Chaetonotus (Hystricochaetonotus), Schwank, 1990

Križanová, Františka Rataj & Vďačný, Peter, 2022, A huge undescribed diversity of the subgenus Hystricochaetonotus (Gastrotricha, Chaetonotidae, Chaetonotus) in Central Europe, European Journal of Taxonomy 840, pp. 1-93 : 71-74

Key to species of the subgenus Hystricochaetonotus Schwank, 1990

The following key contains all species originally assigned to the subgenus Hystricochaetonotus by Schwank (1990) and, later on, by Balsamo (1990), Balsamo & Todaro (1995), Kisielewski (1997a), Kolicka (2016, 2019a), Kolicka et al. (2018), and Todaro (2022). The key uses body shape and size as well as the scale and spine patterns as the main species discriminators following Schwank (1990). The subgeneric assignment of species without associated molecular information needs to be tested (i.e., confirmed or rejected) with phylogenetic analyses in the future. Identification of species within the subgenus Hystricochaetonotus will be most reliable for specimens from which COI and/or ITS2 barcode data are available for comparison of nucleotide sequences with those from type specimens. To avoid any doubts in species identifications, the original descriptions and authoritative redescriptions need to be considered.

1. Dorsal trunk spines gradually or abruptly elongated in a posterior direction ................................... 2

‒ A rather small group of very abruptly elongated dorsal trunk spines ............................................. 31

2. Main dorsal trunk scales rounded and posteriorly incised ................................................................ 3

‒ Main dorsal trunk scales three-lobed or triangular ........................................................................... 4

3. Eight elongated dorsal spines arranged in an arch-like pattern; two elongated lateral spines emerging distinctly above furca base and projecting far behind adhesive tubes; head scales spineless, remaining spines gradually elongated and without lateral denticle ........... Ch. (H.) heterochaetus Daday, 1905

‒ Four elongated dorsal spines form a comparatively small group; two elongated lateral spines emerging at furca base and not projecting behind adhesive tubes; all other spines short and with lateral denticle .................................................................................... Ch. (H.) spinifer Stokes, 1887

4. Dorsal body side covered with evenly and rather densely arranged scales ...................................... 5

‒ Scales absent or reduced in posterior trunk region ......................................................................... 29

5. Dorsal spines gradually elongate posteriorly .................................................................................... 6

‒ A distinct group of abruptly elongated and thick dorsal spines ...................................................... 20

6. Elongated dorsal spines with lateral denticle .................................................................................... 7

‒ Elongated dorsal spines without lateral denticle ............................................................................ 16

7. Lateral denticle emerges near distal end of spine ............................................................................. 8

‒ Lateral denticle emerges in mid-portion of spine ........................ Ch. (H.) polychaetus Daday, 1905

8. Posterior lateral spines not elongated ............................................................................................... 9

‒ Posterior lateral spines elongated and/or thickened .........................................................................11

9. Scale base of elongated dorsal spines three-lobed .......................................................................... 10

‒ Scale base of elongated dorsal spines triangular ........................................ Ch. (H.) gulosus sp. nov.

10. Body larger (ca 160 μm); furca branches run almost in parallel; 10‒11 longitudinal rows of scales; 19‒21 scales per dorsal central row ................................................ Ch. (H.) murrayi Remane, 1929

‒ Body smaller (95‒124 μm); furca branches distinctly diverge posteriorly; 7 longitudinal rows of scales; 15 scales per dorsal central row ................. Ch. (H.) machikanensis Suzuki & Furuya, 2011

11. Posterior elongated lateral spines extend to at least mid-portion of adhesive tubes ...................... 12

‒ Posterior elongated lateral spines extend only to posterior end of furca branches ............................. ............................................................................................. Ch. (H.) hornsundi Kolicka et al., 2018

12. Posterior elongated lateral spines do not extend behind adhesive tubes ........................................ 13

‒ Posterior elongated lateral spines extend far behind posterior end of adhesive tubes .................... 15

13. Interciliary field scales with keels ................................................................................................... 14

‒ Interciliary field scales without keels .................................. Ch. (H.) persimilis Kolicka et al., 2018

14. Furca branches double-keeled scales oval and posteriorly distinctly truncated; ventral central furca base scales oblong with anterior end curved outwards ........... Ch. (H.) borealis Kolicka et al., 2018

‒ Furca branches double-keeled scales circular; ventral central furca base scales oblong .................... ........................................................................................................ Ch. (H.) hystrix Mečnikow, 1865

