Nannopodidae. Body subcylindrical, slightly depressed dorsoventrally, without distinct constriction between prosome and urosome; hyaline frills of somites weak, ornamented with long setules. Rostrum well-developed, triangular, with 1 pair of sensilla. Genital slit ♀ reverse U-shaped, covered by 1 pair of large opercula. Caudal rami elongate, oval in ♀, rectangular in ♂, with 7 setae; principal seta V as long as caudal rami in ♀, slightly shorter than urosome in ♂. Antennule 4-segmented, with elongate first segment in ♀; chirocerate and 6-segmented in ♂. Antennary allobasis without abexopodal seta; exopod represented by single seta. Mandibular palp uniramous, with 1 seta on basis; exopod absent and endopod 2-segmented. Maxillular praecoxal arthrite with 1 surface seta; exopod 1-segmented and endopod absorbed into basis. Maxillary syncoxa with 2 endites; endopod absorbed into basis. Maxilliped with elongate basis; endopod drawn out into long geniculate claw. P1 coxa with 1 coarsely serrate outer projection; inner element on basis displaced onto anterior surface; exopod 2-segmented, with 4 setae on exp-2; endopod arising from well-developed inner pedestal of basis, prehensile, 2-segmented; enp-1 elongate, unarmed; enp-2 with 1 stout claw and 1 long seta. P2–P4 with 1 coarsely serrate outer projection on coxa; intercoxal sclerite hugely broad, separated in P2 and laterally fused to coxae in P3 and P4; outer setophore on basis articulated in P3 and P4; exopod 1-segmented; proximal outer spine on exopod with serrate outer margin, with inner and outer longitudinal rows of setules; endopod absent in P2, represented by 1 small distinct protuberance in P3, 1-segmented in P4; male P3 endopod 1-segmented and armed with 1 stout spine. Setal armature formulae of P1–P4 as follows:

P5 baseoendopod broad; endopodal lobe weakly developed, with 1 seta; exopod 1-segmented, with 4 setae.

P6 represented by 2 setae in ♀; slightly asymmetrical and represented by 3 setae in ♂.

Concilicoxa hispida gen. et sp. nov., by monotypy.

The generic epithet is a combination of the Latin verb concílĭo meaning ‘unite separate parts into a whole’ and the Latin noun coxa, meaning ‘hip’ and alludes to the fusion of the coxae and the intercoxal sclerite in P3 and P4. It is a noun in the feminine singular.

Off the Socheongcho Ocean Research Station (SORS) (37°25'57.16"N, 124°44'56.4"E) in the Yellow Sea of South Korea, sandy sediments, 68 m depth.

Holotype : SOUTH KOREA•♀ dissected and mounted on 11 slides; the Yellow Sea, off SORS; 37°25'57.16"N, 124°44'56.4"E; 68 m depth; 23 Mar 2018; Kim, J.G. leg.; sandy sediments; cat. MInRB-Hr59-S001.

Allotype : SOUTH KOREA•♂ dissected and mounted on 11 slides; same data as for holotype; cat. MInRB-Hr59-S002.

Paratypes : SOUTH KOREA•3♀♀2♂♂ dissected and mounted on 11 or 12 slides each; same data as for holotype; cat. MInRB-Hr59-S003–MInRB-Hr59-S007•3♀♀2♂♂ preserved together in 95% ethanol; same data as for holotype; cat. MInRB-Hr59-L001.

SOUTH KOREA•2♀♀1♂ on a stub for SEM; same data as for holotype.

