A new marine cyclopoid copepod of the genus Neocyclops (Cyclopidae, Halicyclopinae) from Korea

Abstract A new cyclopoid species of the genus Neocyclops Gurney, 1927 is described. Type specimens were collected from a beach on south-western coast of the Korean Peninsula by rinsing intertidal coarse sandy sediments. Neocyclops hoonsooi sp. n. is most characteristic in showing the conspicuous chitinized transverse ridges originating from the medial margins of the coxae of all swimming legs. The new species is most similar to Neocyclops vicinus, described from the Brazilian coast, and Neocyclops petkovskii, from Australia. All three species share a large body size (more than 750 µm long), the presence of an exopodal seta on the antenna, two setae on the mandibular palp, the same seta/spine armature on the third endopodal segment of leg 3 (3 setae + 3 spines), and the fairly long inner distal spine on the third endopodal segment of the female leg 4. However, Neocyclops hoonsooi sp. n. differs from both species by the much shorter caudal rami (less than 1.7 times as long as wide) and the shorter dorsal caudal seta VII. Furthermore, Neocyclops hoonsooi is clearly distinguished from Neocyclops vicinus by the 10-segmented antennule (vs 12 segments in Neocyclops vicinus), and from Neocyclops petkovskii by the elongate inner distal spine on leg 5 exopod and the 3-segmented leg 5 in male (vs 4-segmented in Neocyclops petkovskii). A tabular comparison of characters separating Neocyclops hoonsooi from its closest allies and a key to Neocyclops species from the Indo-Pacific Ocean are provided. This is the first record of the genus Neocyclops from the northern Pacific.


Introduction
Members of the genus Neocyclops Gurney, 1927 (Cyclopidae, Halicyclopinae) typically inhabit marine epibenthic or interstitial environments. The genus is widely distributed in coastal, surface and subterranean (anchialine) habitats of the Northeast and Tropical Atlantic, the Mediterranean, the Black and Red Seas and the Indo-Pacific (West Australia, Papua New Guinea), with an endemism index of about 95% (Pesce 2015). Karanovic (2008) recently revised the genus and added 11 new species, including eight interstitial species from Australia. However, notwithstanding its wide distribution and potentially high diversity, the taxonomy of the genus is in a state of disarray. The primary reasons for this state of affairs are the paucity of useful diagnostic characters to differentiate most species (due to the very conservative morphology within the genus), the significant variability displayed by some of them and the inadequate descriptions of several previously described species (Karanovic 2008). The genus currently accommodates 24 nominal species but it is known that many as yet unnamed species await description (Karanovic 2008, Walter and Boxshall 2015, Pesce 2015. Although cyclopoid copepods constitute an important component of the marine epibenthic and interstitial fauna, our knowledge of their taxonomy and diversity is relatively very poor in comparison with freshwater cyclopoids, especially in the northwest Pacific region (Chang 2011, Karanovic 2014. Chang (2011) recorded a new species of the genus Cyclopinoides Lindberg, 1953 (Smirnovipinidae) from the Korea Strait (Tsushima Island, Japan and Busan, Korea), and recently Karanovic (2014) described a new species of Euryte Philippi, 1843 (Cyclopidae) from the East Sea (Sea of Japan). During field surveys of the marine interstitial cyclopoids from Korea, a new species of the genus Neocyclops was found in a beach on the south-western coast of the Korean Peninsula, representing the first record of the genus from the North Pacific. In this paper we provide a detailed illustrated description of both sexes, including a tabular comparison of the salient characters distinguishing the new species from its closest congeners.

Materials and methods
Collections were made at Holtong beach, located along the south-western coast of the Korean Peninsula, in shallow littoral (about 0.5-1 m deep) by scooping the surface layer of a coarse sand bottom with a long-handled dipper. Sediment samples were gathered into a bucket, subjected to freshwater shock and filtered through a conical plankton net or plankton hand-nets (mesh size 64 µm). Samples were immediately fixed in the field by adding a few drops of 35% formaldehyde. Copepods were sorted in the laboratory, using a micropipette under a zoom-stereomicroscope (Zeiss SV-11, Germany), and transferred to 80% ethanol or to 4% buffered formaldehyde for long-term preservation.
