Three new species of the genus Stephos Scott, 1892 (Crustacea, Copepoda, Calanoida, Stephidae) from Jeju Island, Korea

Abstract During general field surveys carried out recently to collect benthopelagic copepods from near the substrate of the shallow waters off Jeju Island, Korea, a few specimens of three new species of Stephos Scott, 1892, were collected. The new species are placed in the genus Stephos because of the following combination of features: absence of seta on the basal exite of maxillule, and male right leg 5 ending in an unarmed claw-like and/or mitten-like segment. Stephos jejuensissp. nov. can be distinguished from its congeners by body length 0.92 mm, left side of the female genital double-somite with protruding lobes, antennule that extends beyond the distal area of the genital double-somite, and the male leg 5 terminal complex. Stephos concavussp. nov. can be distinguished from its congeners by the genital double-somite with protruding lobes on both sides, and the presence of larger spinules on the distomedial margin of leg 5. Stephos fortipessp. nov. can be distinguished from its congeners by its longer body length, 1.12 mm long in the female, antennules that extend to the end of the genital double-somite, and the presence of a covered row of minute spinules on the ventral surface of the genital operculum in the female. Until now, 35 species of stephids were known worldwide.

During a survey of the copepod fauna of the southern coasts of Jeju Island, the largest island in Korea, a few specimens of stephids were collected from near-bottom shallow waters by vertical tows of 0.1-mm mesh conical nets at high tide in dusk hours. One of these samples contained representatives of several Stephos not known to the benthopelagic environment. This paper reports on three undescribed species of the genus Stephos that are herein described in full and compared with their known congeners around the world.

Materials and methods
Copepods were collected from the shallow waters of Jeju Island, Korea by vertical tows (0.1-mm mesh conical nets) at high tide in dusk hours (Fig. 1). For morphological examination, samples were fixed in a 5% natural formalin-seawater solution and cleared in 70% lactic acid for an hour before dissection in a drop of lactophenol on a wooden slide under the dissection microscope (Humes and Gooding 1964). Dissected body parts and appendages were examined under a compound microscope with magnification up to X1,000. Measurements were made with a stage micrometer from the head to the tip of the caudal ramous, excluding the caudal setae. Drawings were made with the aid of a drawing tube equipped on the microscope. The morphological terminology follows Huys and Boxshall (1991) and Ferrari and Ivanenko (2008). An abbreviation used in the text and figures is ae, for aesthetasc. Specimens are deposited at the National Institute of Biological Resources (NIBR), Incheon, Korea.
Etymology. The specific name of the new species jejuensis refers to the type locality. Description of female. Body ( Fig. 2A, B) robust, length 0.92 mm (mean 0.91 ± 0.03, N = 3). Prosome 5-segmented; cephalosome and first pedigerous somites completely separated; fourth and fifth pedigerous somites incompletely fused ( Fig. 2A, B), Stephos jejuensis sp. nov. Female paratype A habitus, dorsal view B habitus, lateral view C urosome, dorsal view D urosome, lateral view E urosome, ventral view F anal somite and caudal rami. Scale bars in µm. posterior corners of fifth pedigerous somite slightly asymmetric. Rostrum represented by a rounded knob. Prosome-urosome ratio 2.61:1. Urosome 4-segmented, comprising genital double-somite, two free abdominal somites, and anal somite; length ratio of genital double-somite, first free abdominal somite, second free abdominal somite, and anal somite as 48.2: 14.2: 12.6:11.7:13.4 = 100. Genital double-somite (Fig. 2C, E) asymmetric, with protruding lobe on the anterior to posterior of the left side and a projecting lobe to distal margin, with minute spinules patched in lateral view (Fig. 2D); on the right anterior side is a swollen, common operculum bumpy-shaped ventromedially and with ear lobe on the ventrolateral margin. First and second abdominal somites with transverse hyaline frill dorsally and ventrally. Anal somite short. Caudal rami (Fig. 2F), with six setae, symmetric, 1.45 times longer than wide (44 × 31 µm); caudal setae II-VII present (seta I lacking); seta II spiniform, seta III ca. half the length of seta V, seta V longer (right longer than left) than seta IV, both plumose; dorsal seta VII short, plumose.
Maxillule (Fig. 3D): praecoxal arthrite bearing nine stout marginal spines and four elements on posterior surface, rows of tiny spinules on the posterior surface. Coxal epipodite with nine setae; coxal endite with three stiff setae. Basis with cluster of denticles on the anterior surface; proximal basal endite with four setae; distal basal endite indistinct, with five setae; no trace of basal exite. Exopod with eleven marginal setae and a row of setules along the distal portion of the medial margin. Endopod not articulated to basis, indistinctly 3-segmented, setal formula 4, 4, 7.
