Dokdo Island, Korea; 37°14'37.23"N, 131°51'39.61"E; depth 15 m.

Holotype. • One ♀ (MABIK CR00261557) preserved in a vial with 80% ethanol, collected from the type locality, 22 June 2016, T.W. Jeong leg. Allotype. • One ♂ (MABIK CR00261558) preserved in a vial with 80% ethanol, collection data as in holotype. Paratypes. • Two ♀♀ (MABIK CR00261559, MABIK CR00261560) and two ♂♂ (MABIK CR00261561, MABIK CR00261562) each dissected on three H-S slides, collection data as in holotype.

Female. Body length range 795–959 μm (n = 3, mean = 876 μm, holotype = 874 μm) measured from anterior tip of rostrum to posterior end of caudal rami (in lateral view). Habitus (Fig. 2A) subcylindrical, without clear demarcation between prosome and urosome. Prosome (Fig. 2A) slightly shorter than urosome, comprising cephalothorax (cephalosome completely fused with first pedigerous somite) and three free pedigerous somites; dorsal surface ornamented with integumental pores and sensilla as figured (Fig. 2A). Cephalothorax (Fig. 2A) ~ 1.4× as long as wide, greatest width 157 μm measured at posterior end of cephalothorax.

Urosome (Fig. 2A, B) comprising P5-bearing somite, genital double-somite, and three free abdominal somites. Genital somite and first abdominal somite fused, forming genital double-somite; original segmentation indicated dorsally by a faint line on dorsal surface and internal chitinous ribs on ventrolateral regions; ornamentation composed of ventral and dorsal pores and sensilla as figured (Fig. 2A, B). Genital field (Figs 2B, 4F) located medially in anterior half of genital double-somite. Genital apertures fused to median V-shaped slit. Copulatory pore located at approximately midpoint of double-somite length (Fig. 2B). First free abdominal somite (Fig. 2A, B) slightly longer than second free abdominal somite; with dorsal and ventral sensilla and pores. Second free abdominal somite (Fig. 2A, B) with pores only. Anal somite (Fig. 2A, B) longer than penultimate somite, with one pair of dorsal sensilla, one pair of ventral pores, and two pairs of ventrolateral pores; posteroventral margin with minute spinules; anal operculum semicircular, posterior margin with fine spinules.

Caudal ramus (Fig. 2A–D) cylindrical, ~ 2.6× as long as maximum width in dorsal aspect and 2.1× as long as wide in lateral view; dorsomedial surface slightly concave and constricted near insertion of seta VII, but cylindrical in ventral view; with a weak ventroposterior protuberance and three pores, one each on dorsal, ventral, and lateral surfaces; with seven setae: setae I and II positioned on anterior 1/3 of lateral margin; seta I diminutive; seta II slender and naked, 5× longer than seta I; seta III arising from anterior 4/5 of lateral margin, slightly shorter than seta II; setae IV and V fused basally, the former slender; seta V longest; seta VI ~ 1/2 length of seta II; seta VII tri-articulated, inserted dorsally at mid-length.

Rostrum (Fig. 2A) small, subrectangular with rounded corners, completely defined basally, with one pair of subapical sensilla and one apical median pore.

Antennule (Fig. 3A) nine-segmented; first segment elongated, ~ 3× as long as second segment; with strong acute process on outer distal corner, a proximal pore, and several minute inner spinules, and one naked seta; second segment with nine naked setae and a pore; fourth segment with aesthetasc fused basally to one long seta on inner distal corner; seventh and eighth segments small, each with one bi-articulate and one naked seta; ninth segment elongate, with four bi-articulate setae, one naked seta, and apical acrothek consisting of two setae and slender aesthetasc. Armature formula as follows: 1-[1], 2-[9], 3-[8], 4-[3 + (1 + ae)], 5-[2], 6-[4], 7-[2], 8-[2], 9-[5 + acrothek].

Antenna (Fig. 3B). Coxa small, unornamented. Basis elongated, ornamented with rows of spinules along inner and outer margins. Exopod arising distally on basis, one-segmented, ~ 4× as long as wide; ornamented with several rows of minute spinules, and with three pinnate setae (outer distal seta incompletely fused to the exopod), outer distal seta slightly longer than inner distal seta. Endopod two-segmented; first segment as long as basis, unarmed; second segment as long as preceding segment, ornamented with inner and distal spinules, and two hyaline frills (one subdistal and one distal); laterally with two naked setae and two bipinnate spines; distally with one tube pore (indicated by arrowhead in Fig. 3B) and seven elements (one bipinnate spine, one pinnate, one naked, and four geniculate pinnate setae), of which two outermost setae (one geniculate pinnate and one pinnate) fused at base.

Mandible (Fig. 3C). Coxa with a rounded protuberance medially; gnathobase with two tri-cuspidate and several bi- and multi-cuspidate teeth, and one unipinnate seta fused to one small spine; subdistally ornamented with three rows of spinules. Palp biramous; basis elongated, gradually widening towards medial margin, with two rows of long spinules on surface, several spinules on disto-medial corner, and three subequal pinnate setae along inner margin. Exopod two-segmented, extending beyond half the length of endopod; first segment longer than second, with one proximal and one distal seta; distal segment with one subapical and three apical setae. Endopod one-segmented, elongated, with two lateral and seven distal setae, including two pairs of basally fused setae.

Maxillule (Fig. 3D). Praecoxal arthrite well-developed, with two juxtaposed and naked anterior setae, eight distal spines, one naked subdistal seta, one lateral pinnate seta, and two pinnate posterior setae. Coxa with one epipodal seta and one row of spinules anteriorly; cylindrical endite with subdistal spinular row and five distal elements (one spine and four setae). Basis elongate, ornamented with subdistal spinular row, and with one spine and seven setae medially. Exopod elongate, with inner setules and three distal bipinnate setae. Endopod as long as exopod, with outer setules and four pinnate setae (three distal and one subdistal).

Maxilla (Fig. 3E). Syncoxa large, ornamented with several spinules along outer margin and a row of spinules on anterior surface; with four endites: proximal praecoxal endite with a subdistal spinular row, one bipinnate seta and one pinnate spine (fused to endite); distal praecoxal endite with one pinnate seta; proximal and distal coxal endites each with a row of spinules on posterior surface subdistally and three pinnate setae distally. Allobasis drawn out into a curved claw accompanied by one tube pore (indicated by arrowhead in Fig. 3E), with three subdistal setae (one short naked and two long pinnate), and one proximal naked seta; with a subdistal spinular row on posterior surface. Endopod small, three-segmented; first segment with three setae, one of these very short; second segment with one seta; third segment with three setae.

