Bryocamptus (Rheocamptus) sitnikovae, Urbano & Delanira-Santos & Scoarize & Benedito, 2024

Urbano, Vinícius de Andrade, Delanira-Santos, Driele, Scoarize, Matheus Maximilian Ratz & Benedito, Evanilde, 2024, TABLE 4 in Dams and agricultural lands affect energy sources and the trophic position of fish in a floodplain, Arthropoda Selecta 33 (1), pp. 36-52 : 38-46

Bryocamptus (Rheocamptus) sitnikovae sp.n.

Figs 2–10.

urn:lsid:zoobank.org:act:280EFC3D-3C15-46DF-A3D5-60238238FA75

Bryocamptus (Rheocamptus) sp. 6 — Fefilova et al., 2022: 10–11, Fig. 4e–g.

ETYMOLOGY. The species is named after Tatyana Ya. Sitnikova, a collector of this material and a well-known specialist in the deep-water fauna of Lake Baikal.

MATERIAL. Holotype ♀ dissected on two slides, RUSSIA, Baikal Lake , Frolikha hydrothermal seep; 55.517° N, 109.767° E; depth 409 m; 23 July 2010; T. Ya. Sitnikova leg; KFU ZM&G SC-68/1, SC-68/2. Allotype ♂ dissected on one slide; collection data as for holotype; KFU ZM&G SC-69. Paratypes: 1 ♀ dissected on two slides; collection data as for holotype; KFU ZM&G SC-70/1, SC-70/2; 28 ♀ and 1 ♂ undissected, preserved in 4% formalin; collection data as for holotype; KFU ZM&G SC-N/V-1; 3 ♀ and 1 ♂ dissected on four slides; RUS- SIA, Baikal Lake , Frolikha hydrothermal seep; 55.517° N, 109.767° E; depth 432 m; 24 July 2010; T. Ya. Sitnikova leg; IBKSC HRP-29 , HRP-30 , HRP-31 , HRP-32 .

DESCRIPTION. Female. Body subcylindrical ( Fig. 2A). Total body length of holotype from anterior margin of rostrum to posterior margin of caudal rami: 748 µm. Cephalothorax ( Fig. 2B–C), wider than remaining somites, length with rostrum 182 µm, largest width 153 µm. Naupliar eye not discovered. Rostrum ( Fig. 2D–E) small, fused with cephalothorax, with rounded end, with one pair of sensillae and ventral pore. Posterior margin of cephalothorax and all pedigerous somites smooth.

Cephalothorax ( Fig. 2B–C) with dumbbell-shaped dorsal window; with 34 pairs of sensillae and 15 pairs of pores, two of which located on lateral margin of cephalothorax. Second and third pedigerous somites with lateral windows, dorsal unpaired pore, lateral pair of pores and eight pairs of sensillae. Fourth pedigerous somite with dorsal unpaired pore, lateral pair of pores and seven pairs of sensillae. Fifth pedigerous somite with lateral pair of pores and four pairs of sensillae.

Abdomen ( Fig. 3) consisting of fused genital-double somite, two free abdominal somites and anal somite with caudal rami. All somites except anal somite on posterior margin serrated, on surface with rows of small spinules. Genital-double somite consists of last thoracic somite and first abdominal somite; longer than wide; anterior part with three pairs of sensillae, dorsal unpaired pore, lateral paired pores, ventro-lateral rows of spinules; posterior part with four pairs of sensillae, dorsal unpaired pore, pairs of ventral and lateral pores and lateral rows of spinules.

P6 ( Fig. 3A) fused with somite with one pinnate and one naked setae. Genital field ( Fig. 3A) long, laterally with sieves; copulatory pore displaced to posterior part of somite, copulatory duct chitinised, extending proximally to pair of labyrinthic rounded ducts and one chitinised unpaired duct. Egg sac absent in holotype. Paratypes with 10 to 13 eggs in their egg sacs (n = 4: 10, 11, 13, 13).

