Mesopsyllus glacialis Novikov & Sharafutdinova 2021, sp. nov.

Mesopsyllus glacialis sp. nov.

http://zoobank.org/ E7DCBD1A-6E2E-43A0-961E-4DB9A0339052

Etymology. The species is named so because it lives in the Arctic, and its juveniles are painted bluish. The specific epithet is adjective.

Type locality. Laptev sea (75.75833°N, 124.04500°E) in sample with sponges Polymastia grimaldii (Topsent, 1913); Tetilla sibirica (Fristedt, 1887). Depth 45 m.

Material examined. Holotype: female, dissected and mounted on 2 slides (BP 543/1-a; 543/1-b). Allotype, male dissected and mounted on one slide (543/2). Paratypes: two females dissected and mounted on two slides (543/3, 543/4), 5 females, 5 males and 17 copepodites undissected formalin-preserved (543/5).

Description. Female. Body length of holotype: 0.439 mm. Fixed males and females colourless, but copepodites with bluish color. Naupliar eye not discernible. Cephalothorax (Figs. 10A,B) consisting of cephalosome with fused first pedigerous somite, wider as remaining somites, largest width 0.114 mm. Rostrum as in male (Fig. 10C), fused with cephalothorax, with pointed tip; with one pair of sensilla and one pore. Posterior margin of cephalothorax and all somites smooth.

Cephalothorax with 19 pairs of sensilla and 5 pairs of pores. All pedigerous and abdominal somites, except anal somite, with rows of small spinules. Second pedigerous somite with 5 pairs of sensilla and one pair of pore. Third pedigerous somite with 6 pairs of sensilla and one pair of pores. Fourth pedigerous somite with 4 pairs of sensilla and one pair of pores. Fifth pedigerous somite with 4 pairs of sensilla, one pair of pores and dorsolateral rows of large spinules.

Abdomen (Figs. 11 A-C) almost cylindrical, consisting of genital-double somite, two free abdominal somites and anal somite with caudal rami. Genital-double somite with clear border between segments; with rows of spinules ventrally, medially and on posterior margin; with 7 pairs of pores, two dorsal unpaired pores and two ventral pairs of pores. P6 right and left close; with two slender setae. Genital field with sieves; copulatory pore width on posterior margin of first of two somites; copulatory duct elliptical without labyrinthic rounded duct; seminal receptacles located on sides of duct.

First free abdominal somite with three pairs of sensilla, one dorsal unpaired pore and one pair of ventral pores; on posterior margin with ventro-lateral rows of spinules and one latero-dorsal row of spinules. Second free abdominal somite with one pair of ventral pores; on posterior margin with ventro-lateral rows of spinules. Anal somite with one dorsal pair of sensilla, one dorsal, one ventral and two lateral pairs of pores; with spinules at base of caudal rami. Anal operculum short and wide; finely serrated; one ventral side with rows of long spinules.

Caudal rami (Figs. 11 A-C) elongated; length/width ratio 1.6, with two lateral pores; with rows of spinules at base of setae II, III and VII, with row of spinules ventrally at base of apical setae. Seta I very small, next to seta II. Apical setae IV and V (Fig. 10D) long, with spinules, length 0.137 mm and 0.252 mm respectively. Seta VII triarticulated, directed backward.

Antennule (Fig. 12A) 7-segmented. First segment with small seta and three rows of spinules. “Pineapple-setae” located on second, third and seventh segments (1, 2 and 1, respectively). Segment 4 with fused basally large aesthetasc and seta; segment 7 with acrothek consisting of aesthetasc and two bare setae. Armature formula: 1-[1],2-[9],3- [4],4-[1+(1+ae)],5-[1],6-[2],7-[7+acr].

Antenna (Fig. 13A) robust. Coxa with two spinular rows. Allobasis with clear border between basis and first endopodal segment; with one small seta and row of long spinules at its base. Free endopodal segment with two rows of large spinules and two spinulose spines; distally with two rows of spinules, apically with three robust spines and two pinnate geniculate setae. Exopod 1-segmented; with 2 setae distally and one small seta medially.

Mandible (Fig.12D). Coxa with spinules proximally. Gnathobase with strong pars incisiva, lacinia mobilis, spinulose seta and with few sharp teeth. Palp consisting of free basis and 1-segmented endopod. Basis with two rows of spinules, one inner and one outer exopodal “pineapple-setae”. Endopod with row of spinules, one inner “pineappleseta”, two apical bare setae and one apical outer “pineapple-seta”.

Maxillule (Fig. 13B). Praecoxa with outer spinular row. Praecoxal arthrite medially with two rows of spinules and two naked setae; distally with 9 spines. Coxa with outer spinular row; coxal endite reaching third of arthrite, with two bare setae. Basis with two rows of spinules and five bare setae. Endopod and exopod incorporated into basis, represented by two bare and one strong pinnate setae respectively.

