Acontiophorus acuminatus, 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 : 153-156

Acontiophorus acuminatus n. sp. ( Figs. 5 , 6 )

https://zoobank.org/urn:lsid:zoobank.org:act:E4B7321E-

4F22-4C78-A40B-FB6E001F2617

Type material. Holotype (intact ♀, HNIBR IV7753 ), intact paratype (♀, HNIBR IV7754 ), and dissected paratype (♀) from washings of mixed species of sponges, near Biyang-do, Jeju Island (33°24 ʹ 17.0 ʺ N, 126°12 ʹ 16.5 ʺ E), SCUBA, depth 5-10 m, collected by Tae Won Jung and Jong Guk Kim, on 13 June 2023. Both the holotype and intact paratype have been deposited in the Honam National Institute of Biological Resources ( HNIBR), Mokpo. The dissected paratype is maintained in the collection of I.-H. Kim.

Etymology. The specific name of the new species is from the Latin acumin (= a point), alluding to the sharply pointed posterolateral corners of the genital double-somite and first free abdominal somite.

Female. Body ( Fig. 5A ) moderately broad. Body length 770 μm in dissected and figured specimen. Prosome 524 μm long, consisting of cephalothorax and second to fourth pedigerous somites. Cephalothorax 378 × 415 μm, with pointed posterolateral corners. Second pedigerous somite with acutely pointed posterolateral corners. Third and fourth pedigerous somites with nipple-shaped posterolateral corners. Urosome ( Fig. 5B ) 4-segmented. Fifth pedigerous somite 121 μm wide. Genital double-somite slightly wider than long (100 × 105 μm), consisting of slightly expanded anterior third and narrower posterior two-thirds bearing parallel lateral margins; genital apertures located dorsolaterally at posterior region of anterior expansion. Genital double-somite and first free abdominal somites bearing acutely pointed posterolateral corners. Anal somite with minute spinules along posteroventral margin. Caudal ramus ( Fig. 5C ) short, 1.10 times longer than wide (32 × 29 μm), armed with 6 setae and ornament- ed with several spinules on lateral surface, 4 transverse rows of fine spinules on mediodorsal surface, and row of fine spinules along posteroventral margin; seta VI tipped on digitiform process.

Rostrum absent. Antennule ( Fig. 5D ) 212 μm long, 16-segmented; armature formula 2, 2, 11, 7, 2, 2, 2, 2, 2, 2, 2, 1, 1 + aesthetasc, 2, 4, and 7; aesthetasc on thirteenth segment half as long as total segments. Antenna ( Fig. 5E ) slender; coxa unarmed; basis elongate, 4.9 times longer than wide (68 × 14 μm long), unarmed; exopodal segment 3.86 times longer than wide (27× 7 μm), as long as first endopodal segment, armed with 1 naked seta in middle and distally with 1 small, naked and 1 large, pinnate setae; first endopodal segment 27 μm long, unarmed; second endopodal segment 31 μm long, armed with 1 proximal pinnate seta, 1 small, pinnate subdistal seta, 1 naked distal seta, and 1 large spiniform distal seta of 93 μm long.

Oral siphon ( Fig. 5F ) bendable, extending to posterior margin of genital double-somite, consisting of tapering proximal quarter and thin distal three quarters. Mandible ( Fig. 5G ) as in A. enneamerus n. sp. Maxillule bilobed; outer lobe ( Fig. 5I ) bearing 3 pinnate setae; inner lobe ( Fig. 5H ) with protruded medial margin bearing fine spinules, armed distally with 2 large naked setae, 2 large pinnate setae and 1 small naked seta. Maxilla ( Fig. 5J ) slender; proximal segment unarmed; distal segment arched, longer than proximal segment, characteristically bearing 1 large naked seta at proximal third, followed by row of minute spinules along concave margin. Maxilliped ( Fig. 6A ) slen- der, 6-segmented, but articulation between proximal 2 segments incomplete, discernible only on one side; armature formula 0, 1, 1, 2, 1, 1 + claw; basis 114 μm long, ornamented with fine spinules along outer margin, its inner seta small, positioned at proximal third of segment length; terminal segment 32 μm long; terminal claw 77 μm long.

