Gohado (34°45'56.04"N, 126°22'22.38"E), Mokpo, Jeollanam-do, Republic of Korea.

Holotype. • 1♀ (MABIK CR00258872), preserved in a vial with 95% ethanol, collected from washings of marine plastic debris (a gunny sack) at the type locality on 14 March 2022 by GH Han. Paratypes. • 2♀♀ (MABIK CR00258874–00258875), each dissected and mounted on 2 H-S slides; • 2♀♀ (MinRB-Hr104-L001), preserved in a vial with 95% ethanol; • 1♂ (MABIK CR00258876), dissected and mounted on 2 H-S slides; and • 1♂ (MinRB-Hr104-L002) and 1♂ (MABIK CR00258873), each preserved in a vial with 95% ethanol. All material was collected from the type locality.

• 1♀ (MInRB-Hr104-S004), dissected and mounted on 2 H-S slides, collected from washings of marine plastic debris (expanded polystyrene buoy) at Chagwido Port, Jeju (33°19'25.86"N, 126°09'56.88"E) on 20 September 2022 by GH Han.

Female. Body length 441–535 μm (n = 6; measured from anterior margin of rostrum to posterior margin of caudal rami). Habitus broad, moderately flattened and gradually tapering posteriorly in dorsal view, separation between prosome and urosome indistinct (Fig. 1A). Cephalothorax bell-shaped, slightly broadening posteriorly, maximum width 197 µm at posterior margin; dorsally with several paired sensilla, diminutive spinules, and reticulation (observable under high magnification); posteriorly margin fringed with small spinules. Cephalothorax and pedigerous somites laterally ornamented with minute spinules. Free pedigerous somites with dense spinules and scattered sensilla along posterior margins; also, with particular reticulation (observable under high magnification). Urosomites (Figs 1A, 3A) posteriorly fringed with minute spinules (except anal somite); dorsal and ventral surfaces ornamented with transverse rows of fine spinules. Genital double-somite with a transverse spinular row dorsally and laterally, indicating original segmentation; ventrally completely fused. Genital field located near anterior margin of genital double-somite; either side of genital field covered by lobe derived from P6, bearing two setae; each lobe with a small triangular process medially. Anal somite subequal in length to caudal rami, with spinular ornamentation; anal operculum bearing rows of diminutive spinules and pair of sensilla.

Caudal rami (Figs 1A, 3A) ~1.25 × as long as wide; each ramus with seven setae and a ventral tube pore: seta I bare and shortest; setae II and III bare, the latter 1.5 × as long as the former; setae IV and V well-developed, basally fused, and bipinnate; seta VI unipinnate, shorter than seta II; and seta VII tri-articulate, inserted in the distal third.

Rostrum (Fig. 1B, C) completely defined basally, triangular, with a pair of sensilla subdistally; midventral tube pore in subapical position; dorsal surface with reticulate ornamentation.

Antennule (Fig. 2A) 7-segmented; segments 2–6 with several rows of diminutive spinules present on dorsal surface. First segment with several inner spinular rows; second and third segments with inner long spinules. Apical acrothek consisting of an aesthetasc basally fused to two slender setae. Armature formula: 1-[1], 2-[9], 3-[8], 4-[1 + (1 + ae)], 5-[1], 6-[2], 7-[7 + acro].

Antenna (Fig. 2B) 3-segmented. Coxa (not figured) small. Allobasis with one abexopodal seta located at distal third of segment length, and with two rows of spinules. Endopod with a proximal inner tuft of spinules and two subapical frills; laterally with two spines and one slender seta; distally with two strong spines and three geniculate setae (of which most outer one basally fused to a short seta). Exopod 1-segmented, small, ~3 × as long as wide, with four pinnate setae.

Mandible (Fig. 2C) with well-developed gnathobase bearing several blunt teeth around distal margin and one pinnate seta at distal corner. Mandibular palp small, uniramous, with one (exopodal) bare and three (endopodal) pinnate lateral setae, and one (basal) plumose apical seta ~1.7–1.8 × length of palp.

Maxillule (Fig. 2D). Praecoxa with few spinules along outer margin; arthrite with a slender seta on anterior surface and eight elements around distal margin; with one row of long spinules on posterior surface. Coxa and basis each with one cylindrical endite, two spinular rows on anterior surface. Coxal endite with one slender seta and one pinnate spine. Basal endite with two slender setae and one pinnate spine. Endopod incorporated into basis, represented by three setae (one pinnate, two bare). Exopod 1-segmented, cylindrical, with two apical setae and two surface rows of fine spinules.

Maxilla (Fig. 2E). Syncoxa; ornamented with one row of long spinules along distal outer edge, several rows of spinules on posterior surface, and minute spinules around inner margin; with two endites, each with one bipinnate spine (spine on proximal endite fused to segment) and two pinnate setae. Allobasis produced into a stout, distally pinnate claw, accompanied by one pinnate and two bare setae. Endopod incorporated into allobasis, consisting of two long basally fused setae and two short elements.

Maxilliped (Fig. 2F). Syncoxa with two pinnate setae and several rows of spinules. Basis longer than syncoxa, outer margin slightly convex, with one transverse row of spinules; palmar margin bearing a longitudinal row of spinules. Endopod 1-segmented, with a long, curved, proximally pinnate claw; with two anterior accessory setae (one small, one long) and one posterior tube pore.

P1–P4 (Figs 3B, 4A–C) with wide and narrow intercoxal sclerite (not figured for P1); all legs with 3-segmented exopods and 2-segmented endopods.

P1 (Fig. 3B). Coxa large, longer than wide, with several spinular rows on surface. Basis as large as coxa, with one bipinnate seta on outer margin and several spinules, and with one bipinnate seta arising near insertion of endopod. Exopod reaching middle of enp-1, 3-segmented: exp-1 with one bipinnate outer seta; exp-2 with one serrate outer spine; exp-3 with two bare outer spines and two geniculate apical setae. Endopod 2-segmented; enp-1 elongated, ~1.7 × as long as exopod, with long spinules along proximal half of inner margin sparsely; enp-2 short, with one strong, serrate claw and one tiny seta; various-sized spinules along outer margin and few short spinules around inner distal corner.

P2–P4 (Fig. 4A–C). Praecoxae small, transversely elongated-triangular, with several spinules around outer corner. Coxae with several spinular rows along outer margin and on anterior surface. Bases with few spinules on inner margin, several spinules around outer distal corner, and one tube pore on anterior surface. Outer margin of basis with one bipinnate spiniform seta (P2) or one bare seta (P3 and P4). Exopods longer than endopods, gradually tapering distally (especially in P2 and P3); each segment ornamented with spinules along outer and anterior margins; exp-1 with inner spinules subdistally; exp-3 longest. Enp-1 shorter than enp-2, with spinules along outer margin and few (P2) or no spinules (P3 and P4) on inner margin, and with one inner seta in P3 and P4 enp-1 (absent in P2); enp-2 with several spinules along outer margin and few spinules near insertion of proximal seta, and with one (P4) or two (P2 and P3) inner setae, two apical setae and one outer seta in P3 and P4 (absent in P2). P3 and P4 enp-2 with a tube pore near distal margin.

Armature formula of P1–P4 as follows:

P5 (Fig. 3C). Baseoendopod and exopod separate; anterior surface of both segments covered with spinules (ornamentation of exopod denser than that of baseoendopod). Baseoendopod with one bare basal seta arising from a short and thick setophore; endopodal lobe reaching to proximal fifth of exopod, bearing five bipinnate setae, outermost shortest. Exopod oval-shaped, ~1.8 × as long as wide, bearing one bare and five pinnate setae, of which bare one longest and located terminally.

Male (based on the paratypes). Body (Fig. 1D) smaller than female. Body length 349–400 μm (n = 3; measured in the same way as in females), maximum width 148 µm at posterior margin of cephalothorax. Sexual dimorphism present in genital segmentation, antennule, P2–P6.

Urosome (Fig. 3D) narrower than in female, 6-segmented; genital somite and first abdominal somite completely separate. Second to fourth abdominal somites with a row of strong spinules along posteroventral margins. Caudal rami as in female.

Antennule (Fig. 5A) 8-segmented, subchirocer, geniculate between segments 5 and 6. First segment as in females; second segment with nine setae, including two pinnate; fifth segment swollen (Fig. 5a); eighth segment triangular. Armature formula: 1-[1], 2-[9], 3-[8], 4-[2], 5-[9 + 2 pinnate + 2 modified + 2 teethlike elements + (1 + ae)], 6-[3 elements], 7-[1], 8-[8 + acro].

Swimming legs. P1 and P2 similar to those of female, except for P2 enp-1 and enp-2 subequal in length (enp-2 relatively shorter than that of female), with one inner and two apical setae (Fig. 5B).

P3 and P4 (Fig. 5C, D). Exopods more robust than those of females. P3 exp-3 considerably shorter, with thicker spines. P3 endopod not reaching to distal margin of exp-2; enp-1 without inner seta; enp-2 with one inner and two apical setae and one anterior tube pore; outer lateral margin produced into a short apophysis bearing distal cuspidate tip; coarse spinules present near at base of apophysis. P4 endopod reduced, 1-segmented, with two apical setae and one anterior tube pore.

P5 (Fig. 3D, E). Left and right baseoendopods fused medially, each with one outer basal seta; endopodal lobe vestigial, represented by two bare setae; posterior margin with two tube pores. Exopod 1-segmented, oblong, with one inner, one apical and two outer setae; inner seta shortest; spinules present on outer margin and anterior surface.

P6 (Fig. 3D) asymmetrical, one side functional, the other fused to somite; outer sides of both plates produced into cylindrical processes, each bearing one inner pinnate and one outer bare seta.

Morphological variation was observed in the armature formula of the thoracopods in P. setadefectus sp. nov. In females, the P2 enp-2 typically bears two inner setae; however, in one of the five specimens examined, three inner setae were present.

The species name setadefectus is derived from the Latin words seta (bristle) and defectus (lacking, missing), referring to the absence of the proximal endite bearing a seta of maxilla—a distinguishing feature of this species among its congeners. It is an adjective in the singular nominative, gender masculine.

Species of the genus Pseudonychocamptus exhibit marked sexual dimorphism, particularly in the following: the inner seta on P3–P4 enp-1, present in females, is absent in males; and the two-segmented P4 endopod in females is reduced to a single segment bearing two setae in males. Notably, the latter trait represents the most prominent apomorphy of the genus (Huys and Lee 2009). The newly discovered species from Korea exhibits both of these sexually dimorphic characteristics, thereby supporting its assignment to this genus.

The most distinctive feature of P. setadefectus sp. nov., which sets it apart from its six congeners, is the presence of only two endites on the maxilla, lacking the proximal endite represented by a single seta that is present in all other species of the genus. This condition represents a unique and highly unusual trait within Pseudonychocamptus.

The new species, P. setadefectus sp. nov., shares the presence of two inner setae on the female P3 enp-2 with four of the six valid species in the genus. These species are P. abbreviatus (Sars G.O., 1920), P. colomboi Ceccherelli, 1988, P. marinovi Apostolov & Petkovski, 1980, and P. proximus (Sars G.O., 1908). Conversely, P. koreni (Boeck, 1873) and P. spinifer Lang, 1965 possess only one inner seta on the female P3 enp-2. Among these, P. setadefectus sp. nov. is most similar to P. colomboi in that the male P2–P3 enp-2 bears only a single seta. It should be noted, however, that the male of P. marinovi has not yet been described. Previous studies (e.g., Ceccherelli 1988; Huys and Lee 2009) indicate that P. marinovi can be distinguished from the other three species (P. abbreviatus, P. colomboi, and P. proximus) by the broader than long caudal rami.

Although the morphological differences between females of P. setadefectus sp. nov. and P. colomboi are relatively subtle, the new Korean species can be readily distinguished by several characters. The fourth outermost seta on the P5 exopod is ~1.5 × longer than the third outermost seta in P. setadefectus sp. nov. (Fig. 3C), whereas these setae are subequal in length in P. colomboi (Ceccherelli 1988: fig. 8F). Additionally, the apical (basal) seta on the mandibular palp in P. setadefectus sp. nov. is 1.7–1.8 × as long as the total length of the palp (Fig. 3D), compared to P. colomboi, in which it exceeds twice the palp length (Ceccherelli 1988: fig. 7H). In males, the P5 exopod of P. setadefectus sp. nov. possesses an inner seta (Fig. 3D), which is absent in P. colomboi (Ceccherelli 1988: fig. 9G). Furthermore, the P6 is asymmetrical in P. setadefectus sp. nov. (Fig. 3D), whereas it is symmetrical in P. colomboi (Ceccherelli 1988: fig. 9A).

In addition to the aforementioned differences, several minor features further distinguish P. setadefectus sp. nov. from P. colomboi: the maxillary endopod bears four setae in P. setadefectus sp. nov., but three in P. colomboi; the maxillipedal claw is accompanied by two accessory setae in the new species, versus one in P. colomboi; the inner setae on the female P2 enp-2 differ in relative length—the distal seta is ~2 × as long as the proximal in P. setadefectus sp. nov., whereas their length ratio is ~3:1 in P. colomboi; all setae, except for the longest apical seta, on the female P5 exopod are ornamented in P. setadefectus sp. nov., but all are bare in P. colomboi; the two setae of the female P6 differ in length by more than twofold in P. setadefectus sp. nov., but they are subequal in P. colomboi; the male P2 enp-1 and enp-2 are subequal in length in P. setadefectus sp. nov., whereas the enp-2 is slightly shorter in P. colomboi; and P. setadefectus sp. nov. possesses a single pair of sensilla near the apex of the rostrum, compared to two pairs in P. colomboi.

(1) • Gohado (34°45'56.04"N, 126°22'22.38"E), Mokpo, Jeollanam-do, marine rope debris; 01 April 2021; leg. OH Yu, SL Kim and SM Kang: 1♀ (MinRB-Hr105-L001), 3♀♀ (MABIK CR00258880), 1♂ (MinRB-Hr105-L002) and 3♂♂ (MABIK CR00258881), preserved separately in a vial with 95% ethanol; 1♀ (MinRB-Hr105-S003), 2♀♀ (MABIK CR00258882–00258883), 1♂ (MInRB-Hr105-S006) and 2♂♂ (MABIK CR00258884–00258885), each dissected and mounted on 1 or 2 H-S slides.

(2) • Naechi Beach (34°53'17.94"N, 126°00'05.68"E), Sinan, Jeollanam-do, expanded polystyrene buoy; 14 September 2022; leg. GH Han: 1♀ (MABIK CR00258877), dissected and mounted on 2 H-S slides. • Same locality, fishing net; 01 February 2024; leg. SL Kim and SJ Gwak: 5♀♀ (MinRB-Hr105-L003), preserved in a vial with 95% ethanol.

(3) • Masan-ri (36°00'58.99"N, 129°29'05.55"E), Pohang, Gyeongsangbuk-do, green algae; 27 February 1997; leg. J Lee; 1♂ (MinRB-Hr105-L004), preserved in a vial with 95% ethanol. • Same area, coralline algae (36°00'59.90"N, 129°28'58.32"E); 23 April 2005; leg. J Lee: 1♀ (MABIK CR00258878, asymmetry in the number of setae on P2 enp-2), dissected and mounted on 2 H-S slides and 1♀ (MABIK CR00258879), preserved in a vial with 95% ethanol. • Same area (36°00'59.90"N, 129°28'58.32"E), coralline algae; 26 April 2005; leg. J Lee: 2♀♀ (MinRB-Hr105-L005, asymmetry in the number of setae on P2 enp-2), preserved in a vial with 95% ethanol.

Female. Body length 827–1,098 μm (n = 17; measured from anterior margin of rostrum to posterior margin of caudal rami). Habitus slender, cylindrical, and gradually tapering posteriorly in dorsal view, separation between prosome and urosome indistinct (Fig. 6A). Cephalothorax with diminutive spinules and several sensilla dorsally; posterior margin smooth, bearing several sensilla. Free pedigerous somites with diminutive spinular rows on dorsal surface, and one or two rows of spinules and several sensilla along posterior margin. Ventrolateral margins of cephalothorax fringed with a closely set of spinules (Fig. 6a). Urosomites (Figs 6A, 7G) with spinules along posterior margin (except anal somite); dorsal and ventral surfaces with patterns of diminutive spinules. Genital double-somite with a transverse spinular row dorsally and laterally, indicating original segmentation; ventrally completely fused. Genital field located near anterior margin of genital double-somite; each side covered by lobe derived from P6, bearing one inner vestigial and one outer long seta; small triangular process present medially on each lobe. Anal somite as long as caudal rami, ornamented with tiny dorsal spinules and lateral/ventral spinules along posterior margin; semicircular operculum ornamented with marginal spinules and pair of sensilla.

Caudal rami (Figs 6A, 7G) ~1.7 × longer than wide; with seven setae: seta I bare, diminutive, identified at high magnification; setae II and III bare, the former shorter than the latter; setae IV and V well-developed, fused basally to each other, and bipinnate; seta VI bare, shorter than seta II, seta VII tri-articulate, arising dorsally in distal 1/3 of ramus.

Rostrum (Fig. 6A) completely defined basally, triangular, with pair of sensilla near apex; midventral tube pore in subapical position (not figured).

Antennule (Fig. 7A) 7-segmented; segments 1–5 covered with minute spinules on dorsal view. First segment short, with several spinular rows; second segment with outer distal spinules; third segment longest; fourth segment with one seta fused basally to an aesthetasc on a distal peduncle; seventh segment tapering distally, with apical acrothek composed of one aesthetasc and two long setae. Armature formula: 1-[1], 2-[8], 3-[7], 4-[1 + (1 + ae)], 5-[1], 6-[2], 7-[7 + acro].

Antenna (Fig. 7B) 3-segmented, comprising coxa, allobasis, and 1-segmented endopod. Coxa small, with a row of spinules. Allobasis with 1 abexopodal seta located approximately midway along outer margin of segment, and with long setules near base of exopod. Endopod as long as allobasis, ornamented with long spinules along inner proximal margin and rows of spinules along outer distal margin; lateral armature consisting of two pinnate spines and one slender seta; distal armature composed of two strong spines and three geniculate setae (of which most outer one basally fused to short seta). Exopod small, 1-segmented, with three slender setae, middle one shortest inner one longest.

Mandible (Fig. 7C). Coxa transversely elongated; gnathobase with one bicuspidate, three multicuspidate teeth, and one pinnate spine around distal margin and one pinnate seta at distal corner. Palp small, uniramous, with four bare (1 exopodal, 3 endopodal) lateral setae and one pinnate apical (basal) seta; with few small spinules present at base of proximal lateral seta.

Maxillule (Fig. 7D). Praecoxa with a row of outer spinules; arthrite with one subdistal and eight distal elements; posterior surface with a row of spinules. Coxa with a row of long spinules on anterior surface and few spinules around inner distal margin; cylindrical endite bearing one spinulose and one slender seta. Basis with one anterior spinular row; endite with one spinulose and two slender setae. Endopod nearly incorporated into basis, represented by three bare setae, of which middle one smaller than others. Exopod small, 1-segmented, with two long apical setae.

Maxilla (Fig. 7E). Syncoxa with a long spinular row around outer margin and two rows of small spinules on surface; with three endites: proximal endite (praecoxal) bearing one pinnate seta; middle endite with one serrate spine and two pinnate setae, of which spine fused basally to segment; distal endite with one spine and two pinnate setae. Allobasis drawn out into a stout claw, with one spine (bearing three spinules near midlength) and two slender setae. Endopod represented by three bare setae (1 short, 2 long).

Maxilliped (Fig. 7F) 3-segmented, comprising syncoxa, basis, and 1-segmented endopod. Syncoxa with two distal plumose setae and several spinular rows. Basis longer than preceding segment, outer margin slightly convex, with two rows of spinules; inner margin with a row of strong spinules in distal half. Endopod 1-segmented, with a long, curved claw; with one short bare seta anteriorly and a tube pore posteriorly (as in P. setadefectus sp. nov., Fig. 2F); concave inner margin bearing row of spinules.

P1 (Fig. 8A). Intercoxal sclerite transversely elongated and narrow. Praecoxa damaged (not shown). Coxa large, proximal outer margin protruded, with spinular rows along inner and outer margins. Basis with three spinular rows; one outer and one inner seta (displaced anteriorly). Exopod shorter than enp-1, 3-segmented, each segment with outer spinules; exp-1 and exp-2 with one outer spine; exp-2 longest; exp-3 with two outer spines and two geniculate apical setae. Endopod 2-segmented, enp-1 elongated, ~2 × as long as entire exopod; enp-2 short, bearing one strong, serrate claw and one small seta, and several spinules along outer margin.

P2 and P3 (Fig. 8B, C). Intercoxal sclerites subrectangular. Praecoxae transversely elongated triangular, with outer spinular rows. Coxae and bases ornamented with spinular rows along outer margin and on anterior surface. Bases each with one bare outer seta and one anterior tube pore. Exopods 3-segmented, longer than endopods; each segment ornamented with strong spinules as illustrated; exp-2 shortest, exp-3 longest; exp-1 with one outer spine; exp-2 with one outer spine and one inner seta; exp-3 with three outer spines, one apical spine, one apical seta, and one inner seta. Endopods 2-segmented, enp-1 as long as enp-2 in P2, but much shorter than enp-2 in P3, with long inner spinules and short outer spinules, with (P2) or without (P3) one tube pore on outer corner; enp-2 with spinules along outer margin and on anterior surface (P3); with one tube pore located proximally (P2) or distally (P3), bearing two inner (occasionally 3 in P3), two apical and no outer (P2) or one (P3) outer seta.

P4 (Fig. 8D). Intercoxal sclerite elongated and narrow (see Fig. 9E). Praecoxa small, transversely elongated and triangular. Coxa and basis with spinular rows along outer margins and on anterior surface. Basis with one bare outer seta and one anterior tube pore. Exopod 3-segmented, gradually tapering distally, longer than endopod; exp-1 broad, with several spinular rows on anterior surface; exp-2 shortest, with one outer, slender, and elongated spine and one inner seta; exp-3 with three outer, slender, and elongated spines, and two apical elements. Endopod 2-segmented, enp-1 with long inner spinules; enp-2 ~2.5 × as long as enp-1, with spinules on anterior surface, bearing one inner, two apical, and one outer seta.

Armature formula of P1–P4 as follows:

P5 (Fig. 8E) with baseoendopod and 1-segmented exopod, both surfaces covered with fine spinules. Baseoendopod with one bare basal seta arising from long cylindrical setophore; endopodal lobe extending to distal third of exopod, bearing spinules along inner margin, and armed with three inner and two distal bipinnate setae. Exopod widely subquadrate with slightly convex inner margin, strongly sclerotized and dented between innermost distal seta and longest seta; with three bare and three pinnate setae.

Male. Body (Fig. 6B) generally similar to that of female, but with a more or less slender urosome. Body length 688–927 μm (n = 15; measured in the same way as in females). Cephalothorax and pedigerous somites with ornamentation patterns as in females (Fig. 6B, b). Sexual dimorphism in genital segmentation, antennule, P2–P6.

Urosome (Figs 6B, 9B) 6-segmented; genital somite and first abdominal somite completely separate; ornamentation as figured.

Antennule (Fig. 9A) 8-segmented, subchirocer, geniculate between segments 5 and 6. First segment with several spinular rows; fourth segment very small; fifth segment swollen; eighth segment triangular. Armature formula: 1-[1], 2-[9], 3-[8], 4-[2], 5-[8+ 1 pinnate + 2 modified + 2 teethlike elements + (1 + ae)], 6-[3 elements], 7-[1], 8-[8 + acro].

P2–P3 (Fig. 9C, D). Exopods broader than in females; exp-3 considerably shorter, with thicker spines. P2 endopod: enp-1 slightly longer than enp-2, with long spinules along inner and outer margin, and one large pore near distal corner; enp-2 with long spinules along outer margin, one tube pore on proximal anterior surface, and inner element modified as strong bare spine. P3 endopod: enp-1 as in females; enp-2 ornamented with long inner spinules and stout spinules along outer margin proximally; bearing one short apophysis on outer lateral margin, two inner and two distal setae (shorter than in females) and one tube pore on distal anterior surface; apophysis weakly curved outwardly at distal tip, bearing one stout and one small strengthen spinules (protuberance ?) near its base.

P4 (Fig. 9E). Exopod considerably longer than in females, with thicker spines. Endopod as in females, except enp-2 bearing tube pore on distal anterior surface.

P5 (Fig. 9B). Left and right baseoendopods medially fused, baseoendopod and exopod fused, forming small lobe bearing seven setae, of which two inner ones diminutive; posterior margin with four tube pores.

P6 (Fig. 9B) symmetrical, represented by small lobe bearing one median spine and one outer seta, and ornamented with spinular row along distal margin.

Previous descriptions of H. discophora (e.g., Lang 1965: fig. 263i; Itô 1974: fig. 44.3) report three inner setae on the female P3 enp-2. However, examination of 17 Korean specimens revealed notable variation in thoracopod setal armature. Thirteen specimens from Gohado and Naechi exhibited only two inner setae on both left and right rami (Fig. 8C). In contrast, left–right asymmetry in the number of inner setae was found exclusively in specimens from Masan-ri, where three of the four individuals examined showed this trait (Fig. 8F), indicating significant intraspecific variability within the species.

In a female specimen from Gohado, caudal seta IV on the right caudal ramus was underdeveloped and deeply embedded within the ramus (Fig. 7G). Additionally, another female specimen exhibited an abnormal P4 exopod. Compared to the normal morphology of P4 (Fig. 8D), the distal segment of the abnormal exopod was markedly shortened and bore five spines/setae, with two distal spines fused together (Fig. 8G).

Heterolaophonte discophora (Willey, 1929) was originally described by Willey (1929) based on a female specimen collected from St. Andrews, New Brunswick, Canada. The male was subsequently reported from Passamaquoddy Bay, New Brunswick, by Willey (1931). However, these initial descriptions and illustrations were brief and provided limited morphological details, particularly regarding the P5 of both sexes and the P2–P4. Lang (1965) later provide a detailed redescription based on specimens collected from Monterey Bay, near Hopkins Marine Station, and treated H. rotundipes Chappuis, 1958, (originally described from Puget Sound near Seattle, Washington, USA) as a junior synonym of H. discophora. Following Lang’s taxonomic revision, Itô (1974) also redescribed the species based on male and female specimens from Hokkaido, Japan. Given the incompleteness of the original description, the present comparison of the Korean material relies primarily on the comprehensive accounts by Lang (1965) and Itô (1974).

Heterolaophonte discophora was first reported from Korea in the doctoral dissertation by Song (2000). In that study, Song identified specimens collected from macroalgae at Masan-ri, Pohang, as H. discophora, providing a brief species description, with the setal formula of P2–P4 and illustrations of the female P1, P4, P5, the A2 exopod, the caudal ramus, and the male P5. He noticed that the Korean H. discophora machted Itô’s (1974) redescription in all characters except for the relative lengths of the two short inner setae on the male P5. We obtained Korean specimens of H. discophora from MPD stranded along the coasts of Gohado, Mokpo, and Naechi Beach, Sinan, as well as Masan-ri. Contrary to Song’s (2000) observations, our examination of these newly collected Korean specimens reveals additional morphological differences not previously reported.

One of the diagnostic traits of H. discophora is the shape of the P4 exp-3, which is slender and elongated, gradually broadening distally in its proximal part (Willey 1929: fig. 2; Lang 1965: fig. 264a). However, this characteristic appears less pronounced in Japanese specimens (Itô 1974: fig. 264a, a.1) and in the Korean material examined here (Fig. 8D). Nonetheless, the markedly large and broad P4 exp-1 in H. discophora, relative to exp-2 and exp-3, may represent a more reliable diagnostic feature of this species than the morphology of exp-3 when compared to congeners.

Comparative analysis of the Korean specimens with previous records of H. discophora reveals four distinct morphological traits. Lang (1965) reported four setae on the endopodal lobe of the female P5, whereas Chappuis (1958) and Itô (1974) described five; the Korean specimens also possess five setae on this lobe. It is possible that Lang (1965) observed a variable condition of the female P5, considering that most Heterolaophonte species have five setae on the corresponding segment, except for H. exigua (Scott, 1912), H. norvegica Drzycimski, 1968, and H. tupitskyi Chislenko, 1976. In the male P5, Lang (1965) and Willey (1931) described five elements, whereas Chappuis (1958) and Itô (1974) noticed seven elements, including two additional small inner setulae—features also observed in the Korean material. It appears that these two inner ornamentations were interpreted as armatures by earlier authors. Regarding the female P3 endopod, Itô (1974) reported three inner setae on the enp-2; however, most Korean specimens only have two, with occasional asymmetry (bearing 2 or 3 inner setae) between the left and right rami (see variability above). Lang (1965) described the male P3 endopod as three-segmented, and Itô (1974) referred to a tripartite condition, though with indistinct boundaries. Conversely, the Korean specimens consistently exhibit a two-segmented endopod, although a posterior suture line is variably developed—absent in some individuals, faint in others, and distinctly visible in a few—potentially leading to interpretative ambiguity. Similar variable chaetotaxy has occasionally been reported in laophontid copepods, particularly in the genus Quinquelaophonte, which is closely related to Heterolaophonte (Willen 1996; Kim and Lee 2023). Finally, Itô (1974) noted a single triangular, spur-shaped protuberance on the outer distal corner of the male P3 enp-2, which, unfortunately, was not mentioned by Lang (1965). In contrast, the Korean specimens exhibit two triangular, strengthened spinules (protuberances?) of unequal size in the same position, with a similar row of strong spinules also present proximally along the outer margin. This ornamentation was probably overlooked by earlier authors.

In addition, the Korean specimens exhibit several minor differences from previous accounts. They have three setae on the endopod of the maxilla, whereas Lang (1965) described two, and Itô (1974) reported only one. In the female P5 exopod of the Korean material, only three of the five setae are bare, whereas all were described as bare in previous accounts. In the male P4 exopod, the terminal segment is similar in length to the second segment, but it is slightly longer in Lang’s (1965) and Itô’s (1974) material. In the male P2 enp-2, the apical outer seta is longer than the inner distal seta, whereas the reverse condition was described previously. Except for the first difference, these minor disparities are considered herein as intraspecific variability.