. brachypeltatus. In the first mentioned the ds-3 insert immediately adjacent or in the margin of the PD, in the latter within the narrow band of striated integument. The Madagascan individuals belong to P. a. alpinus. In general, all adults in populations of P. alpinus are females. The above mentioned male is one of the very rare exceptions. A former record of a male is one of P. a. brachypeltatus. It was found among more than 2500 slide-mounted adults, all extracted from a population living in Northern Germany which inhabited a small former peat ditch, now filled with rapidly growing Sphagnum sp. (Sphagnales) (Bartsch 1987). The water in the peat ditch was acidic, had a low ionic concentration and a sparse fauna (some few cladocerans, rotifers and testaceans). Differences between the males, from Madagascar (P. a. alpinus) and northern Germany (P. a. brachypeltatus), are the larger size, 286 µm versus 254 µm, and a higher number of pgs, namely 39 versus 20 pgs. The morphological differences between the two forms P. a. alpinus and P. a. brachypeltatus raises the question whether these are two separate species, subspecies or ecotypes? In northern Europe, Porohalacrus a. alpinus inhabits a wide range of substrata, slightly acidic to alkaline and even oligohaline brackish water, oligo- and mesotrophic lakes, ponds and banks of slow flowing rivers, substrata rich in microcaverns, e.g. dense colonies of small mussels (Dreissena sp.), colonies of sponges, bryozoans, mosses, and biofilms on vascular plants, all these habitats have a rich meio- and microfauna and -flora. In contrast, P. a. brachypeltatus has been taken only in strongly acidic Sphagnum peats with sparse associated meiofauna. Judging by the gut content of P. a. alpinus, namely brownish, with darker and lighter particles, these mites are carnivorous. In contrast, the gut content of P. a. brachypeltatus is greenish, suggesting that the mites are phytophagous. Is P. alpinus brachypeltatus a degenerate (impoverished) form, because of its life in an environment with low concentration of cations and meagre diet? Both molecular and experimental studies, namely rearing of the mites over several generations under different environmental conditions, may give an answer. Phenotypic plasticity in freshwater crustacea has been documented more than a century ago (Woltereck 1909), and plasticity is not restricted to arthropods, Cattau et al. (2018) recently described a rapid morphological change in a bird of prey. Geographical distribution (Fig. 7) of both P. a. alpinus and P. a. brachypeltatus (Viets 1956; Green & MacQuitty 1987; Bartsch 2007, 2009, 2011; Tolstikov et al. 2005; Semenchenko et al. 2010; Stolbov et al. 2018; Pepato & da Silva Conceição 2019 in press): Afrotropical Region. – Madagascar (new record); Palaearctic Region. Europe: – Austria – Belgium – Great Britain (England, Scotland, Wales) – Denmark – Finland – France – Germany – Hungary – Iceland – Italy – Norway – Poland – Russia (Saratov —Volga Biological Station) – Sweden – Switzerland – The Netherlands – Ukraine. Northern Africa: – Algeria. Asia: – Turkey – Russia (Kamchatka, Sakha Republic, Tyumen region); Australian Region. – Australia (Western Australia —Esperance). – New Zealand (North Island). Nearctic Region. – United States (New Hampshire, Rhode Island). – Canada (Ontario, Quebec); Neotropical Region. – Brazil (Rio Grande do Sul — Porohalacarus cf. alpinus).

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