Methods for dissection, double-coverglass preparation using H-S slides (see Shirayama et al. 1993), drawing and measurements followed those outlined in Chang (2013Chang ( , 2014. Type specimens are deposited in The Natural History Museum, London (NHMUK) and the specimen room of the Department of Biological Science, Daegu University (DB), Korea.
Prosome oval, about 1.2 times longer than urosome, a little protruding anteriorly. Rostrum reflexed downwards, not discernible in dorsal view, with blunt apex in ventral aspect. Nauplius eye not discernible. Cephalothorax not strongly protruding anteriorly, slightly longer than 4 free thoracic somites combined; first pedigerous somite completely incorporated into cephalosome. Prosomites not showing pronounced lateral expansions, with narrow and nearly smooth hyaline fringe along posterior; ornamented with one pair of minute sensilla medially on dorsal surface of second to fourth pedigerous somites, and a few integumental pores near posterior margin of each prosomite.
Urosomites (Figs 1A, 2B) length ratios, beginning with fifth pedigerous somite, 36: 100: 43: 39: 43; with hyaline membrane along posterior margins both dorsally and ventrally; spinule rows lacking, except for anal somite with about 20 minute spinules along ventral posterior margin; arrangement of cuticular pores as shown in Figs 1A, 2A and 2B. Fifth pedigerous somite slightly narrower than genital double-somite, ornamented with paired middorsal sensilla; posterolateral corner pronounced. Genital double-somite slightly longer than broad, with paired backwardly directed spinous processes in anterior quarter; leg 6 represented by one seta and one small cuticular projection surrounded by cuticular wrinkles dorsolaterally. Copulatory pore small, located midventrally in about proximal quarter of genital double-somite; seminal receptacle fully fused medially; both lateral sides transversely undulating, as shown in Fig. 2B. Anal somite much shorter than wide, about 3/4 times longer than caudal rami; 1 pair of dorsal sensilla just anterior to lateral corners of anal operculum. Anal operculum ( Fig. 2A) situated at halfway the anal somite length, not strongly convex with smooth posterior margin.
Caudal rami ( Fig. 2A, B) nearly parallel, with 7 setae; ramus 1.68 times (ranging from 1.64 to 1.72, standard deviation 0.04, n = 6) longer than wide, in ventral view, slightly shorter than anal somite; dorsal and medial surfaces of rami smooth, without hairs along inner (medial) margin; outer margin nearly smooth, not interrupted by indentations or spinules. Anterolateral seta I vestigial, represented by minute setule, situated in anterior part of ventral surface (Fig. 2B). Lateral seta II located slightly dorsally, issuing from about distal quarter of outer margin of ramus. Outer seta III short, spiniform and bipinnate, about 0.7 times as long as ramus, a little less than 2/3 length of inner seta VI, surrounded by 3-5 minute spinules at base. Terminal setae IV-V with fracture planes, bipinnate. Inner seta VI well developed, plumose, about 1.2 times as long as caudal rami, about 1.7 times longer than outer seta III. Dorsal seta VII slender, plumose, about 2/3 times as long as inner seta VI, and slightly shorter (0.9 times) than caudal ramus. Antennule ( Fig. 1B) short, reaching to about middle of cephalothorax; 10-segmented; segments 3 and 5 with incomplete ventral and dorsal sutures, respectively, indicating original subdivision.  Antenna ( Fig. 3A) slender, distinctly 4-segmented, comprising coxobasis and 3-segmented endopod. Coxobasis about 2.1 times as long as wide, with 1 long outer seta distally (unipinnate proximally and plumose distally), representing exopod, and 2 unipinnate setae at inner distal corner. First endopodal segment about 1.7 times as long as wide, with 1 naked seta at halfway the inner margin. Second endopodal segment small, about 1.5 times as long as wide, with minute spinules along outer margin; armed with 1 short medial, 2 short subapical and 2 long apical setae along inner margin. Third endopodal segment elongate, about 2.5 times as long as wide, ornamented with 1 spinular row along outer margin, bearing 7 apical setae including 4 geniculate and 3 slender setae.