(MInRB-Hr59-S001). Total body length 617 μm (measurement based on holotype and six paratypes: range = 530–626 μm; mean = 588 μm; n = 7); maximum width 86 μm measured at the middle of cephalothorax. Body (Figs 2A, B, 8A) subcylindrical, slightly depressed, without distinct constriction between prosome and urosome; prosome slightly longer than urosome. Rostrum (Fig. 2C) well-developed, triangular, reaching distal fourth of first antennular segment, defined from cephalothorax basally, with 1 pair of sensilla laterally and 1 median anterior pore ventrally; lateral margins convex proximally. Cephalothorax nearly square in dorsal aspect, slightly wider than long; integument covered with paired sensilla, several round depressions and irregular wrinkles (visible at high magnification, 1,000×; see insert in Fig. 2A); posterior margin ornamented with short and fine setules; arthrodial membrane of first pedigerous somite visible dorsally and laterally. Tergites of somites with surface ornamentation composed of 1–3 transverse furrows, with 1 mid pore (absent in penultimate and anal somites) and 1 pair of lateral pores (absent in penultimate somite); posterior margins with several paired sensilla (absent in penultimate somite); hyaline frills weak, with 1 row of long setules posteriorly except for anal somite. Genital somite and first abdominal somite fused ventrally forming genital double-somite, but original segmentation indicated by internal chitinous rib dorsally and laterally; genital field (Fig. 3A, D) with 1 large copulatory pore on midventral depression posterior to genital slit; genital slit reverse U-shaped, covered by 1 pair of large opercula derived from P6 on both sides; P6 represented by 1 long and 1 small seta, with 1 row of spinules subdistally; single midventral egg sac carrying 6 large eggs, as long as 1/4 of total body length. Anal somite (Figs 2A, B, 3A, B) with 1 pair of dorsal sensilla near base of operculum, 1 row of long setules on both ventrolateral margins; operculum semicircular, with smooth distal margin; anal opening with lateral row of small posterior spinules on each side; anal opening with 3 fringes of fine setules (Fig. 3B).

Caudal rami (Figs 2A, B, 3A–C) elongate, oval, about 2.4 times as long as largest width, twice as long as anal somite; with a notch in mid-outer margin below caudal setae I and II; anterior half with a row of outer setules ventrolaterally; distal half with non-chitinous lateral margin; with 7 setae: seta I small naked, inserted in mid-length of outer margin ventrolaterally; seta II dorsal to and closely set to seta I, naked, longer than seta I; seta III naked, as long as seta II, arising from subdistal peduncle with 1 tube pore basally (Fig. 3C); seta IV small, naked, slightly longer than setae II and III, fused to principal seta V basally; principal seta V well-developed, slightly longer than caudal ramus, ornamented with outer spinules distally; seta VI naked, as long as seta IV, inserted in outer distal corner; dorsal seta VII naked, tri-articulate at base, arising subdistally close to inner margin.

Antennule (Fig. 4A) short, 4-segmented. First segment largest, elongate, as long as distal two segments combined, with 1 small naked seta subdistally; inner margin with short row of spinules subdistally; outer margin convex, with longitudinal row of minute spinules. Second segment smallest, with 3 bi-articulate and 5 naked setae; outer margin with 1 weak protuberance. Third segment about twice as long as second one, gradually widening distally; lateral margin with 3 bi-articulate and 3 naked setae; inner distal corner with 3 peduncles, of which two with 1 apical seta each, and one bearing 1 apical seta fused to basally to 1 ae. Distal segment as long as preceding one, tapering distally; lateral margins with 6 bi-articulate and 3 naked setae; distal margin with 1 naked seta and 1 acrothek composed of 1 ae and 2 bare setae. Setal armature as follows: 1-[1], 2-[8], 3-[8+ (1 + ae)], 4-[10 + acrothek].

Antenna (Fig. 5A) with small, unornamented coxa (not shown). Allobasis elongate, 2.8 times as long as wide, exopod represented by 1 naked seta issuing at proximal third; abexopodal seta absent. Free endopodal segment with 1 short row of spinules subdistally and 1 surface frill distally; lateral armature composed of 2 weakly-serrate setae; distal armature comprising 1 small and 1 stout spine, 3 geniculate setae, innermost one of which fused basally to 1 small naked seta.

Mandibular coxa (Fig. 5B) slender, with 1 bulge and 1 row of spinules proximally; gnathobase well-developed, with 1 bicuspid and 3 unicuspid teeth, 1 small spinule and 1 unipinnate seta. Palp well-developed, uniramous; basis elongate, covered with rows of spinules, with 1 plumose seta distally; endopod 2-segmented, with 1 long plumose seta on proximal segment and 1 subapical and 2 apical setae on distal segment.

