Pseudomacrochiron bellum, Moon & Kim, 2025

Pseudomacrochiron bellum View in CoL n. sp.

( Figs. 6 View Fig , 7 View Fig )

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Material examined. 4♀♀ 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 CR00247458 ) and paratypes ( 2♀♀, MABIK CR00247459 ) have been deposited in the Marine Biodiversity Institute of Korea ( MABIK), Seocheon, Korea. Dissected paratype ( ♀) is retained in the collection of I.-H. Kim.

Etymology. The name bellum is derived from the Latin bell (= beautiful).

Female. Body ( Fig. 6A View Fig ) narrow. Body length 898 μm. Prosome 513 μm long, 0.57 times as long as body length. Cephalothorax 334 × 280 μm, with faint dorsal suture line between cephalosome and first pedigerous somite. All prosomal somites bearing blunt posterolateral corners. Urosome ( Fig. 6B View Fig ) 5-segmented. Fifth pedigerous somite 91 μm wide. Genital double-somite 1.42 times longer than wide (136 × 96 μm), consisting of expanded anterior half and narrower posterior half; expanded anterior part with roundly convex lateral margins; genital apertures positioned dorsolaterally at 36% region of double-somite length. Three free abdominal somites 36 × 53, 27 × 47, and 51 × 47 μm, respectively. Caudal ramus ( Fig. 6C View Fig ) 4.48 times longer than wide (94× 21 μm), armed with 6 setae; outer seta (seta II) and dorsal seta (seta VII) naked, other setae pinnate; outer seta positioned at 64% region of ramus length; 2 mid-terminal setae (setae IV and V, Fig. 6B View Fig ) consisting of broadened, tape-like proximal two-thirds and thin distal third.

Rostrum ( Fig. 6D View Fig ) longer than wide, tapering, with rounded distal apex. Antennule ( Fig. 6E View Fig ) 227 μm long, 7-segmented; armature formula 4, 13, 6, 3, 4 + aesthetasc, 2 + aesthetasc, and 7 + aesthetasc; all setae naked. Antenna ( Fig. 6F View Fig ) 4-segmented; armature formula 1, 1, 2 + claw, and 5 + 2 claws; second segment (first endopodal segment) ornamented with fine spinules along inner and outer margins; terminal segment (third endopodal segment) 2.56 times longer than wide (41 × 16 μm), ornamented with fine spinules along outer margin; 2 terminal claws almost equal in length, 49 and 48 μm long, respectively.

Labrum ( Fig. 6G View Fig ) with broad posteromedian incision; posterolateral lobes fringed with membrane along distal margin. Mandible ( Fig. 6H View Fig ) with deep inner proximal notch and elongate, serrate distal lash; inner margin of gnathobase straight, perpendicular to distal lash, with 2 rows of spinules (proximal row of 10 larger spinules and distal row of 6 small spinules); convex margin of gnathobase bearing 6-8 spinules proximally, followed by produced angle and row of small teeth. Maxillule ( Fig. 6I View Fig ) bearing 3 distal pinnate setae and 1 naked setiform inner element. Maxilla ( Fig. 6J View Fig ) 2-segmented; proximal segment (syncoxa) broad, unarmed; distal segment (basis) with 3 setae (setae I- III); inner seta (seta I) strong, spiniform, spinulose along both margins; anterior seta (seta II) finely spinulose along its inner margin; outer proximal seta (seta III) rudimentary; distal lash forming right angle with main axis of basis, with 1 cusp on concave margin, more than 10 spiniform teeth along convex distal margin. Maxilliped ( Fig. 6K View Fig ) 3-segmented; first segment (syncoxa) unarmed; second segment (basis) with 2 very unequal elements, proximal element as small, naked seta, distal element as spine ornamented with fine spinules along its distal margin; small third segment (endopod) terminating in large, spiniform process bearing fine spinules along both margins, with 1 small and 1 larger spiniform setae.