Mandible (Fig. 3C), palp reduced to small protuberance, bearing 2 slender, naked setae apically; longest seta not reaching to gnathobasal teeth, about 3 times as long as shorter one. Coxal gnathobase well-developed; cutting edge armed with innermost complex of 3 stout teeth and 1 spinous element, middle group of 6 teeth and 5 sharp spinules, and outer group of 1 unipinnate spine and 1 outer subapical unipinnate seta.
Leg 5 (Fig. 2D) 3-segmented, with small intercoxal sclerite; coxa unarmed; basis with slightly swollen inner margin, bearing 1 outer plumose seta, with 1 cuticular pore near base of lateral seta. Exopod about 2.3 times as long as wide; seta/spine armature similar to that in female, except for additional seta on inner margin; allotype showing aberrant asymmetrical spine armature on left side, with outer apical spine being replaced by 1 short plumose seta (see Fig. 2D). Leg 6 reduced to operculum with 1 short inner bipinnate spine and 2 plumose setae distally; outer seta slightly longer than inner seta.
Etymology. The proposed specific name is dedicated to the late Professor Hoon Soo Kim in honor of his contribution to the development of invertebrate taxonomy in Korea.
Ecology. This species was found at Holtong beach along the western coast of the Dadohae Oceanic National Park, which is located along south-western coast of the Korean Peninsula. The beach is exposed and fringed with rocks on both sides. The intertidal, coarse to medium sandy sediments contained a little mud. Salinity: 27-32 ‰. The new species co-occurred with other interstitial ones: Cerconeotes japonicus (Itô, 1968), Cyclopina spp. (Copepoda), Xenotrichula sp. (Gastrotricha), and Echinoderes sp. (Kinorhyncha).
The new species is most characteristic in having large scar-like integumental ridges originating from the medial margins of the coxae in all swimming legs. The transverse chitinized reinforcements are very conspicuous, and consistently occurred in all specimens examined. Similar structures have been illustrated for three species that were recently described from Australia by Karanovic (2008): figure 54A and D for leg 1 and leg 4 of N. australiensis, figures 58D and 59B for leg 1 and leg 4 of N. sharkbayensis, and figure 61C for leg 3 of N. trajani Karanovic, 2008. However, all of them are less pronounced, and illustrated as small open-circles or ovals in close connection to the medial margin of the coxae, which showed quite different patterns from those of the new species.
The new species is also unusual in bearing a small setule on the anteroventral surface of the caudal ramus in both sexes. This setule is here identified as the anterolateral accessory seta I. As far as we can ascertain, it was recorded only once before in the genus Neocyclops, i.e. in the description of the female caudal ramus of N. pilbarensis Karanovic, 2008, where it was interpreted as a "sensillum at anterior part ventrally". While the caudal seta I is rarely expressed and usually lacking in members of the Cyclopoida, it can sometimes be quite conspicuous in some marine, and especially ancestral, genera, such as Heterocyclopina Pleşa, 1968. Karanovic (2008 fig. 49A, B) interpreted a similar structure as the "lateral sensillum" in his description of Abrsia misophrioides but did not consider the possibility of it being one of the caudal setae. Based on positional homology we believe that the "sensillum" observed in N. pilbarensis and A. misophrioides represents the vestigial caudal seta I and is homologous with the minute seta described in N. hoonsooi sp. n.
