Canthocamptus ( Baicalocamptus ) longifurcatus Borutzky, 1947

Canthocamptus ( Baicalocamptus) longifurcatus Borutzky, 1947 View in CoL

Canthocamptus ( Baicalocamptus) longifurcatus Borutzky, 1947: 1826 View in CoL (female); Okuneva, 1989: 19 (female, male).

Canthocamptus ( Baikalocamptus) longifurcatus View in CoL : Borutzky, 1952: 154 (female); Okuneva & Evstigneeva, 2001: 470 (in checklist).

Canthocamptus longifurcatus View in CoL : Novikov & Sharafutdinova, 2022: 59 (in key and in generic diagnosis; revision of genus).

( Figs 1–8)

Material examined. Russia, Irkutsk Prov., Lake Baikal nr. Utulik Vill., approx. 51°35′31″N 103°57′43″E, 9 June 1968, 6 females on slides, 1 male on slide ( ZMIB: nno. HRP-1, HRP-4, HRP-7(2), HRP-11); Lake Baikal nr. Bol’shoe Goloustnoe Vill., 52°00′05.5″N 105°21′22.1″E, 3 Oct. 2017, T. Sitnikova leg., 2 males in ethanol samples ( ZMIB: nno. HRP-2, HRP-3(1)); Lake Baikal nr. Bol’shye Koty Vill., approx. 51°54′11″N 105°4′28″E, 15 March 1968, 2 females on slides ( ZMIB: nno. HRP-5, HRP-6) GoogleMaps .

Morphological redescription. Female ( Figs 1–3, 7a, c). Total length (measured from tip of rostrum to posterior margin of caudal rami) 1150–1400µm (n = 8).

Body strongly flattened ( Figs 1a, 7a), raspberry pink in colour. Ornamentation of integument with large tile cells and folds. Body somites wide, at least twice as wide as long.

Cephalothorax ( Figs 1a, 2a) with integumental dorsal window, wider than thoracal somites, with posterior parts considerably projecting laterally; lateral margins of cephalothorax exposed to ventral side ( Fig. 2a). Naupliar eye absent. Rostrum large (length 60 µm, width 90 µm), fused with cephalothorax, subquadrate, with a pair of noticeable sensilla at its anterior apex. Thorax evenly tapering from first somite to abdomen; abdominal somites (except anal somite) wider than thoracic ones, their hind angles laterally protruding as rounded lobes. Posterior margins of thoracic and abdominal somites without spinules; posterior margins of somites dorsally strongly serrate, forming coarse folds looking as sparse blunt teeth. Posterior margins of abdominal somites with discontinuous rows of ventral spinules. Ventral surface of abdominal somites with thin short capilliform spinules.

Genital field ( Fig. 1b) located near anterior margin of genital somite, with rather large copulatory pore located in median depression. Anal somite with large anal operculum. Anal operculum rounded, with numerous short thin spinules. Ventral surface of anal somite with a pair of short triangular projections, covered with thin short capilliform spinules ( Fig. 1c).

Caudal rami conical ( Fig. 1a, c), about 2.2 times as long as wide (length 105 µm, width 47 µm). Integument of caudal rami with longitudinal and transverse folds, similar to that of body somites. Distal part of caudal rami with numerous small spinules at inner and outer margins; proximal part with small group of spinules at inner and outer margins. Each ramus with seven setae (I– VII). Inner terminal seta (seta V) the thickest and longest (in all examined specimens, its apex broken), with basal section expanded. Outer terminal seta (seta III) and inner terminal seta (seta VI) short and thin; outer seta about 0.75 times as long as caudal ramus, inner seta half as long as outer seta. Two lateral setae (setae I and II) and dorsal seta (seta VII) present. Dorsal seta inserted in proximal part of caudal rami.

Antennule relatively short, eight-segmented, far not reaching posterior margin of cephalothorax ( Fig. 2b, c). Antennules often hidden under cephalothorax at ventral side ( Fig. 2a). Aesthetasc of fourth segment cylindrical with rounded tip, almost reaching apex of antennule. Setal formula: 1, 8, 4, 2+Ae, 1, 3, 2, 7+Ae.

Antenna with allobasis, one-segmented Exp ( Fig. 2d) and Endp ( Fig. 2e). Allobasis with one seta. Exp with two apical, one subapical and one lateral setae. Last segment of Endp with two lateral spines, two distal spines accompanied by two geniculated setae and one short thin spine at outer corner.

Mandible ( Fig. 2f) robust; gnathobase with several teeth and pinnate seta at dorsal corner twice as long as each tooth. Mandibular palp two-segmented, with four setae: proximal segment (basis) without seta, distal Endp segment with four setae (one inserted laterally, three apically).

Maxillule ( Fig. 2g) composed of robust praecoxa, coxa and basis. Praecoxal arthrite with seven strong apical spines. Coxa with cylindrical endite and distal long seta. Basis with two distal pinnate setae and several setae proximally. Endp and Exp represented by setae located near base of basis.