Maxillule (Fig. 5C). Praecoxa with 1 row of outer spinules; arthrite with 1 naked seta on anterior surface and 7 spines on distal margin and ornamented with few long spinules on distal margin, 1 row of small spinules on dorsal margin and several spinules on posterior surface. Coxa armed with 1 row of outer spinules; endite elongate, with 2 elements distally and 1 row of small spinules laterally. Basis broad, with 2 endites: distal endite with 1 subapical and 3 apical setae; proximal endite incorporated into basis, represented by 2 long naked setae. Endopod incorporated into basis, represented by 3 long naked setae. Exopod 1-segmented, small, with 1 short and 1 long naked seta.

Maxilla (Fig. 5D). Syncoxa armed with 1 row of stout spinules and 1 row of setules along outer margin, 1 row of minute spinules on surface and 1 patch of spinules near inner margin; with 2 coxal endites: proximal endite with 1 long naked seta and 1 short unipinnate seta (fused to endite basally); distal endite with 2 long naked setae and 1 unipinnate seta (fused to endite basally). Allobasis drawn out into strong claw with 2 accompanying naked setae and few spinules. Endopod incorporated into basis, represented by 2 long naked setae fused basally.

Maxilliped (Fig. 5E) enlarged. Syncoxa elongate, ornamented with 1 group of spinules proximally. Basis elongate, about 3.4 times as long as maximum width, with 1 row of outer spinules proximally. Endopod drawn out into long and geniculate claw bearing 1 small accessory seta proximally.

P1 (Fig. 6A). Praecoxa large, triangular, unornamented. Intercoxal sclerite broad, unornamented. Coxa wide, with outer margin forming 1 large and coarsely-serrated projection. Basis with 1 anterior pore and few spinules proximally; inner pedestal well-developed, with serrate distal margin; outer seta plumose, bi-articulated basally, arising from setophore ornamented with 1 row of small spinules at its base; inner seta naked, arising anteriorly, with 1 group of small spinules at its base. Exopod 2-segmented, short, about 0.3 times as long as enp-1; exp-1 with 1 naked outer seta and 1 row of stout outer spinules; exp-2 with 1 small naked seta and 1 stout unispinulose seta on outer margin and 1 short naked and 1 long geniculate seta on distal margin; anterior surface with 1 small pore. Endopod prehensile, 2-segmented; enp-1 elongate, 3.7 times as long as largest width, ornamented with 1 row of small spinules along outer margin and few long inner spinules; enp-2 short, slightly longer than wide, ornamented with few inner spinules and armed with 1 stout, recurved distal claw and 1 long flexible outer seta, of which distal half with very thin cuticular inner lines.

P2–P4 (Figs 7A–F, 8B, C). Protopods composed of praecoxa, coxa and basis. Praecoxae small, ornamented with 2 rows of spinules. Intercoxal sclerites large, broad, separate in P2, fused to coxae laterally in P3 and P4 (see arrowheads in Fig. 8B, C). Coxae wide, with 1, 3 and 2 groups of spinules on anterior surface in P2–P4, respectively; outer margin drawn out into an elongate and coarsely-serrated projection. Bases wide, with 1 pore on anterior surface; outer setophore elongate, ornamented with 1 row of spinules basally, non-articulated in P2 with 1 plumose seta, bi-articulated in P3 and P4, with 1 naked seta; inner distal corner (near base of inner ramus) with 1 group of spinules. Exopod 1-segmented, ornamented with rows of spinules along outer and distal margins; with 4 stout outer spines, of which proximal one uniserrate and ornamented with 2 rows of setules, others pinnate; distal outer spines of P4 strongly pinnate. Endopod absent in P2, represented by 1 small unarmed protuberance in P3 and 1-segmented, ornamented with distal spinules and armed with 1 stout pinnate distal spine in P4.