Second abdominal somite ( Fig. 3A–C) with three pairs of sensillae, unpaired dorsal pore, pairs of lateral and ventral pores; on posterior margin with lateral row of large spinules. Third abdominal somite ( Fig. 3A–C) with pair of lateral pores, on posterior margin with lateral row of large spinules and ventral row of thin spinules. Anal somite ( Fig. 3A–C) with one pair of sensillae, ventral and lateral pairs of pores. Anal operculum ( Fig. 3C) semilunar, with five small spinules.

Caudal rami ( Fig. 3). Length/width ratio 1.3, with dorsal carina; with one large and one-two small ventral pores; with two large spinules on ventral side at base of seta V, two large spinules on inner side and rows spinules at base of setae II and III. Seta I small, located near seta II. Setae II and III normal, located on outer side of ramus. Apical seta IV ( Fig. 3) modified, with thick proximal part and thin distal part; proximal part with fracture plane and with large hyaline dorsal and ventral protrusions forming circle. Apical seta V ( Fig. 3D) long, bipinnate, with fracture plane. Base of seta VI located slightly under base of seta V. Seta VII triarticulated, located near dorsal crest ( Fig. 3C).

Antennule ( Fig. 4A) eight-segmented. Segment 1 short, with one pinnate seta and two rows of spinules. Other segments with bare setae. Segment 4 with fused basally seta and aesthetasc. Distal segment with acrothek consisting of aesthetasc and two setae fused basally. Armature formula: 1-[1],2-[9],3-[5],4-[1+(1+ae)],5-[1],6-[3],7-[2],8-[5+acr].

Antenna ( Fig. 4B) with allobasis. Coxa with three rows of spinules. Allobasis with spinulose basal seta and bare endopodal seta; with spinular rows at base of endopodal setae. Free endopodal segment with two lateral rows of large spinules, two spinulose spines and slender seta; distally with two rows of spinules; apically with three geniculate setae, two long spines and one small accessory seta; outermost geniculate seta fused basally to small seta. Exopod 2-segmented; first segment with one pinnate seta and row of spinules; second segment with three pinnate setae.

Labrum ( Fig. 4C). On outer side with row of thin setules and large proximal pore. Distal margin with lateral rows of ro- bust spinules, rows of fused spinules into comb and three rows of small spinules. On inner side medially with three unpaired pores, two pared pores, with lateral spinular row, groups of thin setules and spinules.

Mandible ( Fig. 4D). Coxa with proximal rows of spinules. Gnathobase with pars incisiva, lacinia mobilis, complex dental battery and spinulose seta; pars incisiva two-pointed; lacinia mobilis three-pointed. Dental battery ( Fig. 4E) consisting of five fused blocks of small short teeth, innermost one fused at base with seta. Pars molaris sharply-edged. Palp two-segmented; first segment with row of long spinules; second segment with one medial and four apical bare setae.

Paragnaths ( Fig. 4F) with paired lateral lobes and unpaired posterior rounded lobe. Lateral lobes extended in distal part; proximally with lateral pore (probably); with three groups of long spinules on outer side; distally with round group of spinules; with three-four rows of spinules on inner side; anterior side with three medial rows of strong spinules and proximal row of spinules.

Maxillule ( Fig. 5A). Praecoxa with two rows of slender spinules on outer edge and one row of spinules on posterior side. Praecoxal arthrite medially with two rows of spinules and one bare seta; distally with four strong spines with pectinate end, three biarticulate spines, one proximal thick pinnate seta and one thin seta with long spinules. Coxa with row of spinules, coxal endite with one geniculate and one pinnate setae. Basis with two subdistal setae and three distal setae, one of which geniculate. Endopod and exopod incorporated into basis; exo- pod represented by long pinnate and short bare setae; endopod represented by three bare setae.

Maxilla ( Fig. 5B). Basis with several rows of spinules on outer and inner edge as figured, with two endites. Proximal endite with spinular row, one spinulose spine and two pinnate setae, distal endite with one strong pinnate spine and two thin pinnate setae. Proximal endopodal segment with two setae, outer tube pore and massive distal claw. Distal endopodal segment with three naked setae.

