Pseudomacrochiron acutirostre, Moon & Kim, 2025

Pseudomacrochiron acutirostre View in CoL n. sp.

( Figs. 3-5 View Fig View Fig View Fig )

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Material examined. 3♀♀, 1♂ from intertidal invertebrate burrows (inhabited mainly by shrimps and polychaetes), Ko Sireh , Phuket, Thailand (approximately 07°52 ʹ 26 ʺ N, 98°25 ʹ 235 ʺ E), collected by I.-H. Kim & J.-S. Hong, 12 July 2015. Holotype ( ♀, MABIK CR00247456 ) and paratype ( ♀, MABIK CR00247457 ) have been deposited in the Marine Biodiversity Institute of Korea ( MABIK), Seocheon. Dissected paratypes ( 1♀, 1♂) are kept in the collection of I.-H. Kim.

Etymology. The specific name is the combination of the Latin words acut (= sharp) and rostr (=snout), referring to the pointed rostrum of the new species.

Female. Body ( Fig. 3A View Fig ) narrow. Body length 1.16 mm. Prosome 723 μm long, occupying 62% of body length. Cephalothorax longer than wide (495 × 405 μm), with faint dorsal suture line between cephalosome and first pedigerous somite. Second pedigerous somite with acutely pointed posterolateral corners. Urosome ( Fig. 3B View Fig ) 5-segmented. Fifth pedigerous somite 118 μm wide, wider than genital double-somite. Genital double-somite distinctly longer than wide (153 × 107 μm), consisting of narrower anterior and posterior parts and roundly expand- ed middle part; genital apertures positioned dorsolaterally at midway of double-somite. Three free abdominal somites 36 × 62, 31 × 63, and 67 × 62 μm, respectively. Caudal ramus ( Fig. 3C View Fig ) 4.50 times longer than wide (117 × 26 μm), with 6 setae; outer and dorsal setae (setae II and VII) naked, other setae pinnate; outer seta (seta II) positioned at 63% region of ramus length; outer distal seta (seta III) with broad proximal half and thin distal half; 2 mid-terminal setae (setae IV and V) consisting of tape-like, broadened proximal half and thin distal half ( Fig. 3B View Fig ).

Rostrum ( Fig. 3D View Fig ) longer than wide, weakly tapering along proximal half but strongly tapering along distal half, with pointed distal apex. Antennule ( Fig. 3E View Fig ) 332 μm long, 7-segmented; armature formula 4, 13, 6, 3, 4 + aesthetasc, 2 + aesthetasc, and 7 + aesthetasc; all setae naked. Antenna ( Fig. 3F View Fig ) 4-segmented; armature formula 1, 1, 2 + claw, and 5 + 2 claws; terminal segment about 3 times longer than wide (68× 23 μm); 2 distal claws 74 and 65 μm long, respectively.

Labrum ( Fig. 3G View Fig ) with broad, semicircular posterolateral lobes. Mandible ( Fig. 3H View Fig ) with elongate, serrate distal lash, deep inner proximal notch; inner margin of gnathobase straight, perpendicular to distal lash, with row of about 15 spinules; convex outer side of gnathobase with 1 broad proximal seta (setiform scale) bearing row of spinules along its inner margin and small teeth distally. Maxillule ( Fig. 3I View Fig ) lobate, armed with 3 weakly pinnate distal setae and 1 shorter, naked subdistal process. Maxilla ( Fig. 3J View Fig ) 2-segmented; proximal segment (syncoxa) broad, unarmed; distal segment (basis) with 3 setae; inner seta (seta I) bipinnate; anterior seta (seta II) weakly pinnate along its inner margin; outer proximal seta (seta III) vestigial; distal lash elongate, forming right angle with main axis of basis, with 1 cusp near middle of concave margin, 1 small spinule proximally (indicated by arrowhead in Fig. 3J View Fig ) followed by row of teeth along convex outer margin. Maxilliped ( Fig. 3K View Fig ) 3-segmented; first segment (syncoxa) longest but unarmed; second segment (basis) with 2 small, naked setae on inner margin; small third segment (endopod) terminating in spiniform process, with 1 spine and 1 small seta.