Maxilla (Fig. 13C). Syncoxa with rows of spinules as figured, with two endites. Proximal endite with one simple seta, one stout, bluntlypointed, spinulose spine and with modified seta fused basally with endite with wide base and thin “tail”. Distal endite with one simple seta, one long spine and one modified seta as on proximal endite. Allobasis with one seta and massive distal claw. Endopod not separated, with one small seta proximally and two large setae distally.

Maxilliped (Fig. 14A) subchelate. Syncoxa with three rows of spinules and pinnate seta. Basis with one row of large inner spinules and two rows of outer long spinules. Endopod on anterior side with small seta and small protuberance. Endopodal claw elongated, with two pairs of long slender spinules.

P1 (Fig. 14B) with 3-segmented rami. Praecoxa with long spinular row. Coxa with inner fold, with spinular ornamentation as figured. Intercoxal sclerite wide with two pairs of spinular rows. Basis wide, on inner side with large elliptical protrusion; with proximal pore and four spinular rows; with robust inner seta and bare outer seta. All endopodal and exopodal segments with inner and outer spinules. First exopodal segment with outer spine. Second exopodal segment with outer spine and minute inner pinnate seta. Distal segment with two spines, one pectinate seta and one geniculate long seta. Endopod is approximately as long as exopod. First and second endopodal segments with minute inner setae. Third endopodal segment with outer and inner spines and apical seta.

P2 (Fig. 15A). Praecoxa with long spinular row. Coxa with pore, outer spinular row and with rows of spinules on frontal side. Intercoxal sclerite rectangular. Basis with proximal pore, small outer seta; spinular rows located on inner side and at base of endopod and exopod respectively; inner side of basis produced into sharp spinous process. All endopodal and exopodal segments with inner and outer spinules. First exopodal segment with outer spine. Second exopodal segment with outer spine and inner minute pinnate seta. Distal exopodal segment with short inner pinnate seta, two apical long setae and three outer spines; at base of second outer spine with pore. Endopod reaching proximal part of exopod. First endopodal segment with inner minute seta and outer pore; outer margin produced into sharp process. Second endopodal segment with one inner minute seta, one inner and two apical long setae and outer spine; outer side produced into spinous process medially.

P3 (Fig. 15B). Praecoxa, coxa, basis, first and second exopodal segments and first endopodal segment as in P2. Intercoxal sclerite trapezoidal. Third exopodal segment with two inner short setae, two long apical setae and three outer spines. Second endopodal segment with inner pectinate seta, two apical pinnate setae and outer spine.

P4 (Fig. 15C). Praecoxa, coxa, first and second exopodal segments and first endopodal segment almost as in P2. Basis with proximal pore and with two inner, one distal, one medial and one outer rows of spinules; inner margin without process. Third exopodal segment with three outer spines, two apical setae, one proximal inner seta and one distal strong pectinate seta. Endopod short, reaching half of second exopodal segment. Second endopodal segment with inner pectinate seta, two apical pinnate setae and outer spine.

Armature of swimming legs as follows:

P5 (Fig. 14C) of characteristic shape for genus, with separate right and left baseoendopods. Baseoendopod reaching beyond end of exopod, with long pinnate outer seta and proximal pore. Endopodal lobe with distal pore, with robust pectinate inner seta, one bipinnate apical seta, long curved and small unipinnate outer setae. Exopod small, with three long pinnate setae distally and one outer bare seta.

Male. Total body length from tip of rostrum to posterior margin of caudal rami: 0.372 mm. Sexual dimorphism expressed in the antennule, P2–P6, genital segmentation. Ornamentation of abdominal somites (Figs. 17 A-C) as in female, but first somite without ventral spinular rows. P6 fused with first abdominal somite, without setae. Caudal rami as in female.

Antennule (Figs. 12B,C) 9-segmented, haplocer with geniculation between segments 7 and 8. Segment 1 as in female. Segment 2 with “pineapple-seta”. Segment 5 with strong modified setae, large aesthetasc and few distal protrusions; one seta robust with large spinules and fine “tail”; one seta with wide short base and fine “tail”. Segment 6 with two basally fused setae. Segments 7-8 with articular surface. Segment 7 with two simple setae and two modified lamellar setae. Segment 8 with two modified mushroom-shaped lamellar setae and one distal seta. Segment 9 with acrothek. Armature formula: 1-[1],2-[9],3-[5],4-[1],5-[3+2 modified +(1+ae)],6-[2],7-[2+2 modified],8-[1+2 modified],9-[7+acr].

P2 (Fig. 17A) almost as in P 2 female, except for second endopodal segment. Outer spine of second endopodal segment in form of small tip.