P5 (Fig. 6B). Baseoendopod broad, with 1 anterior pore, ornamented with rows of spinules along distal and inner margins; endopodal lobe weak, with 1 plumose distal seta; outer setophore articulate, with 1 long naked seta. Exopod small, with 3 pinnate setae and 1 naked seta.

Male (allotype MInRB-Hr59-S002). Total body length slightly shorter than in female, 525 μm (measurement based on allotype and 4 paratypes: range = 485–556 μm; mean = 512 μm, n = 5); body (Fig. 2D) slightly more slender than in female, maximum width 74 μm measured at the middle of cephalothorax; urosome 6-segmented, comprising P5-bearing somite, genital somite, 3 abdominal somites and anal somite; penultimate somite slightly shorter than its width, without lateral ornamentation. Caudal rami (Figs 2D, 3E) parallel, rectangular, more slender than in female; inner margin straight, outer margin slightly convex; outer margins unornamented, with clear cuticular inner line; additional large pore present on ventral surface; seta III issuing from subdistal lateral margin ventrally; set of setae I and II issuing from proximal third of outer margin; seta V slightly shorter than urosome (Fig. 2D).

Antennule (Fig. 4B) chirocerate, 5-segmented. First segment elongate, with 1 short naked seta subdistally; inner margin with few small spinules; outer margin convex, with 1 row of minute spinules. Second segment slightly longer than wide, with 2 bi-articulate and 7 naked setae and 1 minute protuberance. Third segment partially separated into two parts; proximal one with 2 bi-articulate and 6 naked setae; distal part with 2 setae. Fourth segment swollen, with 1 medial protuberance, 4 naked surface setae and 3 well-developed posterior penduncles: one proximal and one medial peduncle with 1 long naked apical seta each; subdistal peduncle with 1 long naked seta fused to 1 long ae basally. Distal segment elongate, slightly recurved distally, hook-shaped, with 2 naked and 6 bi-articulate setae laterally, 1 long naked seta distally and 1 acrothek composed of 1 ae and 2 naked setae fused basally. Setal armature as follows: 1-[1], 2-[9], 3-[10], 4-[6 + (1 + ae)], 5-[10 + acrothek].

P3 (Fig. 6D) as in female, except for 1-segmented endopod with 1 stout distal spine bearing 2 pointed lateral processes.

P5 (Figs 3E, 6E) as in female except for anterior ornamentation with 2 rows of minute spinules.

P6 (Figs 3E, 6F) asymmetrical (one side completely fused to genital somite basally, other side articulated at base), each represented by a plate bearing 3 plumose setae and 1 row of minute spinules distally on inner extension.

Spermatophore as long as 4/5 length of P5-bearing and genital somites combined (Figs 2D, 3E).

The species epithet “hispida” is derived from the Latin adjective híspĭdus, which means ‘hairy’ and refers to the setulose lateral ornamentation of the anal somite and caudal rami in the female. It is a noun in the feminine singular.

The investigated individuals of Concilicoxa hispida gen. et sp. nov. show intraspecific differences in appendage ornamentation. Dense spinular ornamentation was observed on the mandibular basis in one female paratype (MInRB-Hr-59-S003). This paratype also displays fusion of the coxa and basis of the P2 symmetrically (Fig. 6C).

