Mesopsyllus dimorphus, Mu, Fang-hong & Huys, Rony, 2017

Mu, Fang-hong & Huys, Rony, 2017, New Mesopsyllus species from the Bohai Sea, China, re-evaluation of the validity of Vibriopsyllus Kornev & Chertoprud, 2008 and proposal of Sympodella gen. n. (Copepoda, Harpacticoida, Canthocamptidae), ZooKeys 718, pp. 1-33 : 2

Mesopsyllus dimorphus sp. n. Figures 2, 3, 4, 5, 6, 7

Type locality.

Eastern China, Strait of the Bohai Sea, sampling locality D5 (38°15'N, 121°15'E); 37.0 m depth; very fine sand (Fig. 1; Table 1).

Material examined. Holotype: adult ♂ dissected on 13 slides (NHMUK reg. no. 2013.1033). Paratypes are 1 ♀ dissected on 17 slides (NHMUK reg. no. 2013.1034), and 10 ♀♀ and 2 ♂♂ preserved in ethanol (NHMUK reg. nos 2013.1035-1044); all paratypes were collected from the type locality. Additional material was collected from stations A1 (37°44'N, 121°35'E), A2 (38°N, 121°35'E), A4 (38°25'N, 121°35'E) and D3 (38°15'N, 119°44'E) in the central part and the strait of the Bohai Sea, eastern China (Fig. 1; Table 1). Collected by F.-h. Mu and Y.-q. Guo in September 1998.

Description of male.

Body length 220-280 µm (n = 3, mean = 250 µm). Body slightly tapering posteriorly as in ♀ (compare Fig. 7 B–C). P1-bearing somite fused with cephalothorax. Pleural margins of cephalic shield furnished with long hair-like setules (as shown for female in Fig. 7B). Body covered with pattern of minute pimples (not figured). Hyaline frills plain (as shown for female in Fig. 7 B–C). Posterior margin of anal operculum straight and with fine denticles (as figured for ♀ in Fig. 2H); anus terminal.

Body ornamentation (Fig. 2 A–C). All somites with integumental sensilla, except for penultimate one. Pores present on all somites (positions on urosomites indicated in Fig. 2 A–C). Prosome without spinular ornamentation. Pattern of spinular rows on urosome as follows: urosomite-1 with short paired dorsal rows; urosomite-2 with pairs of short rows dorsally and dorsolaterally; urosomite-3 with a pair of short rows dorsally and a long, continuous row stretching ventrally and laterally; urosomite-4 without dorsal spinules but with a long, continuous row ventrally and laterally; urosomite-5 without dorsal spinules but with an interrupted row ventrally and laterally; anal somite with lateral and ventral spinules at base of caudal rami.

Rostrum (Fig. 2F) not defined at base; triangular with a round apex; with a pair of lateral sensilla subapically and a median pore dorsally.

Antennule (Fig. 3 A–B) 8-segmented, haplocer with geniculation located between segments 6 and 7. Segment 1 with spinules along anterior and subdistal margins and one minute seta. Segment 2 with two stout spinulose spines and six smooth setae. Segment 3 with one stout spinulose spine, two long and three minute, naked setae. Segment 4 moderately swollen, with one stout pinnate spine, three short, naked setae (one of which arising from minute cylindrical process) and one small spiniform process near anterior distal corner; ventral surface of segment 4 with a sub-cylindrical setophore carrying one slender seta and one large aesthetasc. Segment 5 with two naked setae. Segment 6 with two slender setae and two conical elements (modified setae). Segment 7 with three conical elements and one anterodistal seta. Segment 8 with seven naked setae and apical acrothek consisting of two setae and short aesthetasc.

Antenna (Fig. 4A). Coxa well developed, bearing row of spinules. Allobasis without trace of original segmentation; with row of spinules and short smooth seta in proximal half of abexopodal margin. Exopod 1-segmented, about twice as long as wide; with two apical naked setae. Free endopod 1-segmented, bearing two surface rows of stout spinules and two stout pinnate spines along lateral margin; apical armature consisting of five pinnate spines; outer distal corner with few spinules.

Mandible (Fig. 4B). Gnathobase with strong teeth and unipinnate seta at dorsal corner, with spinular ornamentation as illustrated. Palp consisting of basis, 1-segmented endopod and vestigial exopod. Basis short, with few spinules and strong pinnate spine near inner distal corner. Endopod with one pinnate inner spine; apical margin with one pinnate spine, two naked setae and transverse spinular row. Exopod represented by a short pinnate spine.

Maxillule (Fig. 4C). Praecoxa with well-developed arthrite bearing two setae and two spinular rows on anterior surface, and nine spines along distal margin. Coxa with long spinules along outer margin; with cylindrical endite bearing two apical setae. Basis and rami fused, forming elongate palp; with spinules on inner and outer margins as figured; basal armature presumably consisting of five naked setae; endopod represented by small cylindrical outgrowth with two distal setae; exopod represented by one long plumose seta.

