Pseudomacrochiron eurydatum, Moon & Kim, 2025

Pseudomacrochiron eurydatum View in CoL n. sp.

( Figs. 8-11 View Fig View Fig View Fig View Fig )

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Material examined. 14♀♀, 12♂♂ from washings of a submerged fish trap, Yeosu , south coast of Korea, collected by S. Y. Moon, 16 Sept. 2011. Holotype ( ♀, MABIK CR00247460 ) and paratypes ( 11♀♀, 10♂♂, MABIK CR00247461 ) have been deposited in the Marine Biodiversity Institute of Korea ( MABIK), Seocheon. Dissected paratypes ( 2♀♀, 1♂♂) are kept in the collection of I.-H. Kim.

Additional material examined. One ♀ (dissected) 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 .

Etymology. The name of the new species is derived from the Greeks eury (= broad) and dato (= distribute), referring to its occurrences at two disjunct areas, the southern coast of Korea and Phuket, Thailand.

Female. Body ( Fig. 8A View Fig ) narrow. Body length 1.01 mm in dissected and figured specimen. Mean body length 941 μm (908-1030 μm, n = 10). Prosome 644 μm long, 0.64 times as long as body length. Cephalothorax 412 × 354 μm, with faint dorsal suture line delimiting cephalosome and first pedigerous somite. All prosomal somites bearing blunt posterolateral corners. Urosome ( Fig. 8B View Fig ) 5-segmented. Fifth pedigerous somite 110 μm wide. Genital double-somite 1.75 times longer than wide (161 × 92 μm), narrower than fifth pedigerous somite, consisting of broader anterior two-thirds and narrower posterior third (63 μm wide across this region); broader anterior part with almost straight lateral margins; genital apertures positioned dorsolaterally at anterior third of double-somite length. Three free abdominal somites 38 × 62, 29 × 58, and 45 × 57 μm, respectively. Anal somite with fine spinules along posteroventral margin ( Fig. 8C View Fig ). Caudal ramus ( Fig. 8C View Fig ) 1.88 times longer than wide (49 × 26 μm), armed with 6 setae (setae II- VII), ornamented with several fine spinules on posteroventral margin; outer seta (seta II) positioned at 65% region of ramus length; outer and dorsal setae (setae II & VII) naked, all other setae pinnate; 2 mid-terminal setae (setae IV & V) consisting of broad proximal half and thin distal half ( Fig. 8D View Fig ). Egg sac ( Fig. 8E View Fig ) fusiform, 431 × 196 μm; eggs small, 54 μm in diameter.

Rostrum ( Fig. 8F View Fig ) longer than wide, tapering, with nipple-shaped process at distal apex. Antennule ( Fig. 8G View Fig ) 282 μm long, 7-segmented; armature formula 4, 13, 6, 3, 4 + aesthetasc, 2 + aesthetasc, and 7 + aesthetasc; setae naked, relatively large; aesthetasc on fifth segment much longer than other aesthetascs or setae on antennule. Antenna ( Fig. 8H View Fig ) 4-segmented; armature formula 1, 1, 2 + claw, and 5 + 2 claws; all setae naked; terminal segment (third endopodal segment) 2.77 times longer than wide (61 × 22 μm); 2 terminal spines almost equal in length and thickness, distinctly longer than terminal segment.

Labrum ( Fig. 8I View Fig ) with broad posteromedial incision, finely serrate thin membrane along inner margin of posterolateral lobes. Mandible ( Fig. 8J View Fig ) with broad inner proximal notch and elongate distal lash; inner margin of gnathobase with 18 spinules (proximal group of 14 spinules and distal group of 4 spinules); convex side of gnathobase with row of 8 spinules proximally, followed by row of teeth; distal lash spinulose along both margins, longer than remaining proximal part of mandible. Paragnath ( Fig. 8K View Fig ) as small, densely setulose lobe. Maxillule ( Fig. 8L View Fig ) as small lobe bearing 4 setae; distal 3 setae unequal in length, finely setulose; shortest inner margin seta naked. Maxilla ( Fig. 9A View Fig ) with unarmed syncoxa; basis with pinnate inner seta (seta I), longer, spiniform anterior seta (seta II), and vestigial proximal seta (seta III); distal lash forming right angle with main axis of basis, longer than proximal part, with 1 cusp on concave margin, about 20 spinules along convex margin, proximal 6 of these spinules thick, tooth-like. Maxilliped ( Fig. 9B View Fig ) 3-segment- ed; first segment (syncoxa) longest but unarmed; second segment (basis) with 2 spines of equal length (proximal spine ornamented with 1 spinule on its proximal margin and 5 spinules on distal margin; distal spine ornament- ed with 7 or 8 spinules on its distal margin); small third segment (endopod) with 2 small setae, and terminating in spiniform process bearing 2 rows of 3 fine spinules.

Legs 1-3 ( Fig. 9C- E View Fig ) with 3-segmented rami. Leg 4 ( Fig. 9F View Fig ) with 3-segmented exopod and 1-segment- ed endopod. Inner coxal seta small, naked in leg 4, but well-developed, pinnate in legs 1-3. Outer seta on basis small, naked in legs 1-3, but feebly pinnate in leg 4. Leg 4 endopod 2.4 times longer than wide (53 × 22 μm); inner margin smooth; outer margin setulose, with 1 distinct cusp near proximal third; 2 distal spines 46 μm (outer) and 56 μm long (inner). Armature formula for legs 1-4 as in other species described in this paper.