George (2002) established the genus Laophontisochra to accommodate L. maryamae from the Patagonian continental slope (Chile) and Laophontisochra sp. from the Magellan Straits (Chile). He allocated this genus into the family Cristacoxidae Huys, 1990, based on the presence of an outward growth on the coxa of P1, an enlarged maxilliped and atrophy of the antennary exopod and abexopodal seta despite the discrepancies with the following characters of the family Cristacoxidae, which were defined by Huys (1990): the first antennular segment with an outer spinous process, the absence of the exopod and an abexopodal seta in the antenna, the presence of modified seta on the middle endite of maxillary syncoxa and the single plate P5 with the same setae/spines in both sexes, which is considered as a neotenous origin. George (2002) suggested that the Cristacoxidae could be divided into two lineages: a plesiomorphic group comprising only Laophontisochra and a derived group composed of Noodtorthopsyllus Lang, 1965, Cubanocleta Petkovski, 1977 and Cristacoxa Huys, 1990 [the latter was considered as a junior synonym of Noodtorthopsyllus by Huys and Kihara (2010)]. However, Huys and Kihara (2010) transferred the genus Laophontisochra to the family Nannopodidae, based on a re-evaluation of the three fundamental morphological differences between the two groups suggested by George, with their newly-erected genus Acuticoxa within the family Nannopodidae for Laophontisochra sp. sensu George, 2002 (= A. biarticulata Huys & Kihara, 2010) and A. ubatubaensis Huys & Kihara, 2010 from the Brazilian coast. They assumed that both genera differ from the Cristacoxidae with the following evidence: (1) P1 coxa with a pair of serrated cristae (outer projections) in Noodtorthopsyllus and Cubanocleta vs. a single non-serrate, lobate or spinulose outgrowth in Laophontisochra and Acuticoxa; (2) maxillipedal endopod represented by a geniculated claw in Laophontisochra and Acuticoxa vs. non-geniculated in Noodtorthopsyllus and Cubanocleta; (3) antennary exopod consistently absent in Noodtorthopsyllus and Cubanocleta vs. atrophied in Laophontisochra and Acuticoxa (see Huys and Kihara 2010: 34). In addition, Huys and Kihara (2010) suggested that Laophontisochra and Acuticoxa are more closely related to both Huntemannia and Rosacletodes than to the cristacoxid genera, in that they share the presence of a coxal projection on the P1–P4 (vs. the plesiomorphic state of this character expressed in Laophontisochra, which lacks the coxal processes in the P2–P4). Corgosinho (2012) created the genus Talpacoxa, which was first mentioned as “Genus X” by Huys and Kihara (2010) and revealed close relationships amongst the genera of the nannopodid clade–Huntemannia, Rosacletodes, Laophontisochra, Acuticoxa and Talpacoxa–supported by three synapomorphies that are likely morphological adaptations to a burrowing lifestyle: (1) P1 coxa with an outer projection; (2) the P2–P4 exopods one- or two-segmented; and (3) the P2–P4 endopods one-segmented or vestigial.

The new genus Concilicoxa gen. nov. is assigned to the Nannopodidae because, as a member of the nannopodid clade, it exhibits the burrowing adaptation of the thoracopods. Concilicoxa gen. nov. appears to be closely related to both Laophontisochra and Acuticoxa in that they share four-segmented female antennules with elongate first segments, the prehensile P1 endopod, the presence of coxal outer projection on the P1, large and broad intercoxal sclerites on the P2–P4, the general shape of the female genital field (with a large copulatory pore and a well-developed operculum derived from P6) and elongate caudal rami. However, the novel genus is easily distinguishable from Laophontisochra by the distal armature of the antennary endopod with three geniculate and three non-geniculate elements (vs. four geniculate and two non-geniculate elements in Laophontisochra), the presence of coxal outer projections in the P2–P4 (vs. absent in Laophontisochra) and one-segmented exopods in the P2–P4 (vs. two-segmented in Laophontisochra). The new genus is also different from Acuticoxa in the absence of the P2 endopod (vs. one-segmented in Acuticoxa), a serrate coxal outer projection in the P1–P4 (vs. acute in Acuticoxa) and the female P5 exopod and baseoendopod separate (vs. fused into a single plate in Acuticoxa).