Legs 1-3 with 3-segmented rami ( Fig. 7A- C View Fig ). Leg 4 ( Fig. 7D View Fig ) with 3-segmented exopod and 1-segmented endopod. Leg 3 identical to leg 2, except bearing 3 spines and 2 setae on third endopodal segment ( Fig. 7C View Fig ). Inner coxal seta rudimentary, naked in leg 4, but all other setae on legs 1-4 distinct, pinnate. Leg 4 endopodal segment 38 × 14 μm, with smooth outer and inner margins; 2 distal spines 44 μm (inner) and 33 μm long (outer), respectively; distal margin with 1 or 2 cusps (mid-terminal one present or absent). Armature formula for legs 1-4 as in P. gurneyi .

Leg 5 ( Fig. 6B View Fig ) consisting of dorsolateral seta on fifth pedigerous somite and free exopod; exopodal segment ( Fig. 7E View Fig ) nearly oval, 1.80 times longer than wide (27 × 15 μm), with convex outer margin, angular inner distal corner, distally armed with 1 spine (30 μm long) and 1 naked seta (37 μm long). Leg 6 ( Fig. 7F View Fig ) represented by 1 small cusp and 2 unequal, naked setae on genital operculum.

Male. Unknown.

Remarks. The exopod of leg 5 of Pseudomacrochiron is armed distally with two elements. These elements in the female are two setae (formula 2) in P. gurneyi and P. malayense ( Sewell, 1949) , or two spines (formula II) in P. ornatum ( Krishnaswamy, 1952) and P. parvum (Scott A., 1909) , or one spine plus one seta (formula I + 1) in P. bellum n. sp. and other seven known species. Of the latter seven species, four ( P. pocilloporae Kim, 2004 , P. urostenum , P. acutirostre n. sp., and P. eurydatum n. sp.) are comparable with P. bellum n. sp., because these five species bear in common a distinct cusp on the concave (inner) margin of the distal lash of the maxilla ( Table 1).

According to Kim (2004), P. pocilloporae has a short caudal ramus which is 1.84 times longer than wide (cf. 4.48 times longer than wide in P. bellum n. sp.) and an elongate exopodal segment of female leg 5 which is 5.46 times longer than wide (cf. 1.80 times longer than wide in P. bellum n. sp.). Therefore, P. pocilloporae is not confusable with P. bellum n. sp.

Pseudomacrochiron urostenum known from the Yellow Sea ( Kim, 2000) differs from P. bellum n. sp. in the following points: (1) the distal spine on the exopod of female leg 5 is much longer than the distal nearby seta (opposite condition in P. bellum n. sp.), (2) the two setae on the basis (second segment) of the female maxilliped are equal in shape and size (in P. bellum n. sp. the proximal element is a small seta, but the distal element is a strong spine), (3) the posterolateral corners of the second pedigerous somite are acutely pointed (cf. the corners are blunt in P. bellum n. sp.), (4) the rostrum bears an angular distal apex (cf. distal apex is rounded in P. bellum n. sp.), and (5) two mid-terminal caudal setae (setae IV and V) are simple (cf. these setae broadened, tape-like along their proximal two-thirds).

Pseudomacrochiron acutirostre n. sp. was collected together with P. bellum n. sp. at the same locality. In this species the rostrum bears an acutely pointed distal apex, the exopodal segment of female leg 5 bears a patch of spinules, the basis of the female maxilliped is armed with two equally small setae, and the outer margin of the endopod of leg 4 bears an angle. These features are not adjustable to P. bellum n. sp. and serve to distinguish it from P. bellum n. sp.

Pseudomacrochiron bellum n. sp. is not related to P. eurydatum n. sp., since in the latter species the caudal ramus of the female is shorter, 1.90 times longer than wide, the exopodal segment of female leg 5 is longer, 4.50 times longer than wide, and the distal lash of the maxilla bears a cusp on the concave margin.