Yeongheung-ri (33°57.59”N; 126°17.82”E), Chuja-myeon, Jeju-si, Jeju-do, Republic of Korea. English equivalents of political divisions in Korea: ri = village; myeon = township; si = city; do = province.

Specimens were collected by Dr Min Ho Seo (Marine Ecology Research Center, Korea) using a light trap on 11 September 2017 alongside a small harbor at the type locality. The depth at the sampling site was about 3 m. The type specimens are deposited in the National Marine Biodiversity Institute of Korea (MABIK), Seocheon, Korea, with the following accession numbers: male holotype (MABIK CR00244260) dissected and mounted on five slides in lactophenol; six intact male paratypes (MABIK CR00244261) in 99.5% ethanol vial. Five additional specimens were used for molecular analysis.

(male). Total body length 1.12–1.30 mm (mean 1.21; N = 7). Length ratio (lateral view) of cephalothorax: metasome: urosome as 33.1 (32.1–35.7): 40.9 (39.3–42.0): 26.0 (25.0–27.6). Oral papilla inconspicuous, rather retracted inwards, located ventrally within 27.3% (25.1–31.6) of distance from anterior margin of cephalothorax. Length of antennules in relation to total body length 30.2% (27.7–33.8), to cephalothorax length 90.4% (82.8–95.2). Antennular segments relative length (as % of antennule total length) from proximal to distal as 15.7 (14.8–17.4): 19.8 (18.7–20.8): 18.2 (16.6–19.3): 23.0 (21.5–24.9): 23.4 (21.5–24.3). Distal segment lacking branched setae, with unbranched simple setae instead. Fourth segment with prominent accessory spine 4a on inner dorsodistal margin. Distal margin of intercoxal sclerite of legs 1–4 excavated. First urosomal somite with extremely reduced, knob-like fifth legs devoid of setae inserted on posteroventral margin. Genital shaft 0.07 (0.068–0.079) mm long, basal half robust, distal half gradually tapering; proximal and distal parts distinguished by anterior protrusion in lateral view; distalmost part with smooth medial protrusion; two subtriangular lappets arising from distolateral sides of shaft, span of lappets not exceeding width of succeeding postgenital somite. Genital opercular openings covered by hand-like opercular flaps placed at distal end of genital shaft. Caudal rami with 6 setae (II–VII); dorsal apical seta VII conspicuously shorter than rest.

Total body length excluding antennules and caudal rami 1.24 mm in dorsal view, 1.25 mm in lateral view. Body consisting of nine somites: cephalothorax incorporating first pedigerous somite, free somites 1–3, first urosomal somite, genital somite, postgenital somite, penultimate somite, and anal somite (Fig. 1A, B). Length of somites as percentage of total body length: 32: 16: 14: 11: 7: 6: 6: 4: 3 in dorsal view; 35: 17: 15: 11: 6: 6: 6: 4: 2 in lateral view.

Cephalothorax incorporating first pediger rather short, 0.40 mm long in dorsal view, 0.43 mm in lateral view, generally bullet-shaped in dorsal view. Anterior margin convex, without typical forehead sensilla. Length 1.2 times greater than maximal width, narrowest (0.23 mm) at 58.6% of distance from anterior margin. Width of incorporated first pediger 0.37 mm near posterior margin (at 91.6% of distance from anterior margin), this being widest part of cephalothorax. Anterodorsal part of cephalothorax with several pores (Fig. 1A, B). Two pairs of concave depressions posterior to porous region, with anterior pair located closer to central body axis than posterior pair. Ventral side of cephalothorax with three pairs of scars (Fig. 2A): two prominent pairs posterior to antennular bases, and relatively inconspicuous pair placed more laterally at one-third length of cephalothorax. Anteriormost and posteriorly adjacent scars each followed by rounded depression (Fig. 2B). Oral papilla situated ventrally between posteriormost pair of scars, not protrusive at all, almost rudimentary, retracted in lateral view (Fig. 1B). Two pores located behind oral papilla. Cuticle of ventromedial region between two anterior pairs of scars with fingerprint-like whirling pattern with fine wrinkles (Fig. 2A). Tergite of incorporated first pediger with five pairs of pit-setae (Fig. 1A, B): one pair (no. 1) situated dorsally, four pairs laterally (no. 2–5). Two longitudinal rows of four faint pores each located slightly anteriorly to pit-setae 1, arranged in parallel across midline.