Maxilla ( Fig. 2h) two-segmented, composed of proximal syncoxa and basis. Syncoxa with rows of outer spinules and two endites, each with two terminal pinnate setae. Basis drawn out into a strong serrate claw with one accompanying seta. Endp represented by three setae.

Maxilliped ( Fig. 2i) three-segmented, composed of coxa, basis and one-segmented Endp. Coxa with pinnate seta and several spinules at inner margin. Basis 2.5 times as long as wide, with small outer spinules equal in length. Endp drawn out into a strong acutely curved claw with seta at base.

P1–P3 with three-segmented Exps and three-segmented Endps ( Figs 3a–c).

P1 ( Fig. 3a): basis with robust inner and out- er spines. Endp-1 longer than two first Exp segments, with setae on inner margin. Endp-1 more than three times as long as Endp-2 and Endp-3, with pinnate seta on inner margin. Endp-3 with two distal armatures: curved strong unipinnate outer spine and thin seta being twice as long as spine. Exp-1, Exp-2 and Exp-3 with strong unipinnate outer spine. Exp-2 with unipinnate inner seta. Exp-2 and Exp-3 without inner armature.

P2 ( Fig. 3b): Endp with one seta on inner margin in each of two first segments and two setae on inner margin of terminal segment. Endp-3 with two pinnate setae on inner margin, two pinnate setae on distal margin, and one long unipinnate subapical spine on outer margin. Exp-1 without armature on inner margin and one pinnate spine on outer margin. Exp-2 with one pinnate seta on inner margin and one pinnate spine on outer margin. Exp-3 with one bare seta on inner margin, three pinnate spines on outer margin, and one pinnate seta and one long unipinnate spine on distal margin. Exp-2 segment with inner seta about 1.7 times as long as Exp-3.

P3 ( Fig. 3c) with chaetotaxy alike P2 except following details: basis with a strong short spine on margin between Exp and Endp, setae of inner margin of Endp 3 naked (in P2, pinnate) and distal outer armature transformed into a pinnate seta (in P2, this armature represented by long unipinnate spine).

P4 ( Fig. 3d) with three-segmented Exp and two-segmented Endp. Endp-1 with one inner seta, Endp-2 with two inner setae, two long distal setae and one inner-distal spinule being about 0.3 times as long as terminal setae. Exp-2 segment with inner seta being about 1.7 times as long as Exp-3.

Armature formula (Lang system) of Exps of P2–P4: 3, 3, 1; 3, 2, 1; 3, 2, 2.

P5 ( Figs 3e, 6c) with separate Exp and baseoendopod. Baseoendopod wide (width 126 µm), endopodal lobe almost not projecting upon basal margin of Exp, its maximum length 40 µm. Endopodal lobe with five pinnate spiniform setae and one inner short bare seta, half as long as outer-

E.B. Fefilova et al. Integrative redescription of Canthocamptus (Baicalocamptus) longifurcatus most pinnate seta. Third inner seta of endopodal lobe the longest, about twice as long as fourth one. Outer endopodal seta long, bare. Exp oval, longer than endopodal lobe, its length 43 µm, width 40 µm. Outer margin of Exp with numerous small spinules, dorsal and ventral surfaces smooth, without armatures. Exp with five pinnate spiniform setae on outer margin, inner seta tiny, second inner seta the longest, more than twice as long as Exp segment. Three outer setae half as long as second inner seta. Two very short bare spines inserted near margin between shortest and longest setae.

P6 ( Fig. 1b) fused, small, with short robust pinnate seta on each side of copulatory pore.

Male ( Figs 4–6, 7b, c, 8). Total length (measured from tip of rostrum to posterior margin of caudal rami) 1150–1300 µm (n = 6).

Habitus similar to that of female. Ornamentation of cephalothorax, thoracic somites, abdominal somites and caudal rami similar to those of female ( Figs 4a, 5a, 7b, 8a–c). Caudal rami ( Figs 4a, 5a) about 2.1 times as long as wide (length 100 µm, width 48 µm). Anal operculum ( Fig. 4b), antenna, mouthparts, P1 ( Figs 5b, 7b) and Exps of P2–P3 ( Figs 5c, 6a, b) similar to those of female. Spermatophore ( Fig. 4c) wide, sacciform. Antennule ( Fig. 4d) eight-segmented, geniculate. Aesthetasc on segment 4 not reaching distal margin of segment 8.

Endps of P2–P4 two-segmented.

P2 ( Fig. 5c): Endp-1 with one inner seta, one thin long inner setula and outer setulae. Endp-2 three times as long as Endp-1, with one short inner seta, spiniform inner armature, two apical setae and one apical spine; innermost apical seta the shortest, outermost spine the longest.

P3 ( Fig. 6a): Endp three-segmented, barely longer than two first Exp segments (Exp-1 and Exp-2) combined. Endp-1 with a long pinnate seta. Endp-2 with a long inner apophysis being as long as all segments of Endp combined. Apophysis finely serrate at inner distal apex. Endp-3 oval, smooth.