Another unusual characteristic of N. hoonsooi sp. n. is the very short caudal ramus, being slightly less than 1.7 times as long as wide. Caudal rami of Neocyclops species are generally more than twice as long as wide, being about 2.0-2.5 times in N. affinis, N. parvus, N. australiensis andN. ferrarii Rocha, 1995, 2.7-3.0 times in N. magnus andN. vicinus (Herbst, 1955), and even reaching to 3.5-4.0 times in N. remanei (Herbst, 1952). However, in a few species the caudal ramus is much shorter, and less than twice as long as wide, being about 1.8-2.0 times in N. medius and N. dussarti, and slightly less than 1.7 times in N. hoonsooi sp. n. Two genuinely interstitial species from beaches in southern Australia, have extremely short caudal rami (1.5 times in N. tropicus Karanovic, 2008, and 1.3 times in N. trajani), however, these species differ clearly from N. hoonsooi sp. n. by the much smaller body size (546-565 µm long), the 12-segmented antennule, the presence of 3 setae on the mandibular palp, and the very long dorsal caudal seta (1.5-2.4 times longer than caudal rami). The caudal seta VII in N. hoonsooi sp. n. is slightly shorter or nearly as long as the caudal ramus. This condition is shared with N. ferrarii, N. improvisus, N. magnus, N. mediterraneus (Kiefer, 1960), N. remanei and N. vicinus, while most other species have a much longer dorsal seta (more than twice longer than the caudal ramus): N. geltrudeae, N. pilbarensis, N. sharkbayensis and N. tropicus. Neocyclops papuensis Fiers, 1986 clearly differs from all its congeners, including the present new species, by bearing an extremely short dorsal seta (0.4 times as long as the caudal ramus). Seta VI (innermost caudal seta) of N. hoonsooi sp. n. is much longer than seta III (outermost caudal seta), and thus differs from those species that display the reverse condition (seta III longer than seta VI) such as N. affinis, N. vicinus, N. improvisus, N. monchenkoi Karanovic, 2008 and N. australiensis. Neocyclops hoonsooi sp. n. displays the typical seta/spine armature pattern on legs 1-4 found in the majority of species in the genus Neocyclops. The setal formula of the third exopodal segments of the new species is 5,5,5,5, which differs from the 5,5,5,4 pattern in N. herbsti Petkovski, 1986 andthe 4,5,5,5 condition in N. wellsi Petkovski, 1986. The spine formula of the third exopodal segments of N. hoonsooi sp. n. is 3,4,4,3, and differs only from the 2,4,4,3 pattern of N. sharkbayensis. The setal formula on the distal endopodal segments of the new species is 4,3,3,2, and differs from the 4,3,4,2 pattern displayed by N. affinis, N. dussarti and N. improvisus (Pleşa 1961, 1973, Dus-sart 1974. The spine formula of the third endopodal segments of N. hoonsooi sp. n. is 2,3,3,3, and differs from the 2,3,2,3 condition observed in N. monchenkoi. Neocyclops species, including the new species, typically bear two setae along the inner margin of the second endopodal segment of all swimming legs; the only exception to this rule is N. sharkbayensis which displays a single seta only on legs 1-2. Taking into consideration the characters mentioned above, N. hoonsooi sp. n. appears to be most similar to N. vicinus, described from the Brazilian coast, and N. petkovskii, from Australia. All three species share a large body size (more than 750 µm long), the presence of an exopodal seta on the antenna, two setae on the mandibular palp, the same seta/spine armature on the third endopodal segment of leg 3 (3 setae + 3 spines), and the fairly long inner distal spine on the third endopodal segment of the female leg 4. However, N. hoonsooi sp. n. clearly differs from N. vicinus by the follow- ing characters: (1) 10-segmented antennule (vs 12 segments in N. vicinus); (2) shorter caudal rami (less than 1.7 times as long as wide, while about three times longer in N. vicinus); and (3) much shorter dorsal caudal seta VII (about 2/3 times shorter than inner caudal seta VI, while 1.4 times longer in N. vicinus), and much longer inner caudal seta VI (more than 1.5 times longer than outer caudal seta III, while slightly shorter than outer one in N. vicinus). Furthermore, N. hoonsooi sp. n. also clearly differs from N. petkovskii by the much shorter caudal rami (vs 2.4 times as long as wide in N. petkovskii), the shorter inner distal spine on the female leg 5 (vs slightly shorter than the outer distal and lateral spines, and about half the length of the exopod in N. petkovskii), and the 3-segmented leg 5 in male (vs 4-segmented in N. petkovskii). Table 1 shows the character comparison between the new species from South Korea and its closest allies.