Leg 2 ( Fig. 4C) biramous, endopod 2-segmented; coxa and basis unarmed; second endopodal segments with a row of spinules on the medial and distal edges, with a pointed process on the distolateral corner; exopod 3-segmented, with a row of spinules on the medio to distal margins of the distal exopodal segment.
Legs 3 (Fig. 4D) and 4 ( Fig. 4E) biramous, with 3-segmented rami: coxa and basis unarmed; second and distal endopodal segments with a row of spinules on the distal edges, with a pointed process on each of the distolateral corners; exopod with a row of spinules on the medial to distal margins of the distal exopodal segment.
Leg 5 ( Fig. 4F) symmetric, uniramous, 3-segmented with a proximal segment fused to intercoxal sclerite; basis separated, 2.27 times longer than wide (41 × 18 µm), widening distally with minute spinules on the anterior corner and an acute inner process, and unarmed. Distal segment with a transverse row of spinules across near the middle part and an outer seta medially.
Leg 5 (Fig. 5D-F), strongly asymmetric, slender on both sides, developed as a grasping organ on the left. Right leg 4-segmented; coxa and basis are short, unarmed, but thickened proximally; terminal segment comprising a single longer process (see arrowed in Fig. 5F), outwardly directed, curved medially, and acute at its tip. Left leg 5-segmented (see Fig. 5E); proximal segment ca. as long as right proximal segment; second segment with rounded outgrowth on medial margin; third segment elongated, unarmed; fourth segment narrow, shorter than third segment; terminal segment complex, with 5 terminal (long) and 5 subterminal (short) lamella spines.
Variations. Within this new species, there was a minor variation in the number of spinules on the genital double-somite and on the surfaces of legs 1-4 in both sexes.
Remarks. The genital double-somite in most species of Stephos has been found to be symmetric and/or slightly asymmetric in shape. The feature of an asymmetric genital somite in S. jejuensis sp. nov. is shared with five of its congeners, S. lamellatus Sars, 1902;S. tsuyazakiensis Tanaka, 1966;S. exumensis Fosshagen, 1970; S. kurilensis Kos, 1972;and S. robustus Ohtsuka & Hiromi, 1987. Of these, S. jejuensis has a projecting lobe on the distal margin in the lateral side of the genital double-somite; however, the other five species do not have this feature. Stephos jejuensis has been group IV.
In addition, S. jejuensis expresses by two diagnostic features: the fifth pedigerous somite is slightly asymmetric; and a projecting lobe in the lateral side of the genital double-somite. These features are shared by only one other species: S. jejuensis can be distinguished from S. maculosus (Bradford-Grieve 1999) by the following features in the female: the body length is 0.92 mm (vs. 0.62 mm in S. maculosus); dorsally the left side of the genital double-somite has anterior and posterior protruding lobes (vs. without protruding lobe in S. maculosus); the antennule extends beyond the distal area of the genital double-somite (vs. not beyond the distal area in S. maculosus); and the distal segment is less than four times longer than the second segment of leg 5 (vs. more than four times in S. maculosus). In the male: the body length is 0.93 mm (vs. 0.54 mm in S. maculosus); the antennule extends beyond the distal area of the genital double-somite (vs. beyond the anterior margin of the caudal rami in S. maculosus); on the leg 5 fourth segment of the male is narrow (vs. with an finger-like lobe on the medial expansion in S. maculosus); and the leg 5 terminal segment complex consists of five terminal (long) and five subterminal (short) lamella spines (vs. not complex, only with three lamella spines in in S. maculosus).
Leg 2 (Fig. 8E) biramous, endopod 2-segmented; coxa and basis unarmed; second endopodal segments with a row of spinules on medial and distal edges, with pointed process on the distolateral corner; exopod 3-segmented, with a row of spinules on the medio to distal margins of the distal exopodal segment.
Legs 3 (Fig. 8F) and 4 ( Fig. 8G) biramous, with 3-segmented rami: coxa and basis unarmed; second and distal endopodal segments with a row of spinules on the distal edges, with a pointed process on each distolateral corner; exopod with row of spinules on the medio to distal margins of the distal exopodal segment, except for a row of spinules on the posterior surface of leg 4 basis.
Leg 5 (Fig. 6F) symmetric, uniramous, 3-segmented with a proximal segment fused to intercoxal sclerite; basis separated, 2.53 times longer than wide (38 × 15 µm) and unarmed. Distal segment constricted slightly at ca. mid-length with five large spinules and a large seta medially and with two rows of teeth on both lateral each sides as figured.
Male. Not collected.

Remarks. The new species
Stephos concavus sp. nov. is easily recognized by its four diagnostic features in the female: the genital double-somite with a protruding lobe on the anterior to medial part of both lateral sides; the presence of seven large rows of spinules on the left side of the genital double-somite; the basis of leg 5 is separated, 2.53 times longer than wide; and the presence of large spinules mediodistally on distal segment of leg 5.