Maxilliped (Fig. 3F) subchelate, arising from small pedestal. Syncoxa with several spinular rows and three pinnate setae on inner distal corner. Basis longer than syncoxa, with several long spinules and a row of minute spinules, and two pinnate setae along inner margin. Endopod drawn out into strong, slightly curved claw bearing minute spinules in distal half; one anterior tube pore (indicated by arrowhead in Fig. 3F); accessory armature consisting of one pinnate seta and one naked seta.

P1 (Fig. 4A). Intercoxal sclerite small, trapezoidal. Praecoxa not illustrated. Coxa large, with rows of anterior and posterior spinules as figured, and one anterior pore on inner margin and another on outer margin. Basis distinctly smaller than coxa, with two rows of spinules on inner margin and one anterior pore near outer margin; with one outer and one inner bipinnate spine. Exopod three-segmented, each segment with outer marginal spinules; exp-1 and exp-2 with outer bipinnate spines, respectively; exp-2 elongate, with inner setules; exp-3 smallest, with one posterior pore and four elements (two outer pinnate spines and two distal geniculate pinnate setae). Endopod distinctly longer than exopod, two-segmented; enp-1 elongated, as long as exp-1 to exp-3 combined, 2.6× as long as enp-2, with inner setules, outer spinules, and one inner seta with proximal half bearing long spinules and distal half pectinate on one margin and pinnate on the other, and extending far beyond tip of enp-2; enp-2 ~ 3.6× as long as wide, with a row of spinules and three setae (two geniculate pinnate and one short naked).

P2 (Fig. 4B). Intercoxal sclerite nearly rectangular. Praecoxa not illustrated. Coxa large, with several rows of anterior and posterior spinules as figured and one anterior pore near outer margin. Basis with inner marginal spinules, one anterior pore, and one outer pinnate spine; distal margin with two small acute processes (inner and median) and minute marginal spinules between two processes. Exopod three-segmented, each segment ornamented with inner and outer marginal spinules; exp-1 and exp-2 each with inner distal hyaline frill and one outer bipinnate spine; exp-1 with posterior spinules and one inner bipinnate seta; exp-2 with inner setules proximally, one anterior pore, and one inner pinnate seta; exp-3 with one anterior pore, one inner pinnate seta, one apical pinnate spine, one apical pinnate seta, and three outer pinnate spines; inner setae on exp-2 and exp-3 subequal in length, slightly longer than terminal segment length. Endopod two-segmented, reaching mid-length of exp-3; each segment ornamented with inner and outer spinules; enp-1 with one anterior pore and one inner unipectinate seta; enp-2 longer and slenderer than preceding segment, with one outer pinnate spine and two apical pinnate setae; outer spine elongate, longer than supporting segment (enp-2); distal margin with two distinct processes.

P3 (Fig. 4C). Intercoxal sclerite, coxa, and basis as in P2, but basis lacking inner spinules and bearing one elongate and plumose outer seta. Exopod and endopod as in P2, except exp-3 with two anterior pores and only two outer pinnate spines, and enp-2 with one anterior pore; inner seta on enp-1 slightly exceeding distal end of ramus.

P4 (Fig. 4D). Intercoxal sclerite, coxa, and basis as in P3, but outer seta on basis naked and shorter than that on P3. Exopod distinctly elongate, ~ 2.5× as long as endopod; ornamentation of segments as in P2 and P3, but exp-1 without posterior spinules; exp-2 elongate, with one inner bipinnate seta; exp-3 with two anterior pores as in P3 and seven pinnate elements (three inner, two apical, and two outer), of the three inner elements, the two distal ones robust and long; proximal one unipectinate and distal one bipinnate. Endopod as in P2 and P3, but inner seta on enp-1 distinctly exceeding distal end of ramus and outer spine on enp-2 distinctly shorter than enp-2 length, as long as half-length of adjacent distal setae.

Armature formula of P1–P4 as follows:

P5 (Fig. 4E). Baseoendopod and exopod distinct. Baseoendopod consisting of broadened proximal half and triangular distal half; inner margin ornamented with setules in proximal three-quarters and anterior surface with five pores; outer margin extended outwards, with plumose (basal) seta; endopodal lobe with three inner bipinnate setae and two apical setae, of which outer one shortest, naked, and inner one longest, bipinnate, reaching tip of exopod. Exopod elongate, gradually tapering to pointed tip in distal half, ~ 3.1× as long as wide; with one inner and five outer setae, of which second and third outer proximal setae short and basally swollen, and fifth outer seta longest; with one inner and one outer marginal pore, and two anterior pores.

P6 (Fig. 4F) represented by a small segment bearing one plumose and two naked setae; outermost (plumose) seta ~ 2× as long as innermost seta, and middle seta shortest.

Male. Body length range 709–835 μm (n = 3, mean = 778 μm; measurement as in females). Habitus (Fig. 2E) as in females, except for urosomal segmentation. Sexual dimorphism observed in antennule, urosome, caudal rami, and P2–P6.

Urosome (Figs 2E, 5F) six-segmented; genital somite and first abdominal somite completely separated, with pores and sensilla as figured.

Caudal rami (Figs 2E, 5F) cylindrical, tapering distally, ~ 6.5× as long as maximum width; with seven setae; setae I and II positioned on anterior 1/5 of lateral margin; seta III arising from posterior 1/5 of lateral margin; seta VII inserted dorsally at posterior 1/3 of ramus.

Antennule (Fig. 5A) eight-segmented, subchirocer, with geniculation between fifth and sixth segments; first segment as in females; fourth segment smallest; fifth segment swollen, with several rows of spinules and faint suture lines; sixth segment with two unequal elements. Armature formula: 1-[1], 2-[11], 3-[8], 4-[2], 5-[11 + 1 element + (1 + ae)], 6-[2 elements], 7-[1], 8-[9 + acrothek].

P2 exopod as in females, except inner setae on exp-2 and exp-3 longer than those in females. Enp-2 (Fig. 5B) shorter than in females, as long as preceding segment, with one apophysis (basally fused to segment) and two naked setae (inner short and outer long); distal margin without protrusions.

P3 exopod as in females, except inner setae on exp-2 and exp-3 longer than those in the females. Enp-2 (Fig. 5C) ~ 1.3× as long as preceding segment, with one short outer spine bearing unilateral spinules and with bifid tip and two long distal bipinnate setae; outer spine half as long as inner apical seta.

P4 (Fig. 5D) as in females, but exp-2 with modified outer distal corner with concave inner margin and blunt tip (shown separately in Fig. 5D); exp-3 with six elements (two inner, two apical, and two outer). Endopod shorter than in females; enp-2 with inner seta distinctly shorter than that in females and outer spine slightly shorter than outer apical seta.

P5 (Fig. 5E). Baseoendopods fused medially, each with three anterior pores and one outer basal seta; endopodal lobe reaching proximal 1/4 of exopod, with inner marginal spinules, one subdistal bipinnate spine, one distal bipinnate spine, and one distal pinnate seta, of which outermost seta shorter than other spines. Exopod one-segmented, ~ 6× as long as maximum width; with distal end produced into a long extension; with one inner marginal and three anterior pores, and three outer and two inner unequal setae (outer proximal seta longest, ~ 1/2 length of exopod, and inner proximal seta bipinnate distally).

P6 (Fig. 5F) asymmetrical, left plate functional, the other fused to genital somite; outer distal corner bearing one stout spine (innermost) and two slender setae (outer one ~ 3/5 length of inner one).

The specific epithet concavus is derived from the Latin adjective concavus (concave) and refers to the slightly concave dorsomedial surface of the female caudal ramus. It is in the nominative singular. Gender masculine.

Tetragoniceps concavus sp. nov. and four other new species described herein were assigned to Tetragoniceps based on the presence of an elongate first antennulary segment bearing a strong, acute outer process and a non-foliaceous female P5.

The diagnostic characters traditionally used to distinguish Tetragoniceps species (Wells 2007; Mussini et al. 2025) include the segmentation of the female antennule, the number of setae on the antennary exopod, the segmentation of the mandibular exopod, the shape of the caudal rami, the morphology of the female P5, and the armature formulae of swimming legs. Tetragoniceps concavus sp. nov. is readily distinguished from all known congeners by the unique chaetotaxy of its swimming legs, having a single inner seta on exp-1 and exp-2 of P2–P4 and six, five, and seven elements on exp-3 of P2–P4, respectively. This combination of armature patterns has not been observed in previously described congeners.

The species is further distinguished by the shape of the female caudal ramus, with the inner margin irregularly dentate, concave, and constricted at mid-length. In dorsal view, the caudal ramus is cylindrical and 2.6× as long as wide, whereas in lateral view it appears comparatively broader, being 2.1× as long as wide. The male caudal ramus exhibits sexual dimorphism, being considerably more elongate than those of the female and approximately 6.5× as long as wide.

Dokdo Island, Korea; 37°14'23.28"N, 131°52'20.64"E; depth 25 m.

Holotype. • One ♀ (MABIK CR00261563) preserved in a vial with 80% ethanol; collected from the type locality, 28 May 2015, H.S. Rho leg. Allotype. • One ♂ (MABIK CR00261564) preserved in a vial with 80% ethanol, collection data as in holotype. Paratypes. • Three ♀♀ (MABIK CR00261565–CR00261567) and three ♂♂ (MABIK CR00261568–CR00261570) each dissected on three H-S slides, collection data as in holotype.

Female. Body length range 625–846 μm (n = 4, mean = 744 μm, holotype = 846 μm). Habitus (Fig. 6A, B) subcylindrical, without clear demarcation between prosome and urosome, slenderer than body shape of T. concavus sp. nov. Ornamentation composed of numerous integumental pores, small pits (not figured) and sensilla on prosome as figured (Fig. 6A, B). Cephalothorax (Fig. 6A) ~ 1.6× as long as wide, greatest width 122 μm measured at posterior end of cephalothorax.

Genital double-somite (Figs 6A, 6B, 9A) completely fused, original segmentation marked by faint dorsal line and internal chitinous ribs dorsolaterally, laterally, and ventrally. Paired genital apparatus located at anterior 1/6 of genital double-somite; genital pores closed off by reduced P6 (separated from each other), each bearing one long outer plumose seta (longest), one middle naked seta (shortest), and one long inner naked seta; outer seta ~ 1.5× as long as inner seta, and middle seta ~ 1/6 length of inner seta (Fig. 9A). First and second free abdominal somites (Figs 6A, 6B, 9A) subequal in length. Anal somite (Figs 6A, 6B, 9A) slightly smaller than penultimate somite, with one pair of dorsal sensilla, one pair of ventral pores, and two pairs of ventrolateral pores; anal operculum semicircular, ventroposterior margin with minute spinules; anal opening wide.

Caudal rami (Figs 6A–C, 9A) divergent, ~ 1.6× as long as wide in dorsal view, with posterior 2/5 distinctly narrower than anterior 3/5 in lateral view (Fig. 6B, C); ornamented with four pores (Figs 6C, 9A), one row of inner long spinules, and two rows of ventroposterior spinules; with seven setae: setae I and II positioned on anterior 1/5 of lateral margin; seta I diminutive; seta II slender and naked, 2.5× length of seta I; seta III arising from 1/2 on outer margin, as long as seta II; setae IV and V fused basally; seta IV slender, ~ 1.5× as long as caudal ramus; seta V longest; seta VI ~ 2/3 length of seta II; dorsal seta VII tri-articulated basally, inserted in mid-length of ramus.

Rostrum (Fig. 6A) small, separated basally, with one pair of subapical sensilla and one apical median pore.

Antennule (Fig. 7A) nine-segmented and armature formula as in T. concavus sp. nov., but morphological differences detected in the following: first segment ~ 2× as long as second segment; outer protrusion more developed than that of preceding species.

Antenna (Fig. 7B) composed of small coxa, basis, two-segmented endopod, and one-segmented exopod, and armature formula as in T. concavus sp. nov., but morphological difference detected in the following: exopod ~ 3.5× as long as wide, smaller than in T. concavus sp. nov., with three pinnate setae, outer distal seta longer than inner distal seta.

