Stenopontius brevicauda, Lee, 2025

Stenopontius brevicauda View in CoL n. sp. ( Figs. 16-18 View Fig View Fig View Fig )

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Type material. Holotype (intact ♀; HNIBR IV7486 ), intact paratypes (2 ♀♀; HNIBR IV7487 ), and dissected paratypes (2 ♀♀, 1 ♂) from washings of mixed species of sponges, Ullung Island (37°31 ʹ 31.98 ʺ N, 130°56 ʹ 17.35 ʺ E), SCUBA, depth 32 m, collected by Tae Won Jung, 28 November 2022. The holotype and intact paratypes have been deposited in the Honam National Institute of Biological Resources ( HNIBR), Mokpo. The dissected paratypes are retained in the collection of I.-H. Kim. GoogleMaps

Etymology. The name, a noun, derives from Latin brev (= short) and cauda (= tail), referring to the short caudal rami of the new species.

Female. Body ( Fig. 16A View Fig ) narrow. Body length 536 μm in figured and described specimen. Prosome elliptical, 307 μm long. Cephalothorax 175 × 190 μm. Cephalothorax and first pedigerous somites with angular posterolateral corners; third and fourth pedigerous somites with rounded lateral corners. Fourth pedigerous somite with deeply concave posterodorsal margin. Urosome ( Fig. 16B View Fig ) 5-segmented. Genital double-somite 1.7 times longer than wide (95 × 56 μm), widest at proximal 36% region, followed by concave lateral margins; genital apertures indistinct, positioned dorsolaterally at widest region. Three free abdominal somites 35 × 46, 33 × 43, and 18 × 43 μm, respectively. Anal somite ( Fig. 16C View Fig ) characteristically short. Caudal rami ( Fig. 16C View Fig ) widely divergent, widely separated from each other; each ramus 0.94 times longer than wide (16 × 17 μm), quadrangular, armed with 6 setae; setae II, III, and VII naked, other 3 setae pinnate; inner and outer margins naked or ornamented with few spinules. Egg sac ( Fig. 16A View Fig ) 243 × 125 μm, containing single large egg.

Rostrum absent. Antennule ( Fig. 16D View Fig ) 132 μm long, 10-segmented; armature formula 0, 1, 6, 4, 3, 3, 1, 0, 1, and 11 + 2 aesthetascs; third segments with incomplete articulation on ventral surface; terminal segment longest, occupying 40% length of antennule, its proximal major aesthetasc extending slightly over distal end of segment, constricted at region slightly distal to middle; all setae naked. Antenna ( Fig. 16E View Fig ) slender, 5-segmented; coxa (first segment) unarmed and unornamented; basis (second segment) with 2 small setae subdistally representing exopod, and ornamented with minute spinules along outer margin; armature formula of endopod 0, 1, and 1 + spine; terminal spine straight, as long as sum of 3 endopodal segments; first and third endopodal segment with setules on outer margin.

Oral cone ( Fig. 16F View Fig ) stout, 84 μm long, with truncate distal tip. Mandible ( Fig. 16G View Fig ) represented by attenuated stylet of 80 μm long, lacking palp; stylet acutely pointed at tip, lacking any visible tooth. Maxillule ( Fig. 16H View Fig ) bilobed, each lobe with 3 setae distally; inner lobe with spinules on outer margin, its 3 distal setae pinnate, subequal in length, innermost one 45 μm long. Maxilla ( Fig. 16I View Fig ) with smooth proximal segment; distal segment slen- der, arched, bearing row of spinules in subdistal region. Maxilliped ( Fig. 16J View Fig ) 5-segmented; armature formula 0. 0. 2, 1, and 1 + claw; articulation between first and second segments incomplete; second segment with minute, rudimentary cusp on middle of inner margin; second endopodal segment 18 μm long, with inflated outer margin, with few spinules at apical region of outer margin; third endopodal segment 22 μm long; terminal claw straight, spiniform, 58 μm long, with few spinules on inner margin.