Legs 1-3 ( Fig. 4A- C View Fig ) with 3-segmented rami. Leg 4 ( Fig. 4D View Fig ) with 3-segmented exopod and 1-segmented endopod. Inner coxal seta of leg 4 small and naked; oth- er setae on legs 1-4 pinnate. Inner distal corner of basis of leg 4 with small pointed cusp (indicated by arrowhead in Fig. 4D View Fig ). Leg 4 endopodal segment 68 × 23 μm, with angle on outer margin; two distal spines 46 (outer) and 68 μm (inner), respectively. Armature formula for legs 1-4 as in P. gurneyi .

Leg 5 ( Fig. 3B View Fig ) consisting of 1 small, naked dorsolateral seta on fifth pedigerous somite and free exopod; exopodal segment ( Fig. 4E View Fig ) gradually broadening distally, 2.17 times longer than wide (50× 23 μm), distally armed with 1 spine (47 μm long) and 1 naked seta (37 μm long), and ornamented with patch of spinules near inner distal corner. Leg 6 ( Fig. 4F View Fig ) represented by 2 unequal, naked setae and 1 cusp on genital operculum.

Male. Body ( Fig. 5A View Fig ) similar to that of female; Body length 935 μm. Prosome 545 μm long. Cephalothorax 349 × 298 μm. Urosome ( Fig. 5B View Fig ) 6-segmented. Fifth pedigerous somite 88 μm wide. Genital somite subquad- rate, 134 × 117 μm. Four abdominal somites 30 × 52, 29 × 48, 24 × 47, and 41 × 47 μm, respectively. Caudal ramus 4.0 times longer than wide (88 × 22 μm), armed as in female.

Rostrum as in female. Antennule with 3 additional aesthetascs at opposite places of dots in Fig. 3E View Fig . Antenna as in female. Labrum, mandible, maxillule, and maxilla also as in female. Maxilliped ( Fig. 5C View Fig ) consisting of 3 segments and terminal claw; first segment (syncoxa) unarmed; second segment (basis) slightly inflated, armed with 2 setae on inner side (1 simple seta and 1 transformed seta bearing broadened proximal two-thirds), ornamented with 1 cusp near inner distal corner and row of spinules along inner margin; small third segment (endopod) unarmed; terminal claw elongate, gradually narrowing distally, with 2 unequal setae proximally.

Leg 1 endopod ( Fig. 5D View Fig ) with 2 spines and 4 setae (formula I, I, 4) on third segment. Legs 2-4 as in female. Leg 5 exopodal segment ( Fig. 5E View Fig ) 2.07 times longer than wide (29 × 14 μm), setulose on inner distal region, armed with 1 spine and 1 seta, both 33 μm long. Leg 6 ( Fig. 5B View Fig ) represented by 2 small setae on distal apex of genital operculum.

Remarks. As a diagnostic feature of P. acutirostre n. sp., the exopodal segment of female leg 5 is short, 2.17 times longer than wide. The new species is comparable with three congeners, P. aureliae Tang, Yasuda, Yamada & Nagasawa, 2012 , P. gurneyi , and P. urostenum Kim, 2000 , because these three species have a similarly short exopodal segment of leg 5, which is less than three times as long as wide. However, P. acutirostre n. sp. is distinguished from P. aureliae by having four spines and five setae (armature formula III, I, 5, against III, I, 4 in P. aureliae as described by Tang et al., 2012); from P. gurneyi by having roundly convex lateral margins of the genital double-somite (vs. the genital double-somite of P. gurneyi is hexagonal) and by the bearing one spine and one seta on the exopod of female leg 5 (vs. two setae in P. gurneyi ); and from P. urostenum by having shorter caudal rami of the female which are 4.50 times longer than wide (vs. 6.32 times longer than wide in P. urostenum , according to Kim, 2000) and by having a shorter exopodal segment of female leg 5 which is 47 μm long (vs. the 65 μm long in P. urostenum , according to Kim, 2000). The new and other species of the genus may be distinguished from one another, using Table 1.