The new species closely resembles S. cryptospinosus , but it differs in the following features in the female: the body length is 0.93 mm (vs. 0.86 mm in S. cryptospinosus); the presence of seven spinules on the left side of the genital double-somite (vs. absence in S. cryptospinosus); the antennule extends beyond the distal end of the genital double-somite (vs. beyond the posterior margin of the prosome in S. cryptospinosus); the presence of large spinules on the mediodistal margin of leg 5 distal segment (vs. absence in S. cryptospinosus); and the terminal segment with teeth on both sides and large spinules mediodistally on both fifth legs (vs. absence in S. cryptospinosus).
Stephos concavus differs from another congener S. longipes (Giesbrecht, 1902)  Etymology. The specific name fortipes is the combination of Latin words fortis (strong) and pes (leg), alluding to the strong feature of the female fifth leg.
Description of female. Body (Fig. 9A, B) robust, length 1.12 mm. Prosome fivesegmented; cephalosome and first pedigerous somites completely separated; fourth and fifth pedigerous somites incompletely fused (Fig. 9A), posterior corners of prosome slightly asymmetric. Rostrum represented by a rounded knob. Prosome-urosome ratio 2.45:1. Urosome 4-segmented, comprising a genital double-somite, two free abdominal somites, and anal somite; length ratio of genital double-somite, first free abdominal somite, second free abdominal somite, and anal somite as 39.1: 18.7: 17.1:15.1:10.0 = 100. Genital double-somite (Fig. 9C, E) slightly asymmetric with a differing groups of minute spinules on each side, anterior to mid-length; on the left side is a group of minute spinules that tend to be obscured by detritus and difficult to observe, patches and rows of fine spinules on the right side; genital double-somite not produced ventrally, operculum slightly round, with rows of spinules on the ventral surface. First and second abdominal somites (Fig. 9C), with transverse hyaline frill dorsally and ventrally. Anal somite shortest. Caudal rami (Fig. 9F), with six setae, symmetric, 1.19 times longer than wide (56 × 47 µm), with minute spinules on the dorsal surface; caudal setae II to VII present (seta I lacking); seta II spiniform, seta III ca. half the length of seta V, seta V longer (right longer than left) than seta IV, both plumose; dorsal seta VII short, plumose.
Maxillule (Fig. 10D): praecoxa and coxa incompletely fused; praecoxal arthrite with ten marginal spines plus four stiff setae on posterior surface, rows of tiny spinules on posterior surface. Coxal epipodite with nine setae; coxal endite with three stiff setae. Basis with cluster of denticles on the anterior surface; proximal basal endite with four setae; distal basal endite indistinct, with five setae; no trace of basal exite. Exopod with eleven marginal setae. A row of setules along the distal portion of the medial margin. Endopod not articulated to basis, indistinctly 3-segmented, setal formula 4, 4, 7.
Leg 1 (Fig. 11A) biramous, coxa with hairs and spinules on the inner and posterior surfaces; basis with a row of spinules on the inner distal corner and long, curved inner setae, and endopod with a lobe on the outer margin, bearing a minute spinous process; second and distal exopodal segments with patched minute spinules; second and terminal exopodal segment with a row of spinules on the posterior margin.
Leg 2 (Fig. 11B) biramous, endopod 2-segmented; coxa with hairs on the inner margin, row of spinules on the posterior surface; basis unarmed; each first and second endopodal with row of spinules on the medial and distal edge, with pointed process on distolateral corner; exopod 3-segmented, with a row of spinules on the medio to distal margins of distal exopodal segment.
Legs 3 (Fig. 11C) and 4 ( Fig. 11D) biramous, with 3-segmented rami: coxa with hairs on the inner margin and a row of spinules on the anterior surface; first to distal endopodal segments with a row of spinules on distal edges, with pointed process on each distolateral corner; exopod with a row of spinules on the medio to distal margins of distal exopodal segment.
Leg 5 (Fig. 11E) symmetric, uniramous, 3-segmented with proximal segment fused to intercoxal sclerite; basis separated from the single, tapering terminal segment. Second segment (basis) 1.38 times longer than wide (44 × 32 µm), with an anteromedial patch of minute spinules on the anterior surface. Distal segment constricted slightly at ca. mid-length with seven large spinules and inner stout spine and with two rows of denticles along the tapering portion Male. Not collected.
Variations. Within this new species, there was a minor variation in the number of spinules on the genital double-somite and on the surfaces of legs 1-4 in the female.
Remarks. The new species closely resembles its congeners S. angulatus Bradford-Grieve, 1999, S. hastatus, andS. pacificus Ohtsuka &Hiromi, 1987; however, it differs in the following characteristics in the female: the antennule extends to the end of the genital double-somite (vs. first abdominal segment end in S. angulatus, and fifth pedigerous end in S. hastatus and S. pacificus); the operculum is slightly round (vs. triangular in three species); and the stout and present large row of spinules on the terminal tapering part of leg 5 (vs. not stout and absent in three species).