Legs 1-4 ( Fig. 17A- D View Fig ) with 2-segmented protopod and 3-segmented rami; inner coxal seta absent in leg 1 but present in legs 2-4. Basis of leg 1 with spiniform inner distal seta and spinules on inner distal margin. Most of spines, especially those of legs 2-4 broad, leaf-like. Second endopodal segment of legs 1-4 with bicuspid outer distal corner. Third exopodal segment of legs 2 and 3 also with bicuspid outer distal process. Third endopodal segment of leg 4 with 2 denticles at subdistal region of outer margin. Armature formula for legs 1-4 as follows:

Coxa Basis Exopod Endopod Leg 1 0-0 1-1 I-1; I-1; III, 2, 3 0-1; 0-2; 1, 2, 3 Leg 2 0-1 1-0 I-1; I-1; III, I, 5 0-1; 0-2; 1, 2, 3 Leg 3 0-1 1-0 I-1; I-1; III, I, 5 0-1; 0-2; 1, I, 3 Leg 4 0-1 1-0 I-1; I-1; III, I, 5 0-1; 0-2; 0, I, 2

Leg 5 ( Fig. 17E View Fig ) 2-segmented; protopod (proximal segment) wider than long (27 × 40 μm), articulated from somite, bearing 1 naked dorsolateral seta and pointed lateral apex; exopodal segment 2.29 times longer than wide (39 × 17 μm), broadening distally, with 1 subdistal pinnate outer seta and 2 distal setae (naked inner seta and pinnate outer one). Leg 6 ( Fig. 17F View Fig ) probably represented by 1 small spinule on genital operculum.

Male. Body ( Fig. 18A View Fig ) similar in form to that of female. Body length 462 μm. Prosome 282 μm long. Cephalothorax 170 × 173 μm, with almost parallel lateral margins. Urosome ( Fig. 18B View Fig ) 6-segmented. Genital somite 47 × 63 μm, nearly quadrangular; genital operculum with several minute spinules on subdistal inner margin and tipped with 1 seta. Four abdominal somites 28 × 47, 23 × 44, 22 × 41, and 12 × 40 μm, respectively. Caudal ramus 15 × 16 μm, armed as in female.

Rostrum absent. Antennule ( Fig. 18C View Fig ) 11-segmented, geniculate between penultimate and terminal segments; armature formula for first to penultimate segments 0, 1, 6, 4, 3, 5, 2, 2, 0, 2 + aesthetasc; armature of terminal segment uncertain due to detachment of armature elements. Antenna as in female.

Oral siphon, mandible, maxillule, maxilla, maxilliped, and legs 1-4 not different from those of female. Leg 5 ( Fig. 18B View Fig ) armed as in female, but outer subdistal seta on exopod naked. Leg 6 ( Fig. 18B View Fig ) represented by 1 naked seta tipped on distal apex of genital operculum.

Remarks. The genus Stenopontius comprises four species associated with sponges in shallow waters: S. boxshalli Malt, 1991 and S. parvus Boxshall, 1990 , both document- ed in Hong Kong ( Boxshall, 1990; Malt, 1991); S. humesi Murnane, 1969 , from the Atlantic coast of North America ( Murnane, 1967); and S. spinulatus Kim, 2010 from Madagascar ( Kim, 2010). Stenopontius brevicauda n. sp. can be distinguished from its four congeners by its elongate genital double-somite, which is approximately 1.7 times longer than it is wide (in contrast, in the four congeners, the double-somite is wider than long or as long as wide), by its short caudal rami, which are wider than long (whereas the caudal rami in the four congeners are significantly longer than wide), by having three setae on the outer lobe of the maxillule (compared to two setae in the four congeners), and by having an inner seta on the coxa of legs 2-4 (unlike the absence of an inner seta in all legs 1-4 of the congeners).