Hemicyclops rapax, Lee & Chang & Kim, 2022

Lee, Jimin, Chang, Cheon Young & Kim, Il-Hoi, 2022, Symbiotic copepods (Cyclopoida and Siphonostomatoida) collected by light trap from Korea, ZooKeys 1115, pp. 1-71 : 1

Hemicyclops rapax sp. nov.

Figs 3 , 4 , 5 , 6

Material examined.

Holotype ♀ (MABIK CR00250118) and paratype ♀ (MABIK CR00250119) preserved in 90% alcohol, Site 22 (Yesong, Bogil Island, south coast, 34°08'11"N, 126°33'48"E), 31 May 2021, leg. J. Lee; paratype ♂ (MABIK CR00250123, figured) dissected and mounted on a slide, Site 22, 26 April 2021, leg. J. Lee and C. Y. Chang; 1 ♀ preserved in 90% alcohol and 1 ♀ (figured) dissected and mounted on a slide, Site 11 (Yeongdo, Pusan, 35°04'31.0"N, 129°05'08.7"E), 07 Jul. 2020, leg. J. G. Kim.

Description.

Female. Body (Fig. 3A ) moderately stout, dorsoventrally flattened. Body length 1.10 mm in dissected and figured specimen (1.22 mm in holotype). Prosome 680 × 555 μm, ~ 56% as long as body length. Posterolateral corners of all prosomal somites pointed or angular. Urosome (Fig. 3B ) five-segmented. Fifth pedigerous somite 234 μm wide, slightly wider than genital double-somite. Genital double-somite subcircular, flattened ventrally, thin laterally, slightly wider than long (220 × 227 μm) in dissected specimen or slightly longer than wide in smaller other specimens; lateral margin with small denticle (this denticle absent smaller specimens) as indicated by arrowhead in Fig. 3B ; genital apertures positioned dorsolaterally at anterior part of double-somite. Three abdominal somites 84 × 136 μm, 61 × 127 μm, and 45 × 114 μm, respectively. Genital double-somite and first two free abdominal somites with membranous fringe along posterior margin. Anal somite with row of spinules along posteroventral margin. Caudal ramus (Fig. 3C ) short, 1.15 × longer than wide (55 × 48 μm), slightly longer than anal somite, armed with six setae (setae II-VII), and ornamented with setules on distal half of inner margin and fine spinules along posteroventral margin; seta II (outer lateral seta) positioned at midlength of ramus, spiniform in proximal half but setiform in distal half; seta III (outer distal seta) also proximally spiniform and distally setiform; dorsal seta (seta VII) annulated proximally; setae II, III, and VII naked, but other three setae pinnate.

Rostrum small, with convex posterior margin. Antennule (Fig. 3D ) 237 μm long, seven-segmented; armature formula 4, 14, 6, 3, 4+aesthetasc, 2+aesthetasc, and 7+aesthetasc; first segment ornamented with fines setules on anterior surface; setae and aesthetascs slender; most setae naked except single on each fourth and fifth segments and three on terminal segment. Antenna (Fig. 3E ) consisting of coxa, basis and three-segmented endopod; armature formula 0, 1, 1, 4, and 7; basis ornamented with two patches of spinules on inner margin and several setules on outer margin; first endopodal segment with acutely pointed outer distal corner and patch of spinules on inner surface and patch of small spinules at inner distal corner; second endopodal segment with prominent inner distal prolongation (this prolongation distinctly longer than wide; its two apical setae strong, claw-like), ornamented with broad spinules along inner margin and row of small spinules near outer distal corner; third endopodal segment as long as wide, ornamented with two rows of minute setules on outer side; four of seven setae on third endopodal segment claw-like, wrinkled in middle.

Labrum (Fig. 4A ) with denticles and spinules on posterior margin. Labium (Fig. 4B ) denticulated along anterior margin, spinulose subapically. Mandible (Fig. 3F ) distally armed with one stout, denticulate element, one spinulose, plate-like element and two pinnate setae. Paragnath (Fig. 3G ) lobate, ornamented with fine setules on middle and subdistal regions and spinules on distal region, with trace of articulation subdistally. Maxillule (Fig. 3H ) unequally bilobed distally; smaller inner lobe armed with three weakly pinnate setae; larger outer lobe with five pinnate setae. Maxilla (Fig. 4C ) two-segmented; proximal segment (syncoxa) armed with three setae, smallest one setule-like, inserted on proximal region of spiniform largest seta, and ornamented with row of minute spinules on proximal region; distal segment (basis) distally armed with three heavily spinulose or denticulate spines and single seta. Maxilliped (Fig. 5A ) four-segmented; first segment (syncoxa) with two large setae on inner margin; second segment (basis) also with two large setae on inner margin; third segment (first endopodal segment) short, unarmed; terminal segment (second endopodal segment) forming large hook (this hook much longer than proximal three segments), proximally armed with one spine bearing seven spinules on outer margin and four small, naked setae (Fig. 5B ).

