Pseudomacrochiron gurneyi ( Sewell, 1949 ), 2025

Pseudomacrochiron gurneyi ( Sewell, 1949) View in CoL n. comb.

( Figs. 1 View Fig , 2 View Fig )

Syn.: Kelleria gurneyi Sewell, 1949: 117 , text-fig. 31.

Material examined. 4♀♀ collected with a light trap at night, Kosrae, Micronesia ( 05°21 ʹ 27.17 ʺ N, 162°57 ʹ 51.23 ʺ E), collected by J. Lee, 02 July 2016 GoogleMaps .

Female. Body ( Fig. 1A View Fig ) narrow. Body length 1.10 mm. Prosome 420 μm long. Cephalothorax longer than wide (450 × 386 μm), with blunt posterodorsal corners; dorsal suture line between cephalosome and first pedigerous somite faint. Posterolateral corners acutely pointed in second pedigerous somite, but blunt in third and fourth pedigerous somites. Third pedigerous somite with 1 dorsal tubercle in middle of posterior margin ( Fig. 1A View Fig ). Urosome ( Fig. 1B View Fig ) 5-segmented. Fifth pedigerous somite 131 μm wide, as wide as genital double-somite. Genital double-somite slightly longer than wide (140 × 131 μm), characteristically hexagonal in dorsal view, with large seta on lateral apex positioned at 45% length of double-somite; genital apertures positioned dorsally at anterior quarter; pair of oblique rows of minute spinules present on dorsal surface posterior to genital apertures. Three free abdominal somites 45 × 65, 33 × 62, and 62 × 60 μm, respectively. Caudal ramus ( Fig. 1C View Fig ) 4.74 times longer than wide (128 × 27 μm), about twice longer than anal somite, armed with 6 setae; outer seta naked, positioned at midlength of ramus, other setae pinnate; distal margin of ramus tapering, tipped with small pore.

Rostrum ( Fig. 1D View Fig ) spatulate, with almost parallel lateral margins and slightly concave distal margin. Antennule ( Fig. 1E View Fig ) slender, about 360 μm long, 7-segmented; armature formula 4, 13, 6, 3, 4 + aesthetasc, 2 + aesthetasc, and 7 + aesthetasc; setae thin, generally long, all of them naked. Antenna ( Fig. 1F View Fig ) 4-segmented, consisting of coxobasis and 3-segmented endopod; armature formula 1, 1, 2 + claw, and 5 + 2 claws; first endopodal segment ornamented with fine spinules along outer margin; claws thin, setiform, annulated in middle; third endopodal segment 3.89 times longer than wide (74× 19 μm); all setae thin, naked.

Labrum ( Fig. 1G View Fig ) much wider than long, with shallow posteromedian incision. Mandible ( Fig. 1H View Fig ) lacking inner proximal notch; inner margin of gnathobase straight, oblique to distal lash, ornamented with 2 groups of spinules, proximal group consisting of 8 larger spinules, and distal group of 8 smaller spinules; distal lash thin, spinulose along both margins; convex margin of gnathobase with 1 small proximal seta (this seta ornamented with 2 spinules on its proximal margin and 3 spinules on distal margin) followed by 3 spinules and about 15 teeth, latter teeth getting markedly smaller from proximal to distal. Paragnath ( Fig. 1I View Fig ) as tapering, setulose lamella. Maxillule ( Fig. 1J View Fig ) lobate, bearing 3 setae and 1 setiform process; 2 apical setae spinulose; subapical seta spinulose along inner margin. Maxilla ( Fig. 1K View Fig ) 2-segmented; proximal segment (syncoxa) large but unarmed; distal segment (basis) with spiniform distal lash bearing 2 setiform spinules proximally on outer side; inner seta (seta I) completely fused to basis, strong, spiniform, bipinnate along its proximal half; anterior seta (seta II) naked, long, wrinkled; outer proximal seta (seta III) distinct, naked. Maxilliped ( Fig. 2A View Fig ) 3-segmented; first segment (syncoxa) being longest segment, but unarmed; second segment (basis) with 2 large setae, both pinnate, proximal one shorter than distal one; third segment (endopod) small, terminating in elongate, smooth, spiniform process, proximally with 2 naked setae and 1 minute setule.

Legs 1-3 biramous with 3-segmented rami ( Fig. 2B, C View Fig ). Leg 3 same as leg 2, except bearing 3 spines and 2 setae on third endopodal segment ( Fig. 2D View Fig ). Leg 4 ( Fig. 2E View Fig ) with 3-segmented exopod and 1-segmented endopod; endopodal segment small, 2.76 times longer than wide (47 × 17 μm), setulose along inner and outer margins, with blunt angle on outer margin; 2 distal spines 66 μm (inner) and 42 μm (outer) long. Inner coxal seta of leg 4 naked; all other setae on legs 1-4 pinnate. Armature formula for legs 1-4 as follows:

Coxa Basis Exopod Endopod

Leg 1 0-1 1-0 I- 0; I- 1; III, I, 4 0-1; 0-1; I, 1, 4 Leg 2 0-1 1-0 I- 0; I- 1; III, I, 5 0-1; 0-2; I, II, 3 Leg 3 0-1 1-0 I- 0; I- 1; III, I, 5 0-1; 0-2; I, II, 2 Leg 4 0-1 1-0 I- 0; I- 1; II, I, 5 0, II, 0

Leg 5 consisting of large dorsolateral seta on fifth pedigerous somite and free exopod ( Fig. 1B View Fig ); exopodal segment ( Fig. 2F View Fig ) small, 1.65 times longer than wide, with inner subdistal bulge, 1 cusp at distal tip, armed 2 naked setae of unequal lengths on distal margin, both setae longer than segment. Leg 6 ( Fig. 2G View Fig ) consisting of 1 small seta, 1 cusp, and 1 posterolaterally displaced process tipped with large, naked seta.

Male. Unknown.

Remarks. Pseudomacrochiron gurneyi ( Sewell, 1949) was originally recorded as Kelleria gurneyi in the family Kelleriidae by Sewell (1949) who collected it with surface tow-net, at Kurau River, Malaysia. It has never been rediscovered since the original record. Humes & Stock (1973) treated it as a species “insufficiently described or of uncertain position”.

Sewell (1949) observed a single female and his description and illustration reveal some important taxonomic features, as follows: (1) the third pedigerous somite bears a rounded dorsal tubercle; (2) the genital double-somite is hexagonal in dorsal view and bears a large seta on each lateral apex; (3) the forms of the illustrated maxilla and maxilliped are roughly similar to those of our specimens; and (4) the exopodal segment of leg 5 bears an inner subdistal bulge, as in our specimens. Based on the sharing of the above features, we can determine that our specimens and Kelleria gurneyi Sewell (1949) are conspecific. According to the definitions of the families and genera of the superfamily Lichomolgoidea presented by Humes & Boxshall (1996), this species should be placed in the Pseudomacrochiron of the Macrochironidae , and the revised name of this species is Pseudomacrochiron gurneyi ( Sewell, 1949) n. comb.

The maxilla of the female of P. gurneyi is characteristic: in its distal segment (basis) the inner seta (seta I) is transformed to a spiniform process without basal articulation, and the distal lash is ornamented only with two proximal, setiform spinules. These features of the maxilla are very unusual for the genus and thus serve as key characters of the species, along with the characteristic forms of the genital double-somite and exopodal segment of leg 5.