Phyllognathopus vietnamensis, Tran & & Brancelj & Anton, 2025

Phyllognathopus vietnamensis , new species

( Figs. 2–8)

Type locality. Hyporheic zone on the bank of Suoida stream, Son Tra peninsula, Da Nang City, the central part of Vietnam, 16°06′50.9″ N, 108°15′20.6″ E, 40 m a.s.l. GoogleMaps

Material examined. Holotype: adult female, total body length without caudal setae 466 µm, completely dissected, mounted on one slide (ZC-DTU-COPEPODA-0006), hyporheic zone on the bank of Suoida stream, Son Tra peninsula, Da Nang City, Vietnam, 16°06′50.9″ N, 108°15′20.6″ E, 40 m a.s.l, coll. Ngoc-Son Tran, 20 March 2022 GoogleMaps . Allotype: male, total body length without caudal setae 400 µm, completely dissected, mounted on one slide (ZC-DTU-COPEPODA-0007), same collection data as holotype. Paratypes: 3 males, 10 females (ZC-DTU-COPEPODA-0008), same collection data as holotype GoogleMaps .

Description of holotype female. Body length, measured from tip of rostrum to posterior margin of caudal rami, 466µm.

Habitus cylindrical, tapering posteriorly; no clear separation between prosome and urosome ( Fig. 2A). Naupliar eye and hyaline integumental window not discernable in preserved specimens. Rostrum sub-rectangular in dorsal view, large, surpassing the first segment of antennule ( Fig. 2A). First pedigerous somite free and not fused to cephalosome. Genital and first abdominal somites fused, forming genital double-somite ( Fig. 2E). Genital double-somite with three rows of small spinules laterally. All somites dorsally with smooth free margin. Genital field simple, with small simple copulatory pore at the middle of somite. Last urosomite with two rows of spinules, anal somite with one row of spinules ventrally. Additional row of spinules on anal somite near base of caudal rami ventrally and laterally; with sensilum on both sides near anal operculum base ( Figs. 2B, C). Anal operculum convex, with 10 small spines on free margin ( Fig. 2D).

Caudal rami parallel; each ramus sub-cylindrical, slightly tapering towards the end; about 1.3 times as long as wide; six armature elements: two lateral, three apical, one dorsal ( Figs. 2C, D). Ramus with a row of spinules at about ½ of its length, oriented from dorsal side to inner margin. Additional row of spinules close to insertion of dorsal seta (VII) laterally on inner margin. Anterolateral accessory seta (I) not discernable. Anterolateral seta (II) smooth, about ⅔ length of caudal ramus, inserted at ⅓ length of caudal ramus. Posterolateral seta (III) long, pinnate, inserted at distal outer corner of caudal ramus, about 2.5 times as long as seta II, with few spinules close to its insertion. Outer terminal seta (IV) smooth, 0.7 times as long as caudal ramus. Inner terminal seta (V) unipinnate distally, strong, about 3.5 times as long as seta (III), without fracture plane. Terminal accessory seta (VI) shortest, smooth. Dorsal seta (VII) articulated, inserted at ¾ length of caudal ramus, slightly longer than caudal ramus.

Antennule with eight-segmented, stout, not reaching middle of cephalosome ( Fig. 3A). Segment I with strong spiniform seta. Aesthetasc on segment IV large, long, surpassing tip of apical segment; aesthetasc on apical segment short and slim. Armature formula: 1, 10, 3, 2+ae, 0, 2, 2, 6+ae.

Antenna comprising coxa, basis, one-segmented Exp and two-segmented Enp. Coxa unarmed, about 0.5 times as long as wide ( Fig. 3B). Basis about 0.9 times as long as wide, with one spinule on outer margin. Exp about twice as long as wide, with five elements: two short unipinnate setae laterally, one unipinnate seta subapically, two unipinnate setae apically. Enp-1 with few spinules on proximal outer margin. Enp-2 with two rows of spinules on inner margin at ¼ and ½ of its length; a transverse row of spinules at 3/4 of its length dorsally; three spines, unequal in length at ½ length of segment along outer margin; short bare spine and four geniculate setae, unequal in length, apically.

Mandible comprising coxa, basis, one-segmented Exp and one-segmented Enp. Coxa large, elongated; gnathobase with row of sharp chitinised teeth decreasing in size toward distal corner, accompanied by seta on distal corner ( Fig. 3C). Basis with inner spinule row. Exp with a transversal row of spinules at ⅔ length of segment and few spinules sub-apically; one plumose seta laterally, one unipinnate seta apically. Enp with unipinnate seta subapically; three unipinnate setae unequal in length apically.

Maxillule with arthrite incorporated into praecoxa with nine elements: five smooth spiniform setae, one bipinnate seta, one unipinnate seta, one smooth seta and one thin seta distally ( Fig. 3D). Endite small, with two spiniform pinnate setae. Basis with two plumose setae on outer margin. Exp and Enp fused, with seven spiniform setae: three strong, smooth, two unpinnate, two plumose.

Maxilla short, robust; syncoxa with three endites, with four, three and three robust spiniform setae on proximal to distal ones, respectively ( Fig. 3E). Basal endite with two robust curved spiniform setae, one smooth and one with several long spinules on inner margin. Enp two-segmented; Enp-1 with robust bare seta, Enp-2 with eight elements: five robust setae and three thin setae.

Maxilliped one-segmented, with nine elements: two plumose, one pinnate and two bare setae apically; one long and three short stout unipinnate spines subapically; row of spinules on inner margin ( Fig. 3F).

P1–P3 with three-segmented Exp and Enp; P4 with two-segmented Exp and Enp; intercoxal sclerites (couplers) with concave free margin, unornamented. P1–P3 praecoxa with a row of spinules on outer distal corner. P1–P3 coxae with two sets of spinules each, unequal in size; P4 coxa unornamented. Armature formula of P1–P4 as in Table 1.

P1 basis with short robust unipinnate spine on inner and outer margins, accompanied with several rows of spinules unequal in size on outer and distal margins ( Fig. 4A). Exp as long as Enp. Exp-1–Exp-3 as long as wide, with row of spinules on outer distal margin decreasing in size from segments I to III. Exp-3 without geniculate setae apically, with one long and smooth inner seta, one unpinnate seta and one spine apically, one unipinate outer spine. Enp-1 as long as wide, with several strong spinules on outer distal margin. Enp-2 slim, about twice as long as wide; inner margin with bare spiniform seta at ½ of its length. Enp-3 about 1.5 times as long as wide, with three elements: one thin, smooth inner seta; one smooth seta longer than Enp and one unipinnate spine about 0.4 length of seta, apically.

P2 basis with short robust unipinnate spine on outer margin; row of several spinules on outer and distal margins ( Fig. 4B). Exp about 1.3 times longer than Enp. Exp-1 and Exp-2 slightly longer than wide. Exp-3 1.5 times as long as wide. Exp-1 and Exp-2 with strong unipinnate spine; several strong spinules on outer margin and several smaller spinules at distal inner corner each. Exp-3 with two subapical spines, unequal in length, one smooth, one unipinate; two long setae apically, inner one about twice length of Exp. Enp-1–Enp- 3 as long as wide. Enp-3 with long soft seta apically, as long as apical seta on Exp-3, accompanied with one short unipinnate spine and one short unipinnate seta.

P3 basis with smooth outer spiniform seta, accompanied with row of robust spinules at its base ( Fig. 5A). Exp-1 as long as wide, with strong unipinnate outer spine longer than segment bearing it; row of spinules on outer and distal margin equal in size. Exp-2 about 1.3 times as long as wide. Exp-3 1.5 times as long as wide; inner apical seta soft, about 2.5 times as long as Exp, accompanied with unipinnate seta of about ⅓ of its length and two subapical spines. Enp-1–Enp- 3 as long as wide, with row of spinules on distal margin, unequal in length. Enp-3 with three elements apically: one short pinnate seta, one middle pinnate seta about three times as long as Enp, one short spine.

P4 basis with long soft bare outer seta, as long as Exp ( Fig. 5B). Exp as long as Enp; both rami much shorter than in P1–P3, two-segmented. Exp-1 as long as wide, with few strong spinules on distal margin; strong outer spine. Exp-2 about 1.5 times as long as wide; three apical plumose setae apically, unequal in length. Enp-1 as long as wide, with row of spinules distally. Enp-2 with three short stout setae apically: one smooth and two plumose, unequal in length.

P5 Exp and baseoendopod fused, each with a row of spinules on distal margin ( Fig. 5C). Exp slightly surpassing baseoendopod. Baseoendopod with long soft seta on outer margin and shorter one on inner margin. Exp with four setae apically, unequal in length; second seta from inner margin longest, outermost seta shortest, about 0.4 times length of second seta.

P6 with simple plate, with one short, bare spiniform seta on each side of genital pore ( Fig. 2E).

Description of allotype male. The body length is shorter than that of a female; measured from tip of rostrum to posterior margin of caudal rami, 400 µm. Habitus shape and ornamentation of rostrum and caudal rami as in female. First pedigerous somite free, not fused with cephalosome ( Fig. 6A). Anal somite with two rows of spinules ventrally and laterally ( Fig. 6B, C, D). Anal operculum convex, not reaching distal end of anal somite, with a row of 11 small spines on distal margin. Mouthparts, P1, P2, P3, and P4 similar to those of female.

Antennule with eight-segmented, stout, not reaching middle of cephalothorax ( Fig. 7A). Segments V and VI slightly sclerotised ( Fig. 7A). Aesthetasc on segment IV large, long, surpassing tip of apical segment; aesthetasc on last segment short and slim. Armature formula: 1, 9, 4, 4+ae, 1, 1, 4, 9+ae.

Antenna comprising coxa, basis, one-segmented Exp and two-segmented Enp ( Fig. 7B). Coxa unarmed, about 0.3 times as long as wide. Basis about 0.6 times as long as wide, with few spinules on inner margin. Exp with five elements: three unipinnate robust setae laterally, one smooth and one unipinnate setae apically. Enp-1 distinctly separated from basis, with several spinules on inner margin. Armament of Enp-2 as in female but with more robust elements.

P5 Exp and baseoendopod fused, Exp well overreaching baseoendopod; each with a row of robust spinules on distal margin. Baseoendopod with long soft seta on outer margin; strong pinnate seta on inner corner ( Fig. 8A). Exp with five pinnate setae: innermost and outermost short and thin; middle one longest and robust, about 3.2 times as long as Exp.

P6 simple, forming a single lamellar plate, with notch medially and three elements: two spiniform pinnate setae apically, unequal in length, one smooth bare seta laterally ( Fig. 8B).

Variations. Body length ranged from 466 to 588 µm; mean = 525 µm in female (n = 5) and from 392 to 402 µm; mean = 394 µm in males (n = 3). Anal operculum with ten to twelve short spines in females on free margin but no variability in males.

Etymology. The new species is named after Vietnam, where the species was found for the first time. The species epithet, vietnamensis is derived from “ Vietnam ”. The name is used as a noun in apposition.