In contrast to a close resemblance with both genera in habitus and thoracopod morphology, Concilicoxa gen. nov. displays unambiguous autapomorphies that require the formation of a new genus: (1) the loss of the mandibular exopod, as observed in Huntemannia, is more derived than the exopod represented by a single seta; (2) the mandibular endopod is two-segmented, which seems to be secondarily divided, comparing to other related genera with only one-segmented endopod; (3) the P1 exp-2 comprises a total of only four elements, but five or six setae in Laophontisochra and Acuticoxa, respectively (in the original description of L. terueae, this segment was described as having one lateral and three terminal setae, but was depicted as having three outer and three terminal elements; see Björnberg 2014: fig. 11A); and (4) the intercoxal sclerites of P3 and P4 are laterally fused with the coxae in Concilicoxa gen. nov. (Figs 6D, 7C, E, 8B–D), but this fusion has rarely been reported in harpacticoid copepods (i.e. Orthopsyllus sp. of the family Orthopsyllidae Huys, 1990; cf. Huys and Boxshall 1991). By contrast, the presence of the maxillular exopod, as observed in Talpacoxa demonstrates a more plesiomorphic state than the lack of endopod.

The males of Concilicoxa gen. nov. exhibit distinctive potential autapomorphies for the genus as follows: (1) the P3 endopod has a sexual dimorphic distal element that is a robust spine; (2) the shape of P5 is nearly similar to that of the female; and (3) the caudal rami show sexual dimorphisms in the length of caudal seta V, the issuing position of setae I and II and the number of tube pores. However, we could not compare these characters with other related genera, because males of L. maryamae and A. ubatubaensis remain unknown. The sexual dimorphism of thoracopods is one of the most robust characters used to assess the phylogenic relationships between genera and between families because it facilitates comparison of the positions of homologue elements (such as setae or apophyses) of rami in females and males (Huys 1990; Huys and Kihara 2010). In this nannopodid clade, the known males tend to exhibit differences in morphology of the P3 endopod: (1) the male of Rosacletodes has a two-segmented P3 endopod with an elongate inner apophysis on enp-2, instead of a single seta as in the female (Pallares 1982); (2) all known males of the species of Huntemannia have an additional armature element on the P3 endopod, with no differences in segmentation as in Nannopus and Pontopolites (Song et al. 2007; Karanovic and Cho 2018); (3) the male of T. brandini exhibits a distal small apophysis on the one-segmented P3 endopod; and (4) although the male of L. maryamae has yet to be discovered, there is no sexual dimorphism on the P3 in L. terueae, whose taxonomic position seems to be problematic (see below). The male P3 endopod of the new genus presented herein is one-segmented with a stout spine (Figs 6D, 8D), whereas the female P3 endopod is represented by an unarmed protrusion. Such diverse sexual dimorphism of the P3 endopod prevents deeper insight into the systematic position of this clade within the Nannopodidae. We hypothesise that the lack of original outer element on the female P3 endopod in L. maryamae and C. hispida gen. et sp. nov. leads to the absence of the sexual dimorphic apophysis in the male. In contrast, the presence of a small apophysis on the corresponding ramus in T. brandini seems to be derived from a rudimental apical seta in the female.

Harpacticoids generally display sexual dimorphism in the size, shape and setae of the male P5. However, no sexual dimorphism has been observed in the male P5 of Arenopontiidae Martínez Arbizu & Moura, 1994 (Martínez Arbizu and Moura 1994). Additionally, both sexes bear the same number of setae/spines on the P5 of some taxa, such as Metidae Boeck, 1873, Rotundiclipeidae Huys, 1988, Ectinosomatidae Sars, 1903 and Cristacoxidae Huys, 1990 (Huys 1988; Fiers 1992; Clément and Moore 1995; Huys and Kihara 2010). Except for Ectinosomatidae, the P5 of these families is remarkably reduced or represented by a single plate in both sexes. Although this sexual dimorphism is observed in other nannopodid genera, the structure of this leg in our new taxon is very similar in both sexes, except for micro-ornamentation, such as cuticular spinules and pores (Fig. 6B, E). In addition, the male of Concilicoxa gen. nov. expresses rare sexual dimorphisms in the shape of the caudal rami (oval in the female, but rectangular in the male), the length of caudal seta V (slightly longer than the caudal ramus in the female, but slightly shorter than the urosome in the male), the number of pores on the surface (one pore in the female vs. two pores in the male) and the lateral ornamentation (the presence of a row of long setules proximally in the female vs. absent in the male). These sexual dimorphisms could support the erection of a new genus Concilicoxa gen. nov.