Two lateral and one ventral eyes (Fig. 1A, B) placed at anterior quarter of cephalothorax, all moderately developed and pigmented. Ventral eye positioned slightly anterior to lateral eyes. Lateral eyes bean-shaped in dorsal view, 64.8 μm in diameter, placed 0.02 mm apart across midline. Ventral eye round in dorsal view, but oval in lateral view. Ventral eye slightly smaller in diameter (60.1 μm) than lateral counterparts in dorsal view.

First free pedigerous somite to first urosomal somite each with several pore pairs in various regions (Fig. 1A, B). First free somite with three pairs of pit-setae posteriorly (no. 6–8: pair dorsally, two pairs laterally), plus pair of simple pores anterior to dorsal pair of pit-setae. Second free somite with four pairs of pit-setae posteriorly (no. 9–12: two pairs dorsally, other two pairs laterally), plus pair of simple pores anterior to dorsal pair of pit-seta. Third free somite with two pairs of pit-setae posteriorly (no. 13, 14), all aligned transversally across dorsum, plus pair of simple pores anterior to them. First urosomal somite with pair of pit-setae (no. 15) on posterior dorsal surface. Each free somite also with one or two pairs of pores in anterior dorsum.

Antennules distinctly 5-segmented, generally directed straight forward (Fig. 3A, B). Geniculation placed between fourth and fifth segment. Antennule total length 0.36 mm, representing 28.6% of total body length and 82.8% of cephalothorax length. Relative length of five antennular segments (as % of total antennule length) as 15.2: 20.2: 19.3: 21.5: 23.7. First segment armed with spine 1 on inner distal part, arising slightly dorsally. Second segment armed with five spines (2v_1–3, 2d_{1, 2}) and strap-like, bipinnate seta IId: ventral spines (2v-series) subequal in length, 2d₁ shorter than ventral counterparts; long spine 2d₂ reaching midway of fourth segment, setiform and bipinnate with fine setules. Third segment with three elements: spine 3 located distomedially, IIId, IIIv setae bipinnate, disposed medially on segment. Fourth segment with eight elements (4v_1–3, 4d_{1, 2}, 4a, IVv, 4aes): spines 4v_{1, 2} robust, pinnate; spine 4v₃ naked, slightly longer than former ones; naked spines 4d₁ and 4d₂ slightly thinner and shorter than those of 4v-series; spine 4a relatively well developed, subequal in length to members of 4d-series; seta IVv and ventral aesthetasc 4 (4aes) arising on proximal half of segment. Distomedial margin of segment fringed with hyaline frill. Fifth segment armed with 12 elements: short apical aesthetasc (5aes) arising near tip; three spines (5_1–3) placed on distal part of segment (spine 5₁ located distally, naked; 5₂, 5₃ located dorsally and slightly longer than 5₁, bipinnate with fine setules (Fig. 3C)); distolateral margin of segment without branched setae, which are replaced with four well-developed, simple setae A–D, and with two short, thin simple setae a and b; ventral seta (Vv) arising midway of segment; minute spine 5a located proximally on inner margin; distomedial margin of segment with five transverse serrate ridges, proximal part of medial margin with hyaline frill.