P4 ( Fig. 6b): Endp two-segmented, shorter than two first Exp segments combined. Endp-1 with one inner seta, Endp-2 with two inner pinnate setae, two apical setae and apical spine; short outer apical spine not modified.

P5 ( Fig. 6c) two-segmented, with Exp and baseoendopod. Baseoendopod lobe with outer seta and two pinnate spines, inner about 2.5 times as long as outer one. Baseoendopod lobe with group of several small spinules near base of outer seta. Exp conical with 4–5 spines and small spinules on outer margin. Exp length 40 µm, width up to 24 µm. Inner apical spine of Exp the longest, about twice as long as outer apical spine; short inner bare spine present on right leg and absent on left one.

P6 ( Fig. 6c) consisting of two strong pinnate spines and a short thin bare outer seta. Inner spine the longest.

Morphological comparison and variability. Morphology of the examined females agrees with that of C. ( B.) longifurcatus as described by Borutzky (1947, 1952), in the body shape, structure of A1, Exp of A2, P1–P5, caudal rami, and anal operculum. Morphological characters of the males under study conform to those of C. ( B.) longifurcatus as described by Okuneva (1989), in the body shape, structure of P1, P3–P5, caudal rami, and anal operculum. The males examined by us differ from the Okuneva’s description in the structure of Endp-2: Okuneva (1989) described spiniform inner armature as a short spine whereas we describe it a finger-like armature of the segment ( Fig. 5c).

All females and males of C. ( B.) longifurcatus examined in our study were very similar to each other in the important diagnostic characters. In addition to the variability of Endp- 2 in males, we also observed a variability in the number of spines on Exp of male P5. One of the examined males has four spines on Exp of one leg 5 and five on the oth- er, whereas all other 5 males have five spines. According to the description of C. ( B.) longifurcatus by Okuneva (1989), the males have five spines on Exp of P5 as well.

Canthocamptus ( B.) longifurcatus View in CoL can be easily differentiated from C. ( B.) verestschagini View in CoL by the armature of the female caudal rami, as well as by the structure of P 5 in female and male ( Borutzky, 1931, 1952; Okuneva, 1989). According to the description of C. ( B.) verestschagini View in CoL in Borutzky (1931, 1952) and Okuneva (1989), the female of the latter species has the middle apical seta of the caudal rami transformed into a short conical spine or tubercle in contrast to a long strong seta which is present in C. ( B.) longifurcatus View in CoL . Its Exp of P5 is wider than in C. ( B.) longifurcatus View in CoL and covered by the rows of spinules [in C. ( B.) longifurcatus View in CoL , the rows of spinules are located on the margin of Exp]. As distinct from C. ( B.) longifurcatus, Exp View in CoL of P 5 in the male of C. ( B.) verestschagini View in CoL is covered by the rows of spinules, same as in the female

E.B. Fefilova et al. Integrative redescription of Canthocamptus (Baicalocamptus) longifurcatus View in CoL and armed by six spines [ vs. five in C. ( B.) longifurcatus View in CoL ].

Our analysis of the morphology of the females and males of C. ( B.) longifurcatus and other non-Baikal species of the genus Canthocamptus from Europe, the Urals and North America [ C. ( C.) assimilis , C. ( C.) glacialis , C. ( C.) microstaphylinus , C. ( C.) robertcokeri , C. ( C.) staphylinoides , staphylinoides , C. ( C.) staphylinus staphylinus , and C. ( C.) cf. vagus ] generally confirmed the presence of a number of common features important for the diagnostics of both subgenera, Baicalocamptus and Canthocamptus s. str. These characters are the number of segments of the Exps and Endps of P1–P 4 in females and males, the armature and morphology of Endp of P 2 in males. The shape and size proportions of caudal rami in the studied species were similar. Number and length ratio of the spines on Exps and basoendopods of P 5 in females and males were similar as well.

Firstly, our attention is drawn to the difference in structure of P4 Endp in males: in all non-Baikal Canthocamptus an inner spine of P4 Endp is modified into a thin finger-like appendage. However, according to Borutzky (1931) this spine is not modified in C. ( B.) longifurcatus and C. ( B.) verestschagini . There are some differences in P3 Endp of males: most of non-Baikal species of Canthocamptus have long apical setae on the distal segment of P3 Endp, but this segment is smooth in both species of Baicalocamptus and in C. ( C.) microstaphylinus .

DNA sequences . Nucleotide sequences of 636 bp COI mtDNA gene region were obtained from five specimens of C. ( B.) longifurcatus . On the phylogenetic tree, all five sequences formed a single clade located within a larger clade that included other Canthocamptus species. The closest relative species to C. ( B.) longifurcatus was C. ( C.) staphylinus , with the genetic distance of 29.5% between them, while the genetic distances between the specimens of C. ( B.) longifurcatus were 2%. (The matrix of genetic distances is provided in Electronic supplementary material 2; see the section “Addenda”).