A new species of the genus Stylicletodes Lang, 1936 (Copepoda, Harpacticoida, Cletodidae) from South Korea

Abstract Background Marine benthic harpacticoid copepods are poorly known in Korea due to the difficulty in obtaining specimens. Currently, the genus Stylicletodes Lang, 1936, which is known to occur in subtidal sediments, has not been reported in this area so far. During surveys on the subtidal meiofauna, we found a new species of Stylicletodes from several subtidal muddy sediments in the Yellow Sea and South Sea. New information In this study, we describe both sexes of a new species of Stylicletodes collected from the Yellow Sea and South Sea of Korea. Stylicletodestrifidus sp. nov. differs from its congeners in the following characteristics: the trifid rostrum, relative length ratio of the endopods to exopods on legs 1–4, reduced armature formulae on legs 3–4, constricted shape at mid-length of the anal somite, and structure of the sexually dimorphic male leg 3 with a two-segmented endopod. The new species underwent loss of the maxilliped, which is very rare in harpacticoids and is probably an important clue for the phylogeny of the species of Stylicletodes.

A new species of the genus Stylicletodes was found in sublittoral samples taken from the Yellow Sea and South Sea during surveys on the diversity of benthic harpacticoids from Korea. Here we give a detailed description of the new species.

Materials and methods
Samplings were conducted at several stations in the Yellow Sea and South Sea ( Fig. 1; Table 1) on board R/V Eