Cladotanytarsus molestus Hirvenoja, 1962 in Poland: toward the identification of bioindicative Tanytarsini (Diptera: Chironomidae)

Cladotanytarsus molestus Hirvenoja, 1962: 176 (male, pupa; Finland); Paasivirta 1972: 262 (Finland), 2001: appendix (Finland), 2012: 66 (Finland); Bitušík 1993: 193 (pupa, Slovakia); Cobo et al. 2001: 244 (Portugal); Langton & Ruse 2005: 137 (pupa, Great Britain); Giłka 2011b (Europe, distribution).

Material examined. POLAND. Straszyn near Gdańsk, at Lake Straszyńskie (54°16′31“N, 18°32′12“E; UTM CF31/CF41; Fig. 1), 2 May 2015, netting, 15 males, leg. F. Zimny (DIZP). Comparative material. FINLAND. Lapland (Kemin Lappi), Valkenjärvi near Muonio, 24 June 1982, 3 males, ex coll. L. Paasivirta (DIZP); (Sompion Lappi), Seitajärvi ca 60 km NE of Sodankylä, 20 June 1960, 2 males, leg. M. Hirvenoja (DEUM); Yläpostojoki ca 40 km N of Sodankylä, 10 June 2003, 2 males, ex coll. L. Paasivirta (DIZP).

Adult male (n = 15)

Colouration (in alcohol). Antennal pedicel, tentorium, scutal stripes, scutellum, postnotum and sternum dark brown. Antennal flagellum, legs and abdominal segments brown. Head capsule mouthparts, ground colour of thorax, haltere and hypopygium olive brown. Wing membrane with brownish undertone; C, M and radial veins distinctly darker.

Head. Antenna with 13 well-separated flagellomeres, length of distal flagellomere (μm) 442-516 (474), AR 0.96-1.08 (1.00), plume fully developed. Frontal tubercles variable in size and shape (12-28 μm long, 4-8 μm wide at base). Palp slightly variable in length, palpomeres 2-5 (μm): 32-50 (41), 92-116 (109), 100-124 (113), 108-176 (143). Clypeus with 9-17 setae.

Wing. Length (μm) 1590-1790 (1715), width 480-534 (515). Shape, venation pattern and chaetotaxy typical for Cladotanytarsus. Veins C, R, R₁, M₃₊₄, M₁₊₂ in 2/3 distal part and R₄₊₅ in distal half with macrotrichia, several sparse macrotrichia on Cu and An. Membrane covered with macrotrichia in the distal part of cells r₄₊₅ and m₁₊₂, with a row of macrotrichia in 3/4 distal length of r₄₊₅. Remaining veins and cells bare. FCu slightly distal of RM; VR_Cu 1.19-1.37 (1.25). R₄₊₅ ending slightly distally of M₃₊₄ and well proximally of M₁₊₂.

Legs. Tibial spur of fore leg straight or slightly curved, 15-20 μm long. Tibial combs of mid and hind leg distinctly separated, each comb with spur; longer spur distinctly curved (ca. 25 μm long in mid leg, and ca 30 μm long on hind leg), shorter spur straight (ca. 20 μm long in mid leg and ca. 25 μm on hind leg). Basitarsus of mid leg with 2-3 sensilla chaetica. For length of leg segments and leg ratios, see Table 1.

Hypopygium (Figs 2, 3B-D, 4A). Gonostylus slender, 70-115 (90) μm long, slightly shorter than gonocoxite. Anal tergite with V-type bands broadly separated. Anal point with more or less developed nipple-like tip, variable in shape, as shown in Figs 2 and 3B, bearing narrow crests, 3-12 spinulae and dense microtrichia surrounding the base of the anal point. Superior volsella rounded at base, with distinct apical lip, covered with dense microtrichia in basal part; digitus long, extending far beyond superior volsella, with slightly swollen finger-like tip; 3-4 long inner setae placed on distinct protuberance at base of superior volsella (Figs 2, 3C). Stem of median volsella stout, slightly curved at mid-length and posteriorly directed, bearing 7-8 stout furcate lamellae (Fig. 3D). Inferior volsella relatively broad, evenly tapering to round apex, with narrow dorsomedian ridge (Figs 2, 4A).