Maxilliped ( Fig. 5C) subchelate. Syncoxa elongated with several rows of spinules as figured, distally with one pinnate seta. Basis with two longitudinal rows of large spinules on anterior and posterior sides and three transversal outer rows of small spinules. Endopod on posterior side with one seta, on anterior side with small protuberance, probably tube pore. Endopodal claw elongated, with row of small spinules.

Cuticular process between maxillipeds and P1 ( Fig. 4G) larger than tall, with long spinules, 10–13 long setules on each side.

P1–P4 well developed, setae/spine armature formula listed in Table 1.

P1 ( Fig. 6A). Praecoxa with outer spinular row. Coxa rectan- gular, with eight spinular rows, five of which consisting of little spinules. Intercoxal sclerite wide, with one paired spinular rows. Basis with proximal pore, medial row of small spinules, rows of spinules at base of endopod and exopod, row of spinules at base of inner seta, two inner rows of setules; with inner and outer strong spinulose spines. All endopodal and exopodal segments with longitudinal row of spinules on outer margin. Exopod three-segmented; first segment with one outer spinulose spine; second segment with inner pectinate seta and outer spinulose spine; third exopodal segment with two outer spinulose spines and two apical slender geniculate setae. Endopod two-segmented, longer than exopod; first endopodal segment reaching third of distal exopodal segment, with inner pectinate seta and inner spinular row; second endopodal segments proximally with one inner bare seta; distally with outer spinulose spine, apical long geniculate seta and inner bare seta.

P2 ( Fig. 7A). Praecoxa with row of small spinules. Coxa with one lateral row of large spinules and five rows of spinules on anterior side. Intercoxal sclerite with paired spinular rows. Basis with proximal pore, rows of spinules at base of endopod and exopod and row of inner spinules; with outer spine. All endopodal and exopodal segments with longitudinal row of spi- nules on outer margin. Exopod three-segmented; first exopodal segment with outer spinulose spine, with apical frill; second segment with outer spinulose spine, inner pectinate seta, inner setules and apical frill; third segment with three outer spinulose spines, two apical setae and one inner pectinate seta. Endopod two-segmented; first segment with inner seta; second segment on inner side with prominent border between ancestral segments, with outer spinulose spine, two apical pinnate setae and two inner pectinate setae.

P3 ( Fig. 7B). Praecoxa, coxa and basis similar to that in P2, but basis with outer seta instead of spine. Intercoxal sclerite without spinules. Exopod three-segmented; all segments with longitudinal row of spinules on outer margin; first exopodal segment with outer spinulose spine, apically with frill; second segment with outer spinulose spine, inner pectinate seta, inner setules and apical frill; third segment with three outer spinulose spines, two apical setae and two inner pectinate setae. Endopod two-segmented; first segment with inner seta; second segment on medial and posterior side with prominent border between ancestral segments, with outer spinules, outer spinulose spine, two apical pinnate setae and three inner pectinate setae.

P4 ( Fig. 7C). Praecoxa with spinular row. Coxa with one lateral row of large spinules and four rows of spinules on anterior side. Intercoxal sclerite without spinules. Basis with outer seta, proximal pore, rows of spinules at base of endopod and exopod. Exopod 3-segmented; first exopodal segment with outer naked spine, outer spinules, apically with frill; second segment with outer naked spine, outer spinules, inner pectinate seta, inner slender spinules and apical frill; third segment with three outer spinulose spines, two apical setae and two inner pectinate setae. Endopod two-segmented; first segment with inner seta; second segment with outer spinules, outer spinulose spine, two apical pinnate setae and two inner pectinate setae.

P5 ( Fig. 6B) with separate right and left baseoendopods. Baseoendopod reaching about half of exopodal segment; with four pores, two spinules at base of outer seta; outer seta of basis slender. Endopodal lobe with four long bipinnate setae and two short bipinnate setae V and VI on outer side; with small process between setae III and IV. Exopod with outer spinules; with inner pinnate seta, two apical bare setae, and two unipinnate outer setae.

Male. Sexual dimorphism expressed in the antennule, P2– P6, genital segmentation and ornamentation, shape of caudal rami. Cephalothorax and thoracic somites as in female.