First pedigerous somite (incorporated to cephalothorax) and three succeeding free pedigers each with pair of well-developed legs (Fig. 4A–D). Each protopod consisting of large, square coxa and relatively small basis. Border between coxa and basis on anterior face incompletely defined, but posterior diagonal articulation clearly expressed. Coxae of each leg pair joined by long, rectangular intercoxal sclerite, its length in legs 1–4 respectively 1.4, 1.5, 1.9, and 1.7 times longer than corresponding proximal width (mean = 1.6). Distal margin of intercoxal sclerites excavated (Fig. 4A–D). Basis of legs 1, 2 and 4 with short, simple seta proximally on outer margin, barely reaching to proximal part of first exopodal segment; this seta on leg 3 coarsely plumose, much longer, reaching to end of first exopodal segment. Both exopod and endopod 3-segmented; endopod always located anteriorly to exopod. Endopod of all legs shorter than exopod, reaching distal margin of second exopodal segment. First and third exopodal segments twice longer than corresponding second segment. All endopodal segments subequal in length, outer margin of first two segments fringed with fine hairs. Setal armament pattern of legs 1–4 as follows (Roman numerals indicate number of spines, Arabic numerals indicate number of setae):

Spines on first and third exopodal segment pinnate, outermost seta on third exopodal segment serrate along outer margin, while pinnate along inner margin (Fig. 4A). Other setae subequally long except for relatively short inner setae on first exopodal segments: latter reaching end of exopods. Last segment of each ramus with pore on anterior face. Fifth legs knob-like, devoid of setae, reduced, implanted posteroventrally on first urosomal somite (Fig. 2C, D).

Genital somite with well-developed genital field on ventral side, composed of robust genital shaft plus two short, subtriangular lappets (Fig. 2C, D). Shaft protruding at midpoint along anterior face (Fig. 2D), then tapering distally. Hand-like opercular flap located on distal part at each side of genital shaft. Posterodistal part of genital shaft with prominent, smooth medial protrusion.

Caudal rami diverging from posterior margin of anal somite, each ramus 0.07 mm long, 0.04 mm wide, armed with six setae distributed as follows (Fig. 2C): two on outer lateral side (II, III), one terminally (IV), two on inner terminal corner (V, VI), and one on posterodorsal surface (VII). Setae II–VI subequal in length. Dorsal seta VII noticeably shorter than rest. All caudal setae bipinnate. Pore present on posterior ventral surface of each ramus.

The species name is dedicated to Prof. Susumu Ohtsuka (Hiroshima University, Japan) for his remarkable contributions to copepod taxonomy and ecology.

The present male specimens are assignable to the genus Caromiobenella based on the display of the generic features of males proposed by Jeon et al. (2018a): a relatively short cephalothorax not exceeding half of total body length, antennules with a modified fifth segment with the inner distal part bearing five serrate transverse rows of spinules, an inconspicuous oral papilla, a genital apparatus consisting of a robust shaft and two short, subtriangular lappets, and specific ornamentations such as two pairs of prominent crater-like depressions anteriorly and two longitudinal rows of four pores each posteriorly on the dorsum of cephalothorax. Besides such consistent features through the male congeners, some features are presented in two different ways, i.e. two types of male genitalia (McAlice 1985; Suárez-Morales 2000; Jeon et al. 2018a) and having five or six caudal setae (Jeon et al. 2018a). The combination of the latter two variations eventually leads the present species to be unique and differentiated from any other known male congener.

Two types of male genitalia have been reported to occur in this genus (McAlice 1985; Suárez-Morales 2000; Jeon et al. 2018a): one with the genital shaft displaying a deep triangular notch on the posterodistal margin (type I hereinafter), and a second one with a smooth medial protrusion instead of a notch in homologous position (type II hereinafter). This differential criterion divides the Caromiobenella species into two subgroups, with those sharing type I genitalia represented by C. castorea including C. helgolandica, C. pygmaea and C. patagonica, whereas type II genitalia is displayed by C. polluxea including C. serricornis. Only two species remain to be accommodated in this framework: C. hamatapex, whose type of male genitalia has not been reported thus far, and C. arctica, whose male genitalia were not described with enough detail. The present species displays type II genitalia and thus could be more closely related to the C. polluxea species-group.