15. Two pairs of strongly elongated posterior lateral spines extending behind rear end of adhesive tubes .................................................................................... Ch. (H.) paraguayensis Schwank, 1990

‒ One pair of strongly elongated posterior lateral spines extending behind rear end of adhesive tubes ................................................................................................... Ch. (H.) furcatus Kisielewski, 1991

16. Body shape slender, elongated ........................................................................................................ 17

‒ Body tenpin-shaped, stocky ............................................................................................................ 19

17. Cephalic pleurae clearly demarcated .............................................................................................. 18

‒ Cephalic pleurae only indistinctly marked ............................................... Ch. (H.) optabilis sp. nov.

18. About 30–32 scales per dorsal central row; 3 teeth in mouth ring; hypostomium in a form of pentagonal plate with two protuberances .................................................... Ch. (H.) avarus sp. nov.

‒ About 22 scales per dorsal central row; 1 tooth in mouth ring; hypostomium in a form of two horizontal lamellae accompanied by tear-shaped protuberances ................... Ch. (H.) iratus sp. nov.

19. About 13 longitudinal rows of scales; 13 scales per dorsal central row ............................................. ......................................................................................... Ch. (H.) italicus Balsamo & Todaro, 1995

‒ About 29‒31 longitudinal rows of scales; 21–25 scales per dorsal central row ................................. .......................................................................................... Ch. (Ch.) bombardus Kolicka et al., 2018

20. Elongated dorsal spines not clubbed-like thickened ....................................................................... 21

‒ Some elongated dorsal spines clubbed-like thickened ........................... Ch. (H.) lucksi Voigt, 1958

21. Head and neck spines short ............................................................................................................. 22

‒ Head and neck spines comparatively long but distinctly thinner than elongated dorsal spines ..... 28

22. Posterior lateral spines elongated and thickened ............................................................................ 23

‒ Posterior lateral spines not elongated ............................................................................................. 26

23. Two pairs of elongated posterior lateral spines .............................................................................. 24

‒ One pair of elongated posterior lateral spines .......................................... Ch. (H.) mirabilis sp. nov.

24. Lateral denticle of elongated dorsal spines without membrane, elongated posterior lateral spines do not reach posterior end of adhesive tubes ....................................................................................... 25

‒ Lateral denticle of elongated dorsal spines with membrane, elongated posterior lateral spines reach posterior end of adhesive tubes ....................................................... Ch. (H.) horridus Kolicka, 2019

25. Dorsal side of furca branches naked ............................................ Ch. (H.) persetosus Zelinka, 1889

‒ Dorsal side of furca branches carries three pairs of oblong keeled scales and one pair of rounded double-keeled scales ................................................................. Ch. (H.) inaequabilis Kolicka, 2019

26. Lateral denticle distinct and comparatively distant from spine apex (d -ratio> 15%) .................... 27

‒ Lateral denticle indistinct and very close to spine apex (d -ratio <10%) ...... Ch. (H.) luxus sp. nov.

27. Body stocky ............................................................................................... Ch. (H.) arcanus sp. nov.

‒ Body more or less tenpin-shaped .............................................................. Ch. (H.) superbus sp. nov.

28. Body small (90‒120 μm); 25‒27 elongated dorsal spines, ca 15‒25 μm long, not projecting behind adhesive tubes; 2 pairs of posterior lateral elongated spines .............................................................. ................................................................................................ Ch. (H.) macrochaetus Zelinka, 1889

‒ Body larger (140‒190 μm); 16‒18 elongated dorsal spines, ca 60‒70 μm long, projecting far behind adhesive tubes; 5‒6 pairs of posterior lateral elongated spines ... Ch. (H.) euhystrix Schwank, 1990

29. Base of scales present; at most 7‒9 dorsal scale rows on head; 13 elongated dorsal spines; one pair of posterior lateral elongated spines ......................................... Ch. (H.) fujisanensis Sudzuki, 1971

‒ Base of scales reduced or absent .................................................................................................... 30

30. Dorsal group comprises ca 20 elongated spines; one pair of posterior lateral elongated spines ........ ................................................................................................ Ch. (H.) acanthophorus Stokes, 1888

‒ Dorsal group comprises ca 13 elongated spines; two pairs of posterior lateral elongated spines ...... ............................................................................................................ Ch. (H.) enormis Stokes, 1887