Maxilla (Fig. 4D). Syncoxa with three rows of spinules, a row of setules and two (coxal) endites; proximal endite with a fused spinulose process, one spinulose spine and one short naked seta; distal endite with one spinulose spine, one naked spine and one naked seta. Allobasis drawn into slightly curved claw, bearing few spinules near apex and naked seta halfway down the claw. Exopod a minute segment with three naked setae.

Maxilliped (Fig. 4E) subchelate. Syncoxa with row of small spinules near base and naked seta at inner distal corner. Basis with spinular row along most of palmar (inner) margin and few spinules halfway along outer margin; unarmed. Endopod represented by a strong, acutely curved claw, spinulose along inner distal half and with one minute seta near base.

Swimming legs with 3-segmented exopods and 3- (P1, P3) or 2-segmented endopods (P2, P4).

P1 (Fig. 5A). Praecoxa (not illustrated) a well-developed U-shaped. Coxa with two rows of long spinules and additional small spinules on anterior surface as figured; outer distal corner produced into a round bulge, bearing spinules posteriorly. Basis bearing short outer seta (indicated by arrow in inset of Fig. 5A) and stout bipinnate inner spine; anterior surface with pore and three spinule rows; additional spinules along inner margin. Exopodal and endopodal segments with spinules along outer and distal margins, and with sparse setules along inner margin (except exp-3). Exp-1 with pinnate outer spine; exp-2 with plumose inner seta and pinnate outer spine; exp-3 with two outer and one apical pinnate spines, and one subdistal plumose seta. Enp-1 and enp-2 each with plumose inner seta and small spinous process at outer distal corner; enp-3 with plumose inner seta subdistally and two pinnate elements apically.

P2 (Fig. 5B). Praecoxa (not illustrated) a well-developed U-shaped sclerite with spinules along its distal margin. Coxa with a row of long spinules along outer margin and few long setules near outer distal corner; anterior surface with a pore and rows of tiny spinules as figured. Basis with short outer seta; with spinules along inner, distal and outer margins; inner margin also with hair-like setules; with pore on anterior surface; inner distal corner produced into sharp spinous process; distal margin between exopod and endopod with spinous process. Exopodal segments with spinules along outer margin; exp-1 and -2 with setules along inner margin and spinous process at outer distal corner; exp-2 with plumose inner seta and pinnate outer spine; exp-3 with one plumose inner seta, two pinnate spines (with setules on inner and spinules on outer margin) distally and two pinnate outer spines; exp-3 with pore on anterior surface and spinules along distal margin. Endopodal segments with spinules along outer, inner and distal margins; enp-1 with short, plumose inner seta and spinous process at outer distal corner; enp-2 with one short, plumose inner seta, one plumose seta subdistally, one long pinnate spiniform seta distally and one unipinnate outer spine; outer margin of enp-2 with small spike halfway down the segment length, possibly indicating ancestral segmentation boundary. Intersegmental hyaline frills of segments well developed, serrate.

P3 (Fig. 6A). Praecoxa a well-developed U-shaped sclerite with spinules along its distal margin. Coxa and basis as in P2, except for presence of setules along inner margin of basis. Proximal and middle exopodal segments as in P2; distal segment with two plumose inner setae, two pinnate spines apically and two pinnate outer spines. Endopod 3-segmented; enp-1 and -2 with spinules along outer and inner margins; enp-1 with long, plumose inner seta, a row of short spinules along distal margin, and a spinous process at outer distal corner; enp-2 with one short, plumose inner seta; distal margin of enp-2 with outer spinous process and long anterior apophysis extending beyond enp-3; enp-3 (pseudosegment originating from secondary subdivision of ancestral enp-2) with two plumose setae apically.

P4 (Fig. 6B). Praecoxa, coxa and basis as in P2-P3, except spinous process at inner distal corner of basis absent; inner margin of basis without setular ornamentation. Exopod as in P3. Endopod short, reaching just beyond distal margin of exp-1; enp-1 with plumose inner seta and few spinules along outer and inner margins; enp-2 with one plumose inner seta, two plumose setae apically and one short, pinnate outer spine; with sparse spinular ornamentation along outer margin and pore on anterior surface.

P5 (Fig. 2E). Baseoendopods of fifth pair of legs fused medially forming deeply incised plate. Baseoendopod and exopod not fused, the former with outer basal seta. Endopodal lobe conical, reaching to apical margin of exopod; with one plumose inner spine and one short, naked outer seta; with sparse spinules along outer and inner margins. Exopod small, slightly longer than wide; with four elements: one plumose inner seta, two pinnate apical setae and one small, naked outer seta.