Furthermore, members of Caromiobenella can also be divided into another two subgroups based on the display of five or six caudal setae (Jeon et al. 2018a). Thus, the C. castorea species-group display six caudal setae and includes C. helgolandica sensu McAlice, 1985, C. arctica, C. hamatapex and C. patagonica. The C. polluxea species-group displays only five and includes C. helgolandica sensu Huys & Boxshall, 1991, C. serricornis and C. pygmaea. With respect to the number of caudal setae, the present new species can falls in the C. castorea subgroup.

The combination of type II genitalia and six caudal setae (presented in form of “II-6” hereinafter) makes the new species described herein unique, as the rest of congeners known hitherto present a different combination of these two features: I-6 for C. castorea, II-5 for C. polluxea, I-5 or I-6 for C. helgolandica, II-5 for C. serricornis, I-5 for C. pygmaea, and I-6 for C. patagonica (Claus 1863; Sars 1921; McAlice 1985; Huys and Boxshall 1991; Suárez-Morales 2000; Suárez-Morales et al. 2008; Jeon et al. 2018a).

The monospecificity of Caromiobenella helgolandica has been frequently questioned (Grygier and Ohtsuka 1995, Suárez-Morales 2010, 2011). McAlice (1985) considered Monstrilla canadensis as the male of M. helgolandica (= C. helgolandica; Jeon et al., 2018a). This author provided three illustrations of male caudal rami (McAlice 1985: fig. 1, 1–3c) where five caudal setae are clearly shown, but remarked in the main text that one more additional short dorsal seta occurred as well, although it was difficult to observe. In addition, the author also mentioned that the general setation pattern of the male caudal rami was the same as in the female, which is depicted with six caudal setae (McAlice 1985: fig. 2f). In referring to these two descriptions, we understand McAlice’s C. helgolandica specimens have six caudal setae. On the contrary, Huys and Boxshall (1991: fig. 2.5.8a, b) depicted the same appendage as bearing only five caudal setae without any accompanying descriptive text. Despite such morphological discrepancy by different authors and the uncertainty on the actual identities of those taxa, the present new species is clearly distinguished from the previously known C. helgolandica (or C. helgolandica species-complex) by type of the genitalia. The illustrations of male genitalia by McAlice (1985: fig. 1, 1–3d) showed a specific deep notch, and those by Huys and Boxshall (1991: fig. 2.5.8b) were depicted without a medial protrusion as prominent as in C. polluxea, C. serricornis, or in the present new species. Another presumed congener C. arctica, originally reported from the Arctic region (Resolute Bay, Cornwallis Island, Canada), was originally described without including detailed information on its male genital apparatus. This species, however, can be differentiated from the present new species by the display of unusual features such as an anterior rostral projection on the cephalothorax and the presence of three dichotomously branched setae on the last antennular segment (Davis and Green 1974).

Caromiobenella hamatapex is known only from the female; thus, direct comparison with the present male specimens is risky due to the occurrence of strong sexual dimorphism in the order Monstrilloida (Suárez-Morales 2007; Suárez-Morales et al. 2008; Jeon et al. 2018b).

One of the most important morphological key features for the recognition of the new species is the presence of a fifth leg reduced to a small, knob-like, rudimentary protuberance. The absence of the fifth legs in males is one of the diagnostic characteristics for this genus (Sars 1921; Davis and Green 1974; McAlice 1985; Huys and Boxshall 1991; Suárez-Morales 2000; Suárez-Morales et al. 2008; Jeon et al. 2018a) although some specimens of C. polluxea have been reported to display a unilateral small nipple-like structure on the ventral side of the first urosomal somite (see Jeon et al. 2018a). Unlike in C. polluxea, a fifth pair of legs was present in all the type specimens examined; thus, it can be regarded as one of the genuine features for this species, and this feature separates it from the rest of representatives of the genus.