Mandible (Fig. 7C), maxillule (Fig. 7D), maxilla (Fig. 7E), and maxilliped (Fig. 7F) similar to T. concavus sp. nov., but morphological difference detected in the following: mandibular coxa with one row of outer spinules proximally; mandibular palp with three armature elements (most dorsal one uniserrate, spine-like, and longest); mandibular exopod small, 1/3 length of endopod, first segment with one lateral and one subdistal seta, and second segment with two apical setae; maxillular exopod slenderer, but endopod larger than that of T. concavus sp. nov.; maxilla with additional ornamentation of minute spinules on syncoxa, pinnate seta on proximal praecoxal endite much smaller than apical spine (fused to endite basally), and two setae on enp-1, one seta on enp-2, and two setae on enp-3, all unipinnate; syncoxa of maxilliped with one unipinnate (middle and longest) and two bispinulose setae, tip of reflexed endopod reaching proximal 1/6 of basis.

Morphological features of P1–P4 (segmentation and ornamentation) as in T. concavus sp. nov.

P1 (Fig. 8A). Praecoxa triangular, without ornamentation. Coxa with two rows of spinules, several rows of denticles, and one pore on anterior surface. Enp-1 shorter than exopod, 1.6× as long as enp-2, inner seta of enp-1 reaching mid-length of enp-2; enp-2 slender, 6× as long as wide, with only two geniculate pinnate setae (delicate element lacking).

P2 (Fig. 8B). Praecoxa small, transversely elongated. Coxa with two groups of spinules, several rows of denticles and one pore on anterior surface, and two rows of spinules on posterior surface. Two pointed projections on basis distinctly developed. Exp-1 with two rows of spinules (proximal one rounded) on posterior margin; inner seta on exp-3 elongate, 2× as long as that of exp-2. Endopod slightly exceeding tip of exp-2; enp-1 lacking anterior pore; enp-2 as long as exp-2, with several rows of posterior spinules; outer spine on enp-2 ~ 3/5 length of inner distal seta, and outer distal seta elongate (as long as exopod) and plumose.

P3 (Fig. 8C). Praecoxa small, transversely elongated. Coxa with two anterior pores, several rows of spinules and two rows of denticles on anterior surface. Inner seta on exp-3 ~ 2× as long as that of exp-2. Endopod not reaching distal end of exp-2; enp-2 with several rows of posterior spinules, outer spine distinctly shorter than supporting segment, 1/3 length of plumose outer distal seta.

P4 (Fig. 8D). Intercoxal sclerite narrower than in P2 and P3, rectangular. Praecoxa small, transversely elongated. Coxa with three anterior pores, one patch of spinules, three rows of spinules, and several rows of denticles on anterior surface. Two acute processes on basis smaller than those of P2 and P3. Exopod distinctly elongated; exp-2 with one inner plumose seta, ~ 1.2× as long as exp-2, inner margin ornamented with only one row of setules; exp-3 with one small and one large pore on anterior surface, and seven pinnate elements (three inner, two apical, and two outer); two inner distal elements robust and long; outer and apical elements comparatively slenderer than corresponding elements in P2 and P3. Enp-1 small, without inner ornamentation and acute process at outer distal corner; enp-2 with one anterior pore and small outer spine, ~ 1/3 length of apical pinnate setae.

Armature formula of swimming legs as in T. concavus sp. nov., except for P1 endopod with 1.020.

P5 (Fig. 9B). Baseoendopod and exopod distinct, both more elongate and slenderer than in T. concavus sp. nov. Baseoendopod gradually broadening distally, with one outer plumose seta, and ornamented with setules in proximal 2/3 of inner margin and five pores on anterior surface; endopodal lobe triangular in shape, reaching mid-length of exopod; outer distal corner of endopodal lobe with one minute process; with three inner bipinnate setae (distal one slender), one long apical pinnate seta, and one short apical naked seta. Exopod elongate, slender, ~ 5× as long as wide; with one minute inner seta, one long apical seta (2/3 length of exopod), and four outer slender setae, of which second and third proximal setae short; ornamentation with two inner and two outer proximal pores.

Male. Body length range 712–730 μm (n = 4, mean = 724 μm). Habitus (Fig. 6D) as in females, except for antennule, urosome, caudal rami, and P2–P6.

Urosome (Figs 6D, 9D) with pores and sensilla as figured; genital somite and first abdominal somite completely separate. First abdominal somite with minute spinular row on posteroventral surface (Fig. 9D).

Caudal rami (Figs 6D, 9D, 9E) divergent, cylindrical, tapering distally, ~ 3.5× as long as wide, with seven setae: seta III arising from mid-length of lateral margin; seta VII (Fig. 9E) inserted dorsally at anterior 1/3 length of ramus.

Antennule (Fig. 9C) eight-segmented, subchirocer, with geniculation between fifth and sixth segments. First segment 2× as long as wide; fifth segment without suture lines and with spine-like element with bifid tip; sixth segment with two subequal elements. Armature formula: 1-[1], 2-[11], 3-[8], 4-[2], 5-[11 + 1 element + (1 + ae)], 6-[2 elements], 7-[1], 8-[9 + acrothek].

P2, inner setae on exp-2 and exp-3 longer than those in females. Enp-2 (Fig. 8E) shorter than length of enp-1, with one outer apophysis and two distal setae (inner short and outer long); apophysis ~ 1.3× as long as endopod.

P3, inner setae on exp-2 and exp-3 longer than those in females. Enp-2 (Fig. 8F) with two long apical setae and slightly curved outer spine bearing one row of spinules along one margin; outer spine ~ 2/5 length of inner apical seta.

P4, exp-2 (Fig. 8G) with inner seta shorter than in females, and outer distal corner modified into elongate and recurved process with blunt tip (indicated by arrowhead in Fig. 8G); exp-3 (Fig. 8G) with six elements (two inner, two apical, and two outer). Enp-2 (Fig. 8H), inner apical seta distinctly shorter than in female, nearly half as long as outer apical seta.

P5 (Fig. 9F). Baseoendopods fused medially, each with one outer basal seta and four anterior pores; endopodal lobe small and narrow, reaching proximal 1/5 of exopod, with one long inner bipinnate spine, one distal bipinnate spine, and one distal slender seta. Exopods asymmetrical, with unequal segmentation (right leg incompletely two-segmented, with a suture visible only on anterior surface and left leg one-segmented); distal margin strongly produced into a long apical pinnate process; with one inner pinnate seta, four naked, unequal outer setae (first proximal seta longest, second proximal seta shortest), and with two pores on anterior surface.