P3 (Fig. 17B,C). Praecoxa, coxa, basis, first and second exopodal segments as in female. Third exopodal segment as in female, but with pore at base of middle outer spine. Endopod typical for Canthocamptidae. First segment modified, with pore, inner distal corner produced into protrusion, outer distal corner produced into spinous process; on distal margin with row of small tubercles; inner seta pinnate, long. Second segment split into two pseudosegments. First one with outer spinules, distal row of flat spinules, inner minute seta and long bifurcated claw-shaped apophysis. Second one with processes on anterior surface, inner pore and two apical pinnate setae.

P4 (Fig. 18A). Praecoxa, coxa, intercoxal sclerite and exopod as in female. First endopodal segment as in female, but inner seta long and bare. Second endopodal segment with two inner unipinnate setae, two apical bipinnate setae and outer bare spine.

P5 (Fig. 18B) with left and right baseoendopods fused medially. Baseoendopod with proximal pore, long pinnate seta; endopodal lobe with inner and outer rows of spinules, inner and distal pores, with outer unipinnate and inner bipinnate setae. Exopod small, with inner robust pinnate seta, three distal pinnate setae and outer small bare seta.

Ecology. Like H. spongiophilus sp. nov., M. glacialis sp. nov. is probably an optional inhabitant of sponges. The diet of related species has not been studied. However, the shape of the mouthparts, especially the mandible gnathobase, strongly resembles that of Metahuntemannia Smirnov, 1946 and Dahmsopottekina Özdikmen, 2009. The gnathobase has long, sharp teeth, which, according to Dahms & Pottek (1992), may indicate that the species is a predator or scavenger.

Remarks. According to Huys and Thistle (1989), Mesopsyllus is a basal genus of the subfamily Hemimesochrinae Por, 1986. This assumption was made on the basis of the modification of the outer seta of the male P3 endopodite. In most of their species, the subfamilies have modified setae, as in Bathycamptus Huys & Thistle, 1989, Isthmiocaris George & Schminke, 2003, and Dahmsopottekina Özdikmen, 2009 (Huys & Thistle 1989; Dahms & Pottek 1992; Bruch et al. 2011). In general, together with a new species and an undescribed new species from the Laptev Sea, the genus includes seven species. Mesopsyllus glacialis sp. nov., according to the majority of variable characters within the genus, turns out to be plesiomorphic. This is the first representative of the genus with a 7-segmented antennule; it has a complete set of setae on the distal segments of P2-P3 exopods (6, 7, 7) and a complete set of setae on endopod P2 (5). It also differs from other members of the genus in the unique character of sexual dimorphism of swimming legs. Thus, the second segment of the endopod P2 of the male has a modified shape and a greatly reduced outer seta in the form of a tip. Apophysis P3 bifurcated in the form of a claw. Therefore, on the basis of these characters, we assume that the new species is basal within the genus Mesopsyllus.

Unfortunately, only one female was found in the second of the discovered Mesopsyllus species in the Laptev Sea; we did not describe it since the character of sexual dimorphism of swimming legs is too important in the taxonomy of the subfamily. In the future, it will be possible to find a male of this species. However, undoubtedly, it is also of interest because of the highest reduction of the setae of swimming legs among all representatives of the genus and is close in this parameter to the genus Bathycamptus, but differs in the separated baseoendopod and exopod P5.

Spine formula of P2-P4 Mesopsyllus sp. 1

Key to the females of Mesopsyllus:

1 P2 Exp3 with 6 setae.................................................................................. 2

- P2 Exp3 with 5 setae.................................................................................. 5

2 P2 Enp2 with 5 setae, rostrum with smooth margin.......................................................... 3

- P2 Enp2 with 4 setae, rostrum with setulose margin.......................................................... 4

3 Antennule 7-segmented, P4 Exp3 with 7 setae, caudal rami long (l/w ratio 1.7). Laptev Sea*, Kara Sea*.................................................................................................... M. glacialis sp. nov.

- Antennule 6-segmented, P4 Exp3 with 6 setae, caudal rami long (l/w ratio 3.7). White Sea, Laptev Sea*................................................................................. M. curvisetus (Kornev & Chertoprud), 2008

4 P1 Endopod 3-segmented, P4 Exp3 with 6 setae. Black Sea, English Channel.................... M. atargatis Por, 1960

- P1 Endopod 2-segmented, P4 Exp3 with 7 setae. North Sea.............................. M. secundus (Wells, 1965)

5 P2 Enp2 with 5 setae, P4 Exp3 with 6 setae................................................................ 6

- P2 Enp2 with 4 setae, P4 Exp3 with 5 setae. Laptev Sea*............................................... M. sp. 1

6 P3 Enp2 with 5 setae, P4 Enp2 with 5 setae. Bohai Sea.............................. M. spiniferus Mu & Huys, 2017

- P3 Enp2 with 4 setae, P4 Enp2 with 4 setae. Bohai Sea.............................. M. dimorphus Mu & Huys, 2017

* Our data