Acontiophorus decamerus, Lee, 2025

Lee, Il-Hoi Kim and Taekjun, 2025, Ten new species of siphonostomatoid copepods (Crustacea) associated with marine invertebrates from Korea, Journal of Species Research 14 (2), pp. 146-181 : 147-150

Acontiophorus decamerus n. sp. ( Figs. 1 , 2 )

https://zoobank.org/urn:lsid:zoobank.org:act:81ECF01E-

F385-4006-96E6-37C9C19C78DE

Type material. Holotype (intact ♀; MABIK CR002577 85), intact paratypes (10 ♀♀; MABIK CR00257786 ) and dissected paratype (1 ♀) collected from sponge washings at Munseom , Seogwipo, Jeju Island (33°13 ʹ 39.4 ʺ N, 126° 33 ʹ 48.2 ʺ E), SCUBA, at a depth of 56 m, by Taekjun Lee, on 25 April 2023. The holotype and intact paratypes have been deposited at the National Marine Biodiversity Institute of Korea ( MABIK), Seocheon. The dissected paratype is maintained in the collection of I.-H. Kim.

Etymology. The name derives from the Greek deca (= ten) and mero (= part), referring to the 10-segmented antennule in the female.

Female. Body ( Fig. 1A ) narrowing from anterior to posterior. Body length of figured and described specimen 536 μm. Prosome 375 μm long, consisting of cephalothorax and second to fourth pedigerous somites. Cephalothorax 236 × 261 μm. Second pedigerous somite with angular posterolateral corners; other prosomal somites with rounded posterolateral corners. Urosome ( Fig. 1B ) 4-segmented. Fifth pedigerous somite 74 μm wide. Genital double-somite 0.81 times longer than wide (55 × 68 μm); genital apertures positioned dorsolaterally in middle of double-somite; lateral margins of genital region roundly protruded. Two free abdominal somites 25 × 42 and 27 × 40 μm, respectively. Anal somite ( Fig. 1D ) with pair of longitudinal rows of 5 or 6 thick setules on ventral surface; posterior margin unornamented. Caudal ramus ( Fig. 1C ) 2.19 times longer than wide (35 × 19 μm), armed with 6 setae, but no setules or spinules present on ventral surface and distal margin; setae II and VII positioned at distal quarter of ramus length; seta VI inserted on digitiform process.

Rostrum ( Fig. 1K ) small, strongly tapering. Antennule ( Fig. 1E ) short, 88 μm long, 10-segmented; articulation between 2 terminal segments incomplete, discernible only on one side; armature formula 2, 13, 7, 2, 7, 4, 1 + aesthetasc, 1, 3, and 8; setae densely arranged; aesthetasc on seventh segment large, longer than total of segments, with constriction at distal quarter. Antenna ( Fig. 1F ) consisting of coxa, basis, 1-segmented exopod, and 2-segmented endopod; coxa short and unarmed; basis unarmed but ornamented with 3 long setules at inner distal corner; exopod digitiform, 4.0 times longer than wide (24 × 6 μm), armed with 3 setae (1 proximal, 1 small subdistal, and 1 sparsely pinnate, large distal); first endopodal segment 19 μm long, unarmed and unornamented; second endopodal segment 24 μm long, armed with 5 setae (1 proximal, 1 large distal, 3 unequal subdistal) and ornamented subdistally with transverse row of several spinules.