P6 (Fig. 9D) asymmetrical as in T. concavus sp. nov., distally with one spine and two setae, middle seta 2× as long as innermost spine.

The specific epithet spinulifer is derived from the Latin spinula (small spine) and -fer (bearing), and refers to the presence of numerous spinules on the female caudal rami and on the apical process of the male fifth leg exopod. It is in the nominative singular. Gender masculine.

The P5 were asymmetrical (Fig. 9F) in all male specimens examined (n = 4), with a one-segmented exopod on one side and an incompletely two-segmented exopod on the other.

Tetragoniceps spinulifer sp. nov. shares the same armature formulae of P1–P4 with Tetragoniceps concavus sp. nov., except for the presence of only two setae on P1enp-2. The female caudal ramus is relatively short, being only 1.6× as long as wide, a condition shared only with T. brevicauda, T. prima, and T. truncata among known Tetragoniceps species (Scott 1900; Nicholls 1939; Coull 1971). However, the new species differs conspicuously from the latter three species in the shape of the caudal ramus, which tapers distinctly along the posterior two-fifths in lateral view and bears long spinules along the inner margin and ventroposterior surface. Among the five newly described species, the male of T. spinulifer sp. nov. exhibits two characteristic features: the outer distal corner of P4exp-2 is modified into an elongate, recurved process, and the P5 exopod is distally produced into a long, pinnate apical process. The male P5 exopod appears to be two-segmented because a residual suture is retained on one side; however, this condition is interpreted as incomplete fusion rather than true segmentation. This incomplete fusion differs from the condition observed in T. bergensis, T. prima, and T. santacruzensis, in which the P5 exopod is clearly two-segmented and the distal segment bears an apical seta (Por 1965; Coull 1971; Mielke 1997).

Dokdo Island, Korea; 37°14'59.23"N, 131°52'23.28"E; depth 66.5 m.

Holotype. • One ♀ (MABIK CR00261571) preserved in a vial with 80% ethanol; collected from the type locality, 02 November 2018, J.G. Kim leg. Allotype. • One ♂ (MABIK CR00261572) preserved in a vial with 80% ethanol, collection data as in holotype. Paratypes. • One ♀ (MABIK CR00261573) undissected on a H-S slide; • two ♀♀ (MABIK CR00261574, MABIK CR00261575) and two ♂♂ (MABIK CR00261576, MABIK CR00261577) each dissected on three H-S slides, collection data as in holotype.

Female. Body length range 906–968 μm (n = 4, mean = 934 μm, holotype = 906 μm). Habitus (Fig. 10A, B) subcylindrical, without clear demarcation between prosome and urosome; surface covered with small pits (not figured), more conspicuous than in T. concavus sp. nov. Cephalothorax (Fig. 10A) ~ 1.6× as long as wide, greatest width 138 μm measured at posterior end of cephalothorax.

Urosome (Fig. 10A, B) five-segmented, ornamentation as figured. Genital double-somite separated by faint dorsal line and lateral chitinous ribs, but completely fused ventrally. Copulatory pore (Fig. 12F) located at approximately midpoint of double-somite length. P6 (Fig. 12F) with three setae; innermost seta longest and naked, middle seta ~ 1/5 length of innermost seta, and outermost seta plumose and slightly shorter than inner seta. Anal somite (Figs 10A, 10B, 12G) slightly longer than penultimate somite, with one pair of dorsal sensilla, one pair of lateral pores, and three pairs of ventral pores; anal operculum semicircular, wide, ornamented with minute spinules along posterior margin.

Caudal rami (Figs 10A, 10B, 12G) slightly divergent, ~ 4.1× as long as wide, as long as penultimate somite; each ramus with three pores, a minute ventroposterior protuberance, and seven setae: setae I and II positioned on anterior 1/4 of outer margin; seta I diminutive; seta II slender and naked, ~ 1/2 length of ramus; seta III arising from anterior 4/5 of outer margin, slightly shorter than seta II; setae IV and V fused basally; seta IV ~ 1.5× as long as ramus; seta V longest; seta VI short, ~ 1/2 length of seta II; seta VII tri-articulated, inserted dorsally in mid-length of ramus, slightly longer than ramus.

Rostrum (Fig. 10A) distinct, with two subapical sensilla and one apical median pore.

Segmentation and setal armature of cephalothoracic appendages (Fig. 11A–F) as in T. concavus sp. nov. apart from the following features: first segment of antennule ~ 2× as long as second segment (Fig. 11A); antennary (Fig. 11B) basis with three rows of spinules, exopod with only few spinules, inner distal seta on exopod slightly longer than outer distal seta, and distal endopodal segment lacking inner proximal ornamentation; mandibular exopod elongate, exceeding mid-length of endopod, proximal segment 1.2× as long as distal segment, and distal exopodal segment with two apical setae (Fig. 11C); maxillule with one naked epipodal seta on coxa, and two apical setae on exopod (Fig. 11D); maxillipedal endopod with one subdistal tube pore, two unipinnate setae, and distal claw (Fig. 11F).

Segmentation and setation of P1–P4 as in T. concavus sp. nov. (Fig. 12A–D) apart from the following features; P1enp-1 elongate, distinctly longer than exopod, ~ 2.7× as long as enp-2, inner seta of enp-1 reaching slightly beyond mid-length of enp-2, and enp-2 4× as long as wide; inner setae on P2–P4exp-2 and P2–P3exp-3 delicate, difficult to discern; outer spines on P2–P3enp-2 longer than supporting segments and apical setae short; outer spine on P4enp-2 shorter than supporting segment, outer apical seta longest, ~ 1.2× as long as inner apical seta.

P5 (Fig. 12E). Baseoendopod gradually broadening distally, ornamented with inner setules and five anterior pores; triangular endopodal lobe exceeding mid-length of exopod, with one naked seta (subdistal) and two pinnate setae (proximal) on inner margin, and two small naked setae at apical tip. Exopod slenderer than in T. concavus sp. nov., tapering distally to pointed tip, ~ 4.3× as long as wide; with one short inner seta and five outer setae; second and third proximal outer setae short and bulbous at base, and subdistal seta longest, ~ 0.6× as long as exopod; with four pores (three proximal and one distal).