Legs 1-4 biramous, with three-segmented rami (Fig. 4D-F ); basis spinulose along outer margin and posterior margin between bases of rami. Intercoxal plate setulose in leg 1 (Fig. 4D ) but spinulose in legs 2-4 (Fig. 4E, F ). Outer spines on exopod of leg 1 tipped with setule. Leg 3 armed and shaped as leg 2. Two inner setae on third endopodal segment of leg 4 stiff, spiniform. Armature formula for legs 1-4 as follows:

Leg 5 (Fig. 5C ) two-segmented; proximal segment (protopod) with one slender outer seta and row of spinules at outer distal region; distal segment (exopod) 1.95 × longer than wide (86 × 44 μm) densely ornamented with spinules along outer and inner margins, armed with three spines and one weakly pinnate seta; lengths of three spines 54, 50, and 56 μm respectively from outer to inner. Leg 6 invisible.

Male. Body (Fig. 6A ) slightly larger than that of female. Body length 1.28 mm. Cephalothorax distinctly broader than next somites. Posterolateral corners of all prosomal somites blunt or rounded. Urosome (Fig. 6B ) six-segmented. Fifth pedigerous somite with membranous flap on each side of posterodorsal margin. Genital somite much wider than long (152 × 255 μm), with finely serrate posterodorsal corners and single spine on genital operculum. All urosomal somites lacking membranous fringe on posterior margin. Four abdominal somites gradually shorter from proximal to distal. Caudal ramus 1.04 × longer than wide (50 × 48 μm).

Rostrum as in female. Antennule different from that of female in having one additional seta at proximal anterior margin of fourth segment (thus with 4 setae on this segment). Antenna, labrum, mandible, paragnath, maxillule as in female. Maxilla different from that of female; basis armed with two spines and one seta and terminating in stout, claw-like process bearing spinules on outer margin and granule-like papillae on distal region (Fig. 6C ). Maxilliped (Fig. 6D ) four-segmented; first segment (syncoxa) with one large, distally pinnate seta; second segment (basis) strongly tapering distally, armed with one pinnate seta, ornamented with three rows of denticles along inner margin; small third segment (first endopodal segment) unarmed; terminal segment as hook bearing two unequal setae proximally.

Leg 1 different from that of female in absence of inner distal element on basis (Fig. 6E ). Legs 2-4 as in female. Leg 5 consisting of one naked dorsolateral seta on somite (protopod completely fused with somite) and exopod; exopod shaped and armed as in female. Leg 6 represented by single spine on genital operculum (Fig. 6B ).

Etymology.

The specific name rapax is derived from the Latin rapa (grasping), alluding to the grasping form of the female maxilliped.

Remarks.

Hemicyclops rapax sp. nov. is characterized by its peculiar female maxilliped, in which the terminal segment is transformed to a large hook, as in the males of existing species. This form of female maxilliped is very unusual for the genus, since the terminal segment (second endopodal segment) of the female maxilliped in other species of the genus generally terminates in a spiniform process (or a spine). The only other example of this peculiar female maxilliped in Hemicyclops is that of H. cylindraceus (Pelseneer, 1929), as illustrated by Stock (1954), although the terminal hook is much less developed in the latter species. Otherwise, H. cylindraceus differs from H. rapax sp. nov. in having a narrow, almost cylindrical body, five setae on the first antennular segment, four setae (without spines) on the exopod of leg 5, and an inner distal element on the basis of leg 1 ( Stock 1954).

Hemicyclops rapax sp. nov. may be differentiated from its congeners in other ways. In eight species in Hemicyclops the caudal ramus is short, less than 1.5 × longer than wide in the female, as in H. rapax sp. nov. In five of these eight species ( H. apiculus Humes, 1995, H. australis Nicholls, 1944, H. intermedius Ummerkutty, 1962, H. parapiculus Kim & Hong, 2014, and H. vicinalis Humes, 1995), the genital double-somite of the female is distinctly longer than wide (more than 1.2 × longer than wide); in H. tamilensis (Thompson & T. Scott, 1903) the urosome of the female is six-segmented and the exopod of leg 5 is elongated; in H. saxatilis Ho & Kim, 1992 the basis of male leg 1 bears an inner distal spine, the first segment of the male maxilliped is armed with two (rather than one) setae, and the maxilla is not sexually dimorphic. In the remaining species, H. leggii (Thompson & T. Scott, 1903) described based on the male, the third endopodal segment is armed with five armature elements (I, 4, rather than I, 5), the basis of male leg 1 is armed with an inner distal spine, and the first segment of male maxilliped is armed with two setae. These differences are considered sufficient to distinguish the new species from these eight congeners.

Figures

Figure 3. Hemicyclops rapax sp. nov., female A habitus, dorsal B urosome, dorsal (arrowhead indicates a small denticle on lateral margin of genital double-somite) C anal somite and caudal rami, dorsal D antennule E antenna F mandible G paragnath H maxillule. Scale bars: 0.2 mm (A); 0.1 mm (B); 0.05 mm (C, D); 0.02 mm (E-H).

Figure 4. Hemicyclops rapax sp. nov., female A labrum B labium C maxilla D leg 1 E leg 2 F leg 4. Scale bars: 0.02 mm (A-C); 0.05 mm (D-F).

Figure 5. Hemicyclops rapax sp. nov., female A maxilliped B distal part of maxilliped C leg 5. Scale bars: 0.02 mm (A, B); 0.05 mm (C).

Figure 6. Hemicyclops rapax sp. nov., male A habitus, dorsal B urosome, dorsal C distal segment of maxilla D maxilliped E inner proximal region of leg 1. Scale bars: 0.2 mm (A); 0.1 mm (B); 0.02 mm (C); 0.05 mm (D, E).