Discussion
The benthopelagic copepod fauna of the Korean peninsula was previously surveyed (Soh et al. 2013;Moon et al. 2014Moon et al. , 2015. They recorded five species: Sarsarietellus orientalis, Soh et al., 2013, offshore of Yogji and Maemul Island, southern Korea; Stephos geojinensis from the Geojin fishery port in eastern Korea; S. pacificus from the shallow waters of Jangdeong beach in southern Korea; S. projectus from the Naro Island in southern Korea; and Boholina ganghwaensis, Moon & Soh, 2014, from Ganghwa Island in western Korea. The morphological characteristics of the genus Stephos, and a key to identifying the species were provided by Boxshall and Halsey (2004). Taxonomic analysis of closely related species of Stephos is based on the subtle morphological characters by Bradford-Grieve (1999) and Suárez-Morales et al. (2017). Bradford-Grieve (1999) categorized the species of the genus by analyzing the fourth segments of the male left fifth leg into four types. In this study, seven species in the Australian-Western Pacific region belong to "group IV", where the fourth segment of the male left fifth leg is narrow, as follows: S. angulatus Bradford-Grieve, 1999; S. geojinensis Moon, Youn, & Venmathi Maran, 2015;S. jejuensis sp. nov., S. morii Greenwood, 1977;S. pacificus Ohtsuka & Hiromi, 1987;S. pentacanthos Chen & Zhang, 1965;and S. tsuyazakiensis Tanaka, 1967 (Table 1). The zoogeographic analysis presented based on the structural patterns of the female fifth leg by Suárez-Morales et al. (2017), grouped the 29 species of Stephos together. The three new species described here belong to "group A", where lateral setae are present and segments are apically elongate. This primitive "group A" is the most widespread, present in the most diverse regions (Suárez-Morales et al. 2017).
Most species of Stephos are frequently found in hyperbenthic and epibenthic habitats of tropical to polar regions (Boxshall and Halsey 2004;Jaume et al. 2008;Kršinić 2015;Moon et al. 2015;Suárez-Morales et al. 2017), and are occasionally recorded in anchialine caves (Boxshall et al. 1990;Riera et al. 1991;Carola and Razouls 1996;Jaume et al. 2008;Suárez-Morales et al. 2017). However, in this study, the three new species were collected at night using a plankton net in shallow waters. Other stephids have also occurred in plankton samples collected at night in coastal waters (Kos 1972;Ohtsuka and Hiromi 1987;Costanzo et al. 2000;Zagami et al. 2000;Moon et al. 2015). These facts suggest that benthopelagic calanoids could undertake daily vertical migrations Moon et al. 2015) and also diel feeding rhythm, reproduction, molting, dispersal, and niche diversification (Alldredge and King 1980).
The stephids comparisons of morphological features between the three new species of Korean fauna and the all species of genus Stephos are based on both sexes in the World. Stephos shares many of the characteristics of Miostephos Bowman, 1976, but differs in that the right fifth leg in female is 4-segmented and the male right fifth leg ends in an unarmed claw and/or mitten-like segment in Stephos (Boxshall and Halsey 2004;Kršinić 2015). According to Suárez-Morales et al. (2017), the structure of the female fifth leg is of great significance in the taxonomy of stephids. These characteristics were used in the keys to species of Stephos by Suárez- Morales et al. (2017). Here the following combination of features are used in order to separate

Jaume, Boxshall & Gràcia, 2008
Female Body length (mm) (1) the body length in both sexes; (2) the shape of postero-lateral corners in both sexes; (3) the shape and ornamentations of the genital double-somite in females; (4) the presence and/or absence of spinules on the caudal rami in both sexes; (5) the antennule extension in both sexes; and (6) the ornamentation and shape of fifth legs in both sexes. The principal differences between the three new species and their congeners are summarized in Table 1. Although some features occasionally overlap within the all species considered herein, the characteristic combinations proposed are different for each species, showing them to be essential diagnostic elements. These morphological characteristics were very useful and important criteria for identifying each species of stephids.
To date, the genus Stephos consists of 35 valid species, including those described herein (Boxshall and Halsey 2004;Jaume et al. 2008;Kršinić 2012Kršinić , 2015Moon et al. 2015;Suárez-Morales et al. 2017;this study). Additionally, most of the genus Stephos species were not described following modern standards, and most of them need to be redescribed. Thus, the taxonomy, morphological variability, and distribution of stephids are well understood (Bradford-Grieve 1999; Moon et al. 2015;Suárez-Morales et al. 2017). These facts suggest that more detailed research on its taxonomy, biodiversity, and molecular features is necessary for a better understanding of its evolutionary history.