Male. Body length range 660–745 μm (n = 3, mean = 695 μm). Habitus (Fig. 10C) as in females, but with sexual dimorphism in antennule, segmentation of genital somite, caudal rami, and P2–P6.

Genital somite and first abdominal somite completely separate (Figs 10C, 13B).

Caudal rami (Figs 10C, 13C, 13D) divergent, cylindrical, distinctly longer than those of female, ~ 2× as long as penultimate somite, ~ 6× as long as wide, with seven setae: seta III arising from anterior 3/4 of lateral margin; seta VII (Figs 10C, 13C, 13D) inserted at 1/2 of ramus dorsally.

Antennule (Fig. 13A) eight-segmented, subchirocer. Fifth segment swollen, with partial suture on outer surface. Armature formula as in T. concavus sp. nov., except for a modified proximal seta on the fifth segment, composed of a blunt proximal part and a setiform distal part.

P2 enp-2 (Fig. 13E) as long as enp-1, with two distal setae (inner short and outer long) and one outer apophysis, 2.2× as long as enp-2.

P3, outer spine on enp-2 (Fig. 13F) short and modified (bearing minute spinules unilaterally, with bifid tip), ~ 1/2 length of enp-2, outer distal seta distal half bipinnate, ~ 2.3× as long as outer spine.

P4exp-2 (Fig. 13G) with modified outer distal corner with concave inner margin and blunt tip (shown separately in Fig. 13G). Exp-3 (Fig. 13G) with six elements (two inner, two apical, and two outer). Inner distal seta on enp-2 (Fig. 13H) short, 1/4 length of adjacent distal seta.

P5 (Fig. 13I) with baseoendopods fused medially, each with one outer basal seta (damaged) and three anterior pores; endopodal lobe with spinules on inner margin subdistally; with one inner bipinnate spine, one distal bipinnate spine, and one distal pinnate seta; outer distal seta ~ 1/2 length of inner distal spine. Exopod one-segmented, elongate, ~ 6.8× as long as wide, distally produced into a long apical process occupying 40% of exopod; with one short inner seta, one long (proximal) and two short (subdistal) outer setae, and three pores.

P6 (Fig. 13B) asymmetrical as in T. concavus sp. nov., each small plate with one short spine and two slender setae; outermost seta ~ 2/3 length of adjacent inner seta.

The specific epithet brevisetosus is derived from the Latin brevis (short) and setosus (bearing setae), and refers to the markedly short inner setae on the second exopodal segment of the second to fourth swimming legs. It is in the nominative singular. Gender masculine.

The P3enp-1 bears one inner seta in all examined specimens (n = 7), except for the figured male paratype (MABIK CR00261576; Fig. 13F), in which the inner seta is absent on the right side and is represented by a minute insertion mark on the left.

Tetragoniceps brevisetosus sp. nov. shares the same armature formulae of P1–P4 with T. concavus sp. nov., but differs in having extremely short inner setae (difficult to discern at low magnification) on P2–P3exp-2 and exp-3, and P4exp-2. The new species is further distinguished by comparatively short apical setae on P2–P4enp-2. In the female, the outer apical seta on P2–P3enp-2 is markedly shorter than the outer spine, whereas in the other Korean species it is longer than the outer spine. The inner apical seta of the female P4enp-2 is only slightly longer than the outer spine, whereas in the remaining Korean species, it is more than 1.4× as long as the outer spine. In the male, the inner apical seta of P4enp-2 is even shorter, being < 1/2 as long as the outer spine. The male shows sexual dimorphism, and is expressed in the elongate caudal rami, as in T. spinulifer sp. nov. However, it differs from T. spinulifer sp. nov. in the shape of the P5 exopod, which bears an elongate, slender distal extension and reduced armature consisting of one inner and three outer setae.

South Sea of Korea; 33°40'22.57"N, 127°30'01.02"E; depth 93.7 m.

Holotype. • One ♀ (MABIK CR00261578) preserved in a vial with 80% ethanol; collected from the type locality, 29 November 2016, S.L. Kim leg. Allotype. • One ♂ (MABIK CR00261579) preserved in a vial with 80% ethanol, collection data as in holotype. Paratypes. • Three ♀♀ (MABIK CR00261580–CR00261582) and two ♂♂ (MABIK CR00261583, MABIK CR00261584) each dissected on three H-S slides, collection data as in holotype.

Female. Body length 704–769 μm (n = 4, mean = 740 μm, holotype = 769 μm). Habitus (Fig. 14A) smaller and slenderer than in T. concavus sp. nov. Cephalothorax (Fig. 14A) ~ 1.5× as long as wide, greatest width 116 μm measured at posterior end of cephalothorax.

Body segmentation (Fig. 14A, B) as in T. concavus sp. nov., ornamentation as figured. Genital double-somite fused ventrally and dorsally, without a faint dorsal line. Each P6 (Fig. 14C) with three setae; outer plumose seta ~ 3/4 length of inner seta, and middle seta as long as P6. Anal somite (Fig. 14A, B) as long as penultimate somite.

Caudal rami (Fig. 14A, B) divergent, elongate, ~ 4.1× as long as wide, ornamented with a pair of lateral pores; each ramus with seven slender setae: setae I and II positioned at anterior 1/5 of outer margin; seta III arising from anterior 4/5 of outer margin, slightly longer than seta II; setae IV and V fused basally; seta IV ~ 3× as long as seta II; seta V longest; seta VI ~ 2/3 length of seta II; seta VII tri-articulate, inserted dorsally in mid-length of ramus, ~ 4/5 length of ramus.

Rostrum (Fig. 14A) distinct, with two sensilla on apical margin and one apical median pore; lateral margins slightly concave.

Segmentation and setal armature of cephalothoracic appendages (Fig. 15A–F) as in T. concavus sp. nov. apart from the following features: first segment of antennule with small and backwardly recurved process, ~ 1.8× as long as second segment (Fig. 15A); distal endopodal segment of antenna lacking tube pore on distal margin and spinular row on inner margin (Fig. 15B); basis of mandibular palp lacking distal ornamentation of spinules, with three setae, of which one seta ~ 1/2 length of other setae, and distal exopodal segment with only one apical seta (Fig. 15C); maxillular exopod with two setae (Fig. 15D); basis of maxilliped with two naked setae (Fig. 15F).