31. Elongated dorsal spines without lateral denticle; scales absent or not described ........................... 32

‒ Elongated dorsal spines with lateral denticle; scales three-lobed, keeled, and with short spine .... 34

32. Elongated dorsal spines emerge from reduced and rounded scales; distinctly more than 3 elongated dorsal spines .................................................................................................................................... 33

‒ Elongated dorsal spines emerge from well-developed and three-lobed scales; 3 elongated dorsal spines ......................................................................................... Ch. (H.) trispinosus Balsamo, 1990

33. Ten elongated dorsal spines, up to 40 μm long, arranged in two interrupted longitudinal rows, last spine pair projects far behind adhesive tubes .................. Ch. (H.) decemsetosus Marcolongo, 1910

‒ Six elongated dorsal spines, up to 23 μm long, arranged in two horizontal rows, last spine pair does not reach furca base ........................................................................... Ch. (H.) vargai Rudescu, 1967

34. Scale base of elongated dorsal spines ± hemispherical .................................................................. 35

‒ Scale base of elongated dorsal spines ± three-lobed, triangular, plow-like, or reduced (highly variable in shape) .......................................................................................................................................... 38

35. Only posterior dorsal body half bears some scales, eight elongated dorsal spines ............................ ........................................................................................................ Ch. (H.) octonarius Stokes, 1887

‒ Whole dorsal body side covered with scales .................................................................................. 36

36. Eight elongated dorsal spines; last pair of elongated dorsal spines does not reach adhesive tubes; lateral denticle comparatively short and emerging near distal end of spine ................................... 37

‒ Less than 8 elongated dorsal spines; last pair of elongated dorsal spines reaches adhesive tubes; lateral denticle rather long and emerging distinctly ahead of distal end of spine ............................... ........................................................................................... Ch. (H.) paucisetosus Marcolongo, 1910

37. Five elongated dorsal spines .......................................................... Ch. (H.) pungens Balsamo, 1990

‒ Seven elongated dorsal spines ................................................. Ch. (H.) schlitzensis Schwank, 1990

38. Dorsal and lateral scales well-developed ........................................................................................ 39

‒ Scales reduced except for dorsal scales carrying elongated spines ................................................ 43

39. Body larger (180‒190 μm); elongated dorsal spines curved, long and reaching at least furcal indentation; head spined ................................................................................................................. 40

‒ Body smaller (80‒120 μm); elongated dorsal spines straight, comparatively short (15‒25 μm) and not reaching furca base; head spineless .......................................................................................... 42

40. Scale base of elongated dorsal spines reduced; posteriormost elongated spines almost reach rear end of adhesive tubes; first two anterior horizontal rows of head spines distinctly elongated and thicker ......................................................................................................................................................... 41

‒ Scale base of elongated dorsal spines well-developed; posteriormost elongated spines reach furcal indentation; head spines of same width ....................................... Ch. (H.) anomalus Brunson, 1950

41. One lateral denticle on each of nine elongated dorsal spines ..... Ch. (H.) novenarius Greuter, 1917

‒ Two subsequent lateral denticles on each of nine elongated dorsal spines ........................................ ............................................................................................... Ch. (H.) balsamoae Kisielewski, 1997

42. Seven to nine elongated dorsal spines ....................................... Ch. (H.) aemilianus Balsamo, 1978

‒ Five elongated dorsal spines ........................................................ Ch. (H.) ferrarius Schwank, 1990

43. Elongated dorsal spines long and curved, often extend behind adhesive tubes ............................. 44

‒ Elongated dorsal spines comparatively short and straight, do not reach furca base ....................... 46

44. Body slender and bigger (150 μm) ............................................ Ch. (H.) trilineatus Valkanov, 1937

‒ Body stocky and smaller (70‒120 μm) ........................................................................................... 45

45. Eight elongated dorsal spines arranged in 3‒4 horizontal rows and projecting behind adhesive tubes .................................................................................... Ch. (H.) trichodrymodes Brunson, 1950

‒ Seven or eight elongated dorsal spines arranged in 2 horizontal rows and not projecting behind adhesive tubes .......................................................................... Ch. (H.) longispinosus Stokes, 1887

46. Five to seven elongated dorsal spines, lateral body spines present .................................................... ........................................................................................................ Ch. (H.) spinulosus Stokes, 1887

‒ Five elongated dorsal spines, lateral body spines absent ...... Ch. (H.) quintospinosus Greuter, 1917