Figure 2

Figure 3

Figure 4

Variability in the diagnostic structures in the male of Cladotanytarsus molestus, so far known from the original description based on three specimens (Hirvenoja 1962), applies both to metric and meristic characters of the head (Fig. 3A), wings and legs, but first of all to the hypopygial features: the anal point (Fig. 3B), the superior volsella and digitus (Fig. 3C) and the stem and lamellae of the median volsella (Fig. 3D). Adult males of C. atridorsum Kieffer, 1924 and C. matthei Giłka, 2001 have a similar shape of these structures (but never in combination as shown in Figures 2-4). The crucial diagnostic character best separating the above species is the shape of the inferior volsella: broad, evenly tapering to a round apex, with a narrow dorsomedian ridge in C. molestus (Figs 2, 4A), arcuate with a broad and long, strongly pigmented dorsomedian ridge in C. matthei (Fig. 4B) or U-shaped in transversal section, with a concave apex and with a broad but short dorsomedian ridge in C. atridorsum (Fig. 4C) (cf. Giłka 2001). The median volsella, with its stem somewhat curved at the mid-length can also be treated as a key character for C. molestus (Fig. 3D). The Polish specimens of C. molestus are slightly different from those collected in Finland in having a shorter wing (ca. 1.6-1.8 vs 2.0 mm), lower AR (0.96-1.08 vs 1.05-1.11) and higher LR (1.43-1.69 vs 1.35-1.50).

Cladotanytarsus molestus is a boreal (boreo-al-pine) species with a distribution center in northern Europe and with a few localities in central and southern Europe (Giłka 2011b). To date, C. molestus has been recorded mainly in the northern bioprovinces of Finland, including all the bioprovinces of Lapland (Paasivirta 2012). Apart from Finland, this species has been noted in the highlands of Scotland (Langton & Ruse 2005), Slovakia (Bitušík 1993) and in Portugal (Aveiro district) (Cobo et al. 2001). Now it has been recorded for the first time in Poland, at Lake Straszyńskie (Fig. 1) situated in the Kashubian Lakeland mesoregion of the Southern Baltic Lakeland subprovince.

According to the data hitherto available, the preferred habitats of Cladotanytarsus molestus immatures are oligotrophic lakes, such as those typical in northern Finland (Hirvenoja 1962; Paasivirta 1972; 2001; 2012). Pupae of this species were found in a similar habitat at Loch na Bruthaich in the Scottish Highlands (Langton & Ruse 2005), in a calcareous fen in Slovakia (Rakšianske rašelinisko, Turčianska valley) (Bitušík 1993) and from the Río Vouga (in the middle course) near Albergaria-á-Velha in central Portugal (Cobo et al. 2001).

Lake Straszyńskie (Goszyńskie), where Cladotanytarsus molestus has now been found, is a small reservoir (area ca. 75 ha, volume 3.5 million m³) supplied by the waters of the River Radunia. Since 1983 it has been used as a source of drinking water for the inhabitants of Gdańsk. The waters of Lake Straszyńskie, and also of the Radunia’s catchment area upriver of the reservoir, are of a much higher quality than those of other, neighbouring waterbodies and are routinely analyzed to monitor their physicochemical parameters; biomonitoring is also a standard practice (Stepnowski et al. 2010; Radtke et al. 2011). This is not to say they are entirely free of toxic compounds and contaminants of various origin (e.g. Kobos et al. 2013; Caban et al. 2015): the quality of the water is classified as A3 according to the overall assessment of the Provincial Inspectorate of the Environment (WIOŚ 2015).

Adult males of Cladotanytarsus molestus were sampled from a site situated ca 200 m above the point where the Radunia enters Lake Straszyńskie (Fig. 1). They were found in a large sample of Cladotanytarsus atridorsum, an eurytopic species, in Poland typical of meso- and eutrophic lakes (Giłka & Podlesińska 2009). Sympatric species recorded at Lake Straszyńskie in the 2015 season were also represented by species from the Cladotanytarsus mancus group (Walker, 1856) (eurytopic, >39% of specimens collected), Tanytarsus verralli Goetghebuer, 1928 (eutrophic/mesotrophic lakes, ca. 20%), T. mendax Kieffer, 1925 (eutrophic/mesotrophic lakes, ca. 10%), T. occultus Brundin, 1949 (eurytopic, ca. 9%) and Paratanytarsus inopertus (Walker, 1856) (eurytopic, ca. 9%) (Paasivirta 2001; Giłka & Dominiak 2007; Giłka & Podlesińska 2009; Zimny, unpublished data). These data are similar to those obtained from Lake Raduńskie and Lake Żarnowieckie. The trophic status of the former lake can be described as mesotrophic, whereas that of the latter as intermediate between β-mesotrophic and eutrophic, with eutrophication tending to intensify; both lakes compared are dominated by the same species with a high tolerance to the trophic level (Dubrawski et al. 2003; Giłka & Dominiak 2007, Giłka & Podlesińska 2009); however, C. molestus has not been recorded so far at the two studied lakes. A synthesis of all the information hitherto gathered about the habitat preferences of C. molestus indicates that its range of tolerance to the trophic level is somewhat broader than originally thought.

The data obtained throughout the season indicate that adults of Cladotanytarsus molestus make their appearance in early May (they were not found at other times), which in this region is probably univoltine.