Leg 5 ( Fig. 8B View Fig ) consisting of dorsolateral seta on fifth pedigerous somite and free exopod; exopodal segment ( Fig. 9G View Fig ) slender, almost rectangular, 4.50 times longer than wide (63 × 14 μm), with smooth margins, distally armed with 1 spine (70 μm long) and 1 seta (36 μm long). Leg 6 ( Fig. 9H View Fig ) represented 1 cusp, 1 small seta, and 1 longer seta on genital operculum.

Male. Body ( Fig. 10A View Fig ) moderately narrow. Body length 683 μm in dissected specimen. Mean body length 690 μm (638-754 μm, n = 10). Prosome 406 μm long. Cephalothorax 283 × 255 μm; dorsal suture line obscure. Urosome ( Fig. 10B View Fig ) 6-segmented. Fifth pedigerous somite 50 μm wide, much narrower than genital somite. Genital somite 135× 145 μm. Four abdominal somites 16 × 48, 16 × 45, 14 × 42, and 28 × 43 μm, respectively. Caudal ramus 1.85 times longer than wide (37 × 20 μm), armed as in female.

Antennule different from that of female in having 3 additional aesthetascs at places of dots in Fig. 8G View Fig . Antenna ( Fig. 10C View Fig ) with numerous minute spinules on inner surface of first endopodal segment; terminal segment 2.68 times longer than wide (51 × 19 μm).

Labrum, mandible, maxillule, and maxilla as in female. Maxilliped ( Fig. 10D View Fig ) consisting of 3 segments and terminal claw; first segment (syncoxa) narrowing distally, unarmed; second segment (basis) inflated, armed with 2 small setae, distal one of them finely spinulose along its distal margin, ornamented with proximal and distal longitudinal rows of spinules on inner surface; small third segment (endopod) unarmed; terminal claw narrow, bearing 1 seta proximally.

Leg 1 ( Fig. 10E View Fig ) different from that of female in having 2 spines and 4 setae (formula I, I, 4) on third endopodal segment. Legs 2-4 as in female. Leg 5 exopodal segment ( Fig. 10F View Fig ) 2.42 times longer than wide (29 × 12 μm), much shorter than that of female, with convex inner and outer margins; distal spine 34 μm long, distal seta 33 μm long. Leg 6 ( Fig. 10G View Fig ) represented by 2 unequal setae and 1 cusp on distal apex of genital operculum.

Female from Phuket, Thailand. Body ( Fig. 11A View Fig ) as in type specimens in form. Body length 910 μm. Genital double-somite ( Fig. 11B View Fig ) shaped as in female type specimens. Caudal ramus 1.91 times longer than wide (44 × 23 μm) .

Rostrum, antennule, antenna, and labrum as in type specimens. Mandible ( Fig. 11C View Fig ) almost identical to that of type specimens, but inner margin of gnathobase bearing proximal group of 12 spinules and distal group of 5 short- er spinules; convex side of gnathobase bearing 7 proximal spinules. Maxillule ( Fig. 11D View Fig ) and maxilla ( Fig. 11E View Fig ) as in type specimens. Maxilliped ( Fig. 11F View Fig ) as in type specimens, but proximal spine on basis ornamented with 1 spinule on its proximal margin and 2 spinules on distal margin; distal spine on basis ornamented with 5 spinules on its distal margin; terminal segment ornamented with 2 spinules on outer margin and 3 spinules on inner margin.

Legs 1-4 as in type specimens. Leg 5 exopodal segment ( Fig. 11G View Fig ) 4.08 times longer than wide (63 × 13 μm); distal spine and seta 54 and 33 μm long, respectively. Leg 6 ( Fig. 11H View Fig ) as in type specimens.

Remarks. As selected diagnostic features of P. eurydatum n. sp. for a comparison with its congeners, (1) the caudal ramus is short, with the length/width ratio less than 2.5: 1; (2) the distal lash of the maxilla bears a cusp on its concave margin; (3) the exopodal segment of female leg 5 is elongate, more than four times as long as wide; and (4) the exopod of female leg 5 is armed with one spine and one seta. Within the genus, only a single species, P. pocilloporae recorded from the Pacific coast of Panama ( Kim, 2004), shares the above four diagnostic features with P. eurydatum n. sp., thus needs to be closely compared with the new species. A careful comparison reveals that differences between the two species are very small but not neglectable, as follows: (1) the lateral margins of the expanded anterior part of the female genital double-somite are consistently straight in both Korean and Thai specimens of P. eurydatum n. sp., but rounded in P. pocilloporae ; (2) the two setae on the basis of the female maxilliped are spinulose in P. eurydatum n. sp., but naked in P. pocilloporae ; (3) the exopodal segment of female leg 5 of P. eurydatum n. sp. is shorter than that of P. pocilloporae , 4.50 times longer than wide (63 × 14 μm), compared to 5.47 times longer than wide (71 × 13 μm) in the latter species ( Kim, 2004); (4) the two distal armature elements on the exopod of female leg 5 are very unequal in length in P. eurydatum n. sp. (spine 70 μm long and seta 36 μm long), whereas they are subequal in P. pocilloporae (spine 46 μm long and seta 42 μm long). These differences prevent them from being treated as a same species.

The single female specimen collected at Phuket, Thailand exhibits no significant differences from the female type specimens from Korea. The only noteworthy differences are in the number of spinules on the inner and outer margins of the mandible, in the ornamentation of the spines on the basis of the maxilliped, and in the size of the exopodal segment of leg 5. These differences seem not sufficient to separate them as distinct species. The finding of this species from two remotely separated places may inform that it is widely distributed at least in the Indo-West Pacific .