Oral siphon ( Fig. 1G ) bendable, extending to posterior margin of genital double-somite, consisting of tapering proximal one-fifth and thin distal four-fifths. Mandible ( Fig. 1H ) consisting of thread-like stylet and short, 1-segmented palp tipped with 1 minute seta and 1 large, heavily pinnate seta. Maxillule ( Fig. 1I ) bilobed; smaller outer lobe with 1 small, naked and 3 pinnate setae; inner lobe with 5 unequal distal setae (2 pinnate setae, 2 equally long, naked setae, and 1 small naked seta). Maxilla ( Fig. 1J ) 2-segmented; proximal segment (syncoxa) unarmed; distal segment (basis) arched, about 1.5 times longer than proximal segment, ornamented with rows of spinules. Maxilliped ( Fig. 2A ) 5-segmented; first segment (syncoxa) 52 μm long, partially divided by faint line into precoxal and coxal regions, with 1 seta subdistally on inner margin and row of spinules on outer margin of coxal region; basis 47 μm long, unarmed, but with few spinules on outer margin; armature formula of 3 endopodal segments 2, 2, and 1 + claw; terminal claw arched, 44 μm long, about 2.8 times longer than terminal segment, ornamented with minute spinules along concave inner margin.

Legs 1-4 ( Fig. 2B- E ) biramous, with 3-segmented rami. Outer seta on basis small in legs 1-3, but large in leg 4. Inner distal spine on basis of leg 1 setiform, with minute spinules on margins. Second endopodal segment of legs 1-4 with bicuspid outer distal corner. Distal spine on third endopodal segment pinnate in leg 2 but serrate in legs 3 and 4. Armature formula for legs 1-4 as follows:

Coxa Basis Exopod Endopod

Leg 1 0-1 1-I I-1; I-1; III, 2, 3 0-1; 0-1; 1, 2, 3 Leg 2 0-1 1-0 I-1; I-1; III, I, 4 0-1; 0-2; 1, 1 + I, 3 Leg 3 0-1 1-0 I-1; I-1; III, I, 3 0-1; 0-2; 1, I, 3 Leg 4 0-1 1-0 I-1; I-1; III, I, 3 0-1; 0-2; 1, I, 2

Leg 5 ( Fig. 2F ) consisting of protopod and 1-segmented exopod; protopod not articulated from somite, wider than long, with 1 large outer and 1 small inner setae, both setae naked; exopodal segment 1.13 times longer than wide (17 × 15 μm), armed with 5 naked setae (1 outer subdistal, 2 distal, and 2 inner). Leg 6 ( Fig. 1B ) represented by 1 seta and 1 spinule on genital operculum.

Male. Unknown.

Remarks. Acontioporus decamerus n. sp. is most closely related to A. estivalis Lee, Chang & Kim, 2022 also found in Korean waters. Both species exhibit many similar morphological features, including proportional lengths of caudal rami and oral siphon, segmentation and ornamentation of the maxilliped, and identical armature formulae for legs 1 to 5. However, the new species is not conspecific with A. estivalis , due to several differences: (1) the body length of the female distinctly shorter, 536 μm versus 938 μm in A. estivalis as reported by Lee et al. (2022); (2) the female’s cephalothorax is wider than it is long, contrasting with the equally proportioned cephalothorax of A. estivalis ; (3) the genital double-somite of the new species features a centrally positioned genital aperture and the rounded posterolateral corners, whereas A. estivalis has its genital apertures at the anterior of the double-somite with angular posterolateral corners; (4) the new species has an unornamented caudal ramus compared to the ornamaneted caudal ramus of A. estivalis ; and (5) the female’s antennule is 10- segmented in the new species, in contrast to the 11-segmented antennule in A. estivalis .

Figures

Fig. 1. Acontiophorus decamerus n. sp., female.A, habitus, dorsal; B, urosome, dorsal; C, left caudal ramus, dorsal; D, anal somite and caudal rami, ventral; E, antennule; F, antenna; G, oral siphon; H, mandible; I, maxillule; J, maxilla; K, rostrum. Scale bars:A, 0.1mm; B-F, I, J, 0.02 mm; G, H, 0.05mm; K, 0.01mm.

Fig. 2. Acontiophorus decamerus n. sp., female.A, maxilliped; B, leg 1; C, leg 2; D, leg 3; E, leg 4; F, leg 5. Scale bars: 0.02mm.