Legs 1-4 ( Fig. 6B- E ) segmented and armed as in A. decamerus n. sp.; outer seta on basis pinnate, well-developed. Leg 5 ( Fig. 6F ) consisting of protopod and free exopod; protopod completely fused with somite, armed only with 1 pinnate outer distal seta, ornamented with 3 rows of minute spinules on serrate inner margin; inner distal corner of protopod acutely projected; exopodal segment 1.72 times longer than wide, armed with 5 pinnate setae (1 outer, 2 distal, and 2 inner) and ornamented with several spinules on outer and inner margins. Leg 6 ( Fig. 6G ) represented by 1 pinnate and 1 smaller naked setae and 1 spinule on genital operculum.

Male. Unknown.

Remarks. Existing species of Acontiophorus can be classified into two groups. The first group consists of eight species including A. scutatus (Brady & Robertson, 1873) , while the second group also comprises eight species including A. ornatus (Brady & Robertson, 1876) and A. acuminatus n. sp. In the “ ornatus group”, the second segment (basis) of the maxilla bears a large seta, the caudal ramus is relatively short, typically not exceeding 1.1 times its width, and the inner margin of the protopod of leg 5 is serrate. In contrast, in the “ scutatus group”, there is no seta on the second segment of the maxilla, the caudal ramus is longer, at least 1.6 times its width, and the inner margin of the protopod of leg 5 is smooth.

The segmentation of the antennule appears to be a reliable taxonomic character in Acontiophorus . The female antennule of existing species is nine-segmented in three species, ten-segmented in two species, 11-segmented in four species, 13-segmented in two species, and 16-segmented in four congeners. Acontiophorus acuminatus n. sp. has a 16-segmented female antennule, aligning it with four other species, namely, A. armatus Brady, 1880 , A. brevifurcatus Stock, 1966 , A. maldivensis Sewell, 1949 , and A. ornatus . All these five species pertain to the ornatus group. Despite the incomplete descriptions of most ornatus group species, they can be distinguished from A. acuminatus n. sp. based on distinct differences.

In A. armatus , the body is significantly larger, nearly twice as long as that of A. acuminatus n. sp., measuring 1.5 mm in females according to its original description by Brady (1880). The maxillule bears three “pinnate” setae on the inner lobe and two setae on the outer lobe. In A. brevifucatus , the cephalothorax and the second pedigerous somite have blunt or rounded posterolateral corners. The caudal ramus is wider than long, the second endopodal segment of legs 1, 3 and 4 features a bicuspid outer distal corner, the outer lobe of the maxillule has 1 pinnate and 2 naked setae, and the exopod of leg 5 is armed with four setae, as Stock (1966) described or illustrated. In A. maldivensis , the genital double-somite and the first free abdominal somite exhibit blunt posterolateral corners. The second segment (basis) of the antenna is three times longer than it is wide, according to Sewell (1949) (compared to approximately five times in A. acuminatus n. sp.). The second endopodal segment of the antenna is shorter than the first (contrary to the reverse condition in A. acuminatus n. sp.), and the exopod of leg 5 is armed with 4 setae. Acontiophorus acuminatus n. sp. more closely resembles A. ornatus than other species. Sars (1921) illustrated the more extensively studied the European species A. ornatus , which has a genital double-somite longer than it is wide, an exopodal segment of the antenna shorter than the first endopodal segment, a distal seta on the exopod of the antenna significantly shorter than that of the new species, extending only slightly beyond the distal margin of the second segment, and the protopod of leg 5 is articulated from the fifth pedigerous somite. These distinctions establish A. acuminatus as a new species.

Figures

Fig. 5. Acontiophorus acuminatus n. sp., female.A, habitus, dorsal; B, urosome, dorsal; C, left caudal ramus, dorsal; D, antennule; E, antenna; F, oral siphon; G, mandible; H, inner lobe of maxillule; I, outer lobe of maxillule; J, maxilla. Scale bars:A, 0.1mm; B, F, G, J, 0.05mm; C-E, H, I, 0.02mm.

Fig. 6. Acontiophorus acuminatus n. sp., female.A, maxilliped; B, leg 1; C, leg 2; D, leg 3; E, leg 4; F, leg 5; G, right genital aperture, dorsal. Scale bars:A, F, G, 0.02mm; B-E, 0.05 mm.