Segmentation and setation of P1–P4 as in T. concavus sp. nov., and ornamentation as in Fig. 16A–D. Distinct differences from T. concavus sp. nov. in the following: P1 (Fig. 16A), enp-1 ~ 2.3× as long as enp-2, inner seta of enp-1 short, reaching proximal 2/3 of enp-2, and enp-2 ~ 4× as long as wide; outer spine on P2–P4enp-2 relatively short, ~ 2/3 length of inner apical seta in P2, ~ 1/2 length in P3, and ~ 2/5 length in P4 (Fig. 16B–D).

P5 (Fig. 16E) as in T. concavus sp. nov. apart from the following features: baseoendopod with three anterior pores; distal part of endopodal lobe narrowing abruptly, three setae on inner margin gradually becoming shorter towards distal end, and inner distal seta slightly shorter than length of endopodal lobe; exopod elongate, ~ 4.5× as long as wide, slightly recurved outwardly, with one inner seta, one long apical seta, and three outer setae (first proximal one ~ 1/2 length of exopod, second proximal one shortest and bulbous at base); tip of exopod blunt, with one small inner protrusion.

Male. Body length 606–628 μm (n = 3, mean = 616 μm). Habitus (Fig. 14D) as in females, but with sexual dimorphism in antennule, genital segmentation, caudal rami, and P2–P6.

Genital somite and first abdominal somite completely separate (Fig. 14D).

Caudal rami (Figs 14D, 17B), length-to-width ratio as in females, but additionally with one dorsal and one ventral pore on each ramus.

Antennule (Fig. 17A) eight-segmented, subchirocer. Outer process on first segment small and recurved outwardly as in female. Fifth segment slightly swollen, with partial suture line on outer surface. Armature formula as in T. concavus sp. nov.

P2 as in female, but inner setae on exp-2 and exp-3 longer than those in females. Enp-2 (Fig. 17C) as long as enp-1, with one long outer apophysis (2× as long as enp-2) and only one long apical naked seta (slightly longer than apophysis); inner distal corner modified into an acute process.

P3 as in female, but inner setae on exp-2 and exp-3 longer than those in females. Outer spine on enp-2 (Fig. 17D) modified, slender and bearing lateral projections proximally, and inner apical seta not exceeding half-length of outer apical seta.

P4 (Fig. 17E) as in female, but exp-2 with modified outer distal corner as in T. concavus sp. nov. (shown separately in Fig. 17E). Exp-3 with six elements (two inner, two apical, and two outer). Enp-2 with slightly short inner apical seta, not exceeding half-length of outer apical seta.

P5 (Fig. 17F) similar to T. concavus sp. nov., but outer apical seta on endopodal lobe short, naked, ~ 1/3 length of adjacent apical spine. Exopod elongate, ~ 5.4× as long as wide, with two inner setae and three outer setae; relative length and position of outer setae similar to T. concavus sp. nov., but two inner setae diminutive (difficult to discern, slightly shorter than middle outer seta), located close to each other; inner margin with one pore proximally and one row of spinules between pore and set of setae, anterior surface with one pore.

P6 (Fig. 17G) similar to T. concavus sp. nov., except innermost seta slender and short, ~ 1/4 length of outermost seta.

The specific epithet unisetus is derived from the Latin unus (one) and seta (bristle), and refers to the presence of a single seta on the second segment of the mandibular exopod. It is in the nominative singular. Gender masculine.

The inner seta on the P2 enp-2 exceeds the outer spine in all examined females (n = 4), except for the figured female paratype (MABIK CR00261580), in which the inner seta on the P2 enp-2 is as long as the outer spine on one side. The P2 enp-1 bears one inner seta in all examined males (n = 3), except for the figured male paratype (MABIK CR00261583), with two inner setae on the P2 enp-1, asymmetrically.

Tetragoniceps unisetus sp. nov. is easily distinguished from the other four new species by the comparatively small process on the first antennulary segment, the presence of a single apical seta on the mandibular exopod, two setae on the maxillular exopod, and five setae on the female P5 exopod, as well as the absence of sexual dimorphism in the caudal ramus. The morphology of the female P5 exopod provides the most useful character for distinguishing the five new species. The length-to-width ratio of the female P5 exopod is lower in T. protuberans sp. nov. (3.1) and T. concavus sp. nov. (3.1) than in the remaining species (4.5 in T. unisetus sp. nov., 4.3 in T. brevisetosus sp. nov., and 5 in T. spinulifer sp. nov.). The apical tip is pointed in T. concavus sp. nov., T. brevisetosus sp. nov., and T. protuberans sp. nov., whereas it bears an apical seta in T. spinulifer sp. nov. and T. unisetus sp. nov. The male of T. unisetus sp. nov. exhibits two distinctive characteristics. The P2 enp-2 bears only one outer apophysis and one long apical seta. Notably, a conspicuous process occurs adjacent to the long apical seta, a feature not observed in other congeners. Given its insertion point, this process likely represents a modified inner apical seta associated with sexual dimorphism. This sexually dimorphic modification appears to be unique within the genus. Furthermore, the male P5 exopod is characterized by two remarkably short inner setae positioned close together. In T. spinulifer sp. nov. and T. brevisetosus sp. nov., only a single inner seta is present.

Gwangyang, Korea; 34°56'10.26"N, 127°36'06.95"E; depth 0~1 m.

Holotype. • One ♀ (HNIBRIV25103) preserved in a vial with 95% ethanol, collected from the type locality, 15 October 2016, J.G. Kim leg. Allotype. • One ♂ (HNIBRIV25104) preserved in a vial with 95% ethanol, collection data as in holotype. Paratypes. • Two ♀♀ (HNIBRIV25105, HNIBRIV25106) and two ♂♂ (HNIBRIV25107, HNIBRIV25108) each dissected on three H-S slides, collection data as in holotype.

Female. Body length range 737–749 μm (n = 3, mean = 745 μm, holotype = 737 μm). Habitus (Fig. 18A) subcylindrical, without clear demarcation between prosome and urosome; cephalothorax (Fig. 18A) ~ 1.4× as long as wide, greatest width 134 μm measured at posterior end of cephalothorax.