P6 ( Fig. 8B) two asymmetric flaps fused to the somite, with three long naked setae. Differences from female in abdomen structure as follows ( Fig. 8): first abdominal somite free; first and second abdominal somites with spinular row encircling somite ventrally and laterally; anal somite with ventral spinules; caudal rami with seta IV not transformed.

Antennule ( Fig. 9) ten-segmented, haplocer with geniculation between segments 7 and 8. Segment 5 with large aestetasc fused at base with long seta, with one strong “caudate” seta. Segment 7 with articular plate, with one filiform seta, one small “caudate” seta and with two modified laminar setae. Segment 8 with proximal dentate plate and three strong modified laminar setae. Segment 10 with acrothek consisting of slender aestetasc and two setae. Armature formula: 1-[1],2-[9],3-[8],4-[2],5- [6+(1+ae)],6-[2],7-[2+2 modified],8-[3 modified],9-[1],10- [7+acr].

P2 as in female, except basis and endopod. Basis similar to that in female, but with small inner process. Endopod ( Fig. 10A) two-segmented, reaching third of distal exopodal seg- ment. First segment with outer spinules and inner pectinate seta. Second segment with notch on distal outer margin, outer spinules, two apical pinnate slender setae and two inner pectinate setae.

Praecoxa, coxa, intercoxal sclerite of P3 as in female. Basis as in female, but with small inner process ( Fig. 10B–C). Exopod as in female, but third segment with subapical tube pore ( Fig. 10D). Endopod ( Fig. 10B–C) three-segmented, reaching third of distal exopodal segment. First endopodal segment with strong bare seta. Second endopodal segment with small posterior seta and long apophysis with bifid tip; length of apophysis 98 µm, ratio between apophysis and third endopodal segment 2.28. Third segment with inner round protrusion; with two small inner setae, posterior pore and two apical setae. Inner apical seta displaced to posterior side, bipinnate, short, with wide base. Outer apical seta modified brush-like: evenly thick, with two rows of spinules distally and thin apex.

P4 ( Fig. 10E): praecoxa, coxa, intercoxal sclerite, basis, exopod as in female. Endopod two-segmented; reaching two thirds of second segment of exopod; first segment short un- armed; second segment with outer spinules, spinulose spine, outer apical spiniform spinulose seta, inner apical bipinnate seta and inner naked seta.

P5 ( Fig. 6C) right and left fused medially. Baseoendopod with four pairs of pores, outer spinular row and outer long slender seta; endopodal lobe with two strong spinulose apical spines, ratio between inner and outer spines 1.37. Exopod with one spinule on distal edge and spinules on outer side; with long inner pectinate seta with long setules, two subapical inner spinulose setae, two apical naked setae and outer spinulose seta.

VARIABILITY. The left P4 Exp3 of one of the females has only one inner seta. The right exopod P5 of the holotype has seven setae instead of five, the right endopodal lobe P5 of one of the females has only four setae instead of six. All these characters were found only once on one side of the body, and therefore can be considered as morphological abnormalities.

The ornamentation of the anal operculum is variable. In total, out of 12 individuals studied (10 females and 2 males), 4 individuals completely lack spinules; if spines are present, then their number ranges from three to seven (n = 8: 3, 5, 5, 5, 5, 6, 6, 7).

Variability in the ornamentation of the caudal rami. The holotype has both ventral and inner groups of spines on the rami (arrowed in Fig. 3A, C). However, all possible combinations occur: with both groups, with either one of them, and generally without ventral and inner spinules. Including one female may have different options on right and left rami. Of the examined 10 females, two caudal rami each (total 20 rami) showed the following variants: there are both groups present: 6 (30%); only the ventral group present: 8 (40%); only the inner group present: 2 (10%), both groups absent: 4 (20%).

ECOLOGY. The species has been found so far in the one locality, on the Frolikha hydrothermal seep at a depth of 409– 432 m. Bryocamptus (Rheocamptus) sitnikovae sp.n. is a dominant species of this habitat, since in four marked samples from this seep it was 94% of the abundance of Copepoda [Fe- filova et al., 2023].