Chaetonotus (Hystricochaetonotus) hystrix Metschnikoff 1865

Chaetonotus (Hystricochaetonotus) hystrix Metschnikoff, 1865

(Fig. 13)

Localities: Lake Stäringen, Småland (N 57º 20’ 14’’; E 16º 28’ 18’’), May 1, 2010; Abisko Scientific Research Station, Abisko, Lapland (N 68º 21’ 46’’; E 18º 46’ 73’’), July 5, 2010; Small stream, Abisko Tourist Station, Abisko, Lapland (N 68º 21’ 23’’; E 18º 47’ 59’’), July 3, 2010; A small pond close to lake Torneträsk at Abisko Scientific Research Station, Lapland (N 68º 21’ 19’’; E 18º 49’ 21’’), June 30, 2010; Gravel pit, Åhus, Skåne (N 55º 55’ 43’’; E 14º 15’ 08’’), April 10, 2009.

Material: 7 specimens.

TL, 120–130 µm; FL, 17–18 µm; AL, 9–10 µm; PhL, 30–33 µm; MD, 4–5 µm; CW, ~12 µm; TNC, 12–14; DC, 7–8; DR, ~14–16; VLC, 4–6; HS, 2– 3 x 2–4 µm; NS, 4– 5 x 3–5 µm; DS, 9– 10 x 8–10 µm; HSp, 2–3 µm; NSp, 3–6 µm; DSp, 10–14 µm; VC, 5–6; VR,>20; VTS, 7– 8 x 2–4 µm.

Head rounded to weakly five-lobed with two pairs of cephalic sensory ciliary tufts. Posterior pair of tufts longer than anterior pair. Cephalion and pleurae weakly developed. Hypostomium absent. Two pairs of dorsal sensory bristles present, posterior pair emerging from specialized three-lobed double-keeled scales at U75–76. Furca straight with adhesive tubes constituting approximately half of the total furca length. Dorsal body surface covered by three-lobed scales with deep posterior incision. Scales with barbed spines that increase in size towards the posterior end. An area of the dorsal posterior trunk region with scales that lack spines. At the base of each furcal appendage a pair of rounded to almost squared double-keeled scales surrounded by three pairs of elongated almost rectangular three-lobed scales. Two pairs of thicker parafurcal spines present.

Ventrolateral areas with scales with barbed spines, similar in shape to those of the dorsal surface but finer. Ventral interciliary area covered by small rounded keeled scales. A pair of keeled sub-rectangular ventral terminal scales, with short fine simple spine up to approximately 6–7 µm in length. Ventral ciliation in two longitudinal bands that merge on the head.

Mouth subterminal. Pharynx with weak swellings. PhIJ at U27–29. Intestine straight with anus at U82–85. The examined specimens were all adults in parthenogenetic phase and fall well within the ranges previously reported for the species. C. (H.) hystrix is a cosmopolitan species and is one of the most commonly encountered freshwater gastrotrichs. It can be morphologically distinguished from C. (H.) persetosus and C. (H.) macrochaetus in that it has shorter dorsal spines and a pair of round to squared double-keeled scales at the base of each furcal branch. Moreover, C. (H.) hystrix does not present such a sudden increase in spine length from head-neck region to the trunk region as C. (H.) persetosus and C. (H.) macrochaetus (Schwank 1990).

Previously reported from Austria (Zelinka 1889), Bulgaria (Konsuloff 1913), Denmark (Grilli et al. 2010), Estonia (Järvekülg 1961), France (Grilli et al. 2009), Great Britain (Martin 1981), Hungary (Varga 1949), Italy (e.g. Balsamo 1983), Poland (Kisielewski 1981), Romania (Rudescu 1967), Russia (Preobrajenskaja 1926), Sweden (Kånneby et al. 2009; 2013), Brazil (Kisielewski 1991), Israel (Kisielewski 1999), Paraguay (Daday, 1905).