Urosome (Fig. 18A, B) as in T. concavus sp. nov., ornamentation as figured. Genital double-somite separated laterally by internal chitinous ribs, but fused dorsally and ventrally, lacking a faint dorsal line; genital apertures located near anterior margin and median copulatory pore small, situated in anterior 1/3 of genital double-somite; P6 (Fig. 18C) similar to T. concavus sp. nov., except for naked inner seta slightly shorter than outer plumose seta and middle seta small, not exceeding mid-length of inner seta.

Caudal ramus (Figs 18A, 18B, 20F) cylindrical, slightly shorter than anal somite, ~ 3× as long as wide; ventroposterior margin with a small protuberance at outer distal corner (indicated by arrowhead in Fig. 20F), more prominent than in T. concavus sp. nov.; with three pores, one each on dorsal, ventral, and lateral surfaces; with seven setae: setae I and II positioned on anterior 1/4 of lateral margin; seta I diminutive; seta II 2/3 length of ramus; seta III arising from anterior 3/4 of lateral margin, slightly shorter than seta II; setae IV and V fused basally; proximal part of seta V swollen and slightly recurved outwardly; seta IV slender, approximately as long as penultimate somite and anal somite combined; seta V longest, as long as urosome excluding P5-bearing somite; seta VI small, slightly shorter than seta II; seta VII inserted at anterior 2/5 of dorsal surface.

Rostrum (Fig. 18A) similar to T. concavus sp. nov., with one apical median pore and two subapical sensilla.

Segmentation and setation of cephalothoracic appendages (Fig. 19A–F) as in T. concavus sp. nov. apart from the following features: first segment of antennule (Fig. 19A) ~ 2× as long as second segment, with a stout outer process, and third segment with eight or nine setae, the additional seta indicated by an asterisk in Fig. 19A (see Variability); exopod of antenna (Fig. 19B) ornamented with only one row of spinules proximally; mandibular coxa with one row of spinules proximally, basis with three unequal setae (one of these distinctly shorter than other setae), and endopod with two subequal lateral setae (Fig. 19C); maxillular coxa with one naked epipodal seta, endopod slightly longer than exopod (Fig. 19D); syncoxa of maxilla with less ornamentation (one group on outer margin and two groups on anterior and posterior surfaces, respectively) (Fig. 19E); basis of maxilliped ornamented with inner spinules except for proximal 1/5, claw-like endopod ornamented with stouter spinules than in T. concavus sp. nov. (Fig. 19F).

Segmentation and setation of P1–P4 as in T. concavus sp. nov. (Fig. 20A–D) apart from the following features: coxae and bases of P1–P4 with surface ornamentation as in Fig. 20A–D; P1enp-1 (Fig. 20A) slightly exceeding end of exopod, inner seta extending slightly beyond mid-length of enp-2, enp-1 ~ 2.2× as long as enp-2; P2–P3exp-2 and exp-3 (Fig. 20B, C) with short and naked inner seta, respectively, not exceeding distal margin of corresponding segment; outer spine on enp-2 subequal in length to supporting segment in P2 and P3, but only ~ 1/3 as long as in P4.

P5 (Fig. 20E) similar to T. concavus sp. nov. except for the following features: endopodal lobe, inner apical seta beyond tip of exopod; exopod 3.1× as long as wide.

Male. Body length range 618–688 μm (n = 3, mean = 641 μm). Habitus (Fig. 18D) as in females, except for urosomal segmentation. Sexual dimorphism observed in antennule, urosome, caudal rami, and P2–P6.

Urosome (Figs 18D, 21F) six-segmented, with pores and sensilla as figured; genital somite and first abdominal somite completely separated.

Caudal rami (Figs 18D, 21F) cylindrical, more distinctly tapering distally than in female, 3.1× as long as maximum width; with seven setae: seta III arising from anterior 5/6 of lateral margin; seta VII inserted at anterior 2/5 of ramus dorsally.

Antennule (Fig. 21A) eight-segmented, with same armature formula as in T. concavus sp. nov.; first segment 2.3× as long as wide, outer process strongly developed and recurved backwardly.

P2 enp-2 (Fig. 21B) as long as enp-1, with one apophysis and two naked setae; apophysis and outer apical seta subequal in length, ~ 2.1× as long as enp-2, and inner apical seta short, 2/3 length of enp-2.

P3enp-2 (Fig. 21C) with relatively short outer spine bearing unilateral spinules and with bifid tip, distinctly shorter than enp-2; outer spine ~ 2/3 length of inner apical seta.

P4exp-2 (Fig. 21D) with one short and naked inner seta, and outer distal corner modified as in T. concavus sp. nov. (shown separately in Fig. 21D); exp-3 with six elements (two inner, two apical, and two outer).

P5 (Fig. 21E) similar to T. concavus sp. nov., except for the presence of four anterior pores on baseoendopod. Exopod ~ 6× as long as maximum width and with one marginal and two anterior pores; spinous distal extension slender, elongate, and occupying 45% of exopod length.

P6 (Fig. 21F) as described for T. concavus sp. nov.

The specific epithet protuberans is derived from the Latin protuberans, meaning “protruding”, and refers to the conspicuous protuberance on the outer distal corner of the female caudal ramus. It is in the nominative singular. Gender masculine.

The third segment of the antennule bears eight setae in all examined females (n = 2), except for the figured female paratype (HNIBRIV25105), with both third antennulary segments bearing nine setae (the additional seta indicated by an asterisk in Fig. 19A). In all examined males (n = 3), caudal seta III arises from the posterior 1/5 of the lateral margin of the caudal ramus and the P5 exopod bears two inner setae. However, the figured male paratype (HNIBRIV25107) differs, with caudal seta III of the left caudal ramus (Fig. 21F) inserted at the distal end and the left P5 exopod (Fig. 21E) bearing only one inner seta, resulting in asymmetrical P5 setation.

Tetragoniceps protuberans sp. nov. shares the short inner setae on P2–P3exp-2 and exp-3 with T. brevisetosus sp. nov., but it is readily distinguished from that species by the presence of a well-developed inner seta on P4exp-2 and by its unique caudal ramus morphology. The female caudal ramus is cylindrical and bears a conspicuous ventroposterior protuberance on the outer distal corner. The principal setae IV and V are proximally swollen and curved outward. Furthermore, the male caudal ramus is the shortest among the five new species, being only 3.1× as long as wide. Unlike the other four new species, T. protuberans sp. nov. was collected from a brackish habitat where the river meets the sea.