P6 (Fig. 2C). Fused to genital somite; represented by a median lobe without armature.

Caudal ramus (Fig. 2D). About 3.1 times as long as maximum width; with long spinules around insertion sites of setae IV–VII and short spinules around base of seta II. Ventral surface with pore near seta III and tube-pore near distal outer corner. Armature consisting of six setae (seta I apparently absent); setae II–III slender and bare, positioned along distal half of outer margin; seta IV–V well developed, pinnate; seta V about twice as long as seta VI and about half as long as the body length; seta VI small and naked; seta VII tri-articulated at base, laterally displaced and inserting near distal inner corner.

Description of female.

Body length 240-330 µm (n = 10, mean = 292 µm). General body shape (Fig. 7 B–C) as in male. Body covered with pattern of minute pimples (not figured). Sexual dimorphism in antennule, P2-P6, and urosomal segmentation and ornamentation.

Urosome (Fig. 7 A–C). Genital and first abdominal somites fused forming genital double-somite; original segmentation marked by internal, transverse chitinous ribs laterodorsally, laterally and ventrally. Spinular ornamentation as follows: urosomite-1 with short paired dorsolateral rows; genital double-somite with short, paired lateral rows in anterior half (urosomite-2) and paired lateral rows extending laterodorsally and lateroventrally in posterior half (urosomite-3); urosomite-4 and urosomite-5 each with a long row, extending ventrally and ventrolaterally; anal somite with lateral and ventral rows around bases of caudal rami. Gonopores (Fig. 7D) fused, forming common median genital slit. P6 represented by two minute setae. Copulatory pore large, located in centre of genital double-somite; anterior half of genital double-somite with paired rows of minute spinules either side of genital slit.

Antennule (Fig. 3 C–D) short, 6-segmented. Segment 1 with two spinule rows and one minute seta; segment 2 with eight naked setae (two with bi-articulated base) and one spinulose spine; segment 3 with two stout spinulose spines and two slender setae (one with bi-articulated base); segment 4 with large aesthetasc fused basally to short seta and one slender bi-articulated seta; segment 5 with few spinules and one bi-articulated naked seta along anterior margin; distal segment with one stout spinulose spine, nine naked setae (four bi-articulated at base) and apical acrothek consisting of slender seta and short aesthetasc.

P2 (Fig. 5C). Coxa, basis and exopod as in ♂. Endopod 2-segmented. Enp-1 with one minute inner seta (indicated by arrow) and spinules along outer, distal and inner margins; outer distal corner produced into spinous process. Enp-2 with spinules along inner and outer margins and a sharp hook halfway along outer margin (indicating ancestral segmentation); inner margin with two setae, proximal one (homologue of inner seta in ♂) minute (indicated by arrow); armature around distal margin as in ♂ except for plumose inner distal seta distinctly shorter.

P3 (Fig. 6 C–D). Coxa, basis and first two segments of exopod as in ♂. Exp-3 with two plumose inner setae as in ♂ but distal one markedly shorter. Endopod 2-segmented, with spinules along inner and outer margins of both segments. Enp-1 with minute inner seta (indicated by arrow) and outer distal corner produced into spinous process. Enp-2 with one plumose inner seta, two plumose distal setae and one pinnate outer spine; distal margin with small spinous process; anterior surface with pore.

P4 (Fig. 6 E–F). Coxa and first two segments of exopod as in ♂. Basis inner distal corner and distal margin between exopod and endopod with spinous process. Exp-3 (Fig. 6F) with two inner setae; proximal one considerably shorter than in ♂, distal one long and thick, unipinnate (rather than plumose as in ♂) in distal half. Endopod (Fig. 6E) 2-segmented, with spinules along outer margin of both segments and inner margin of enp-1. Enp-1 with small, plumose inner seta and spinous process at outer distal corner. Enp-2 with one plumose inner seta, two plumose distal setae and one pinnate outer spine, the latter much longer than in ♂; anterior surface with pore.

Seta and spine formulae of P1-P4 as follows:

Formulae in parentheses denote female condition.

P5 (Fig. 2G). Fifth pair of legs not fused medially. Baseoendopod and exopod discrete. Endopodal lobe trapezoid with stepped distal margin; with spinules as figured and pore on anterior surface; armature consisting of four elements: innermost two spiniform, serrate and subequal in length, 3rd one very long, pinnate and typically bent medially, and innermost one (indicated by arrow) minute, naked and setiform. Exopod small, longer than wide; with three setae: outermost one (indicated by arrow) minute and naked, middle one longest and unipinnate, and outermost short and naked.

Variability.

One female specimen shows an asymmetrical armature pattern on P4 exp-3, having one inner seta on one side and two on the other.

Etymology.

The species name is derived from the Greek dis, meaning twice, and morphe, meaning form, and alludes to the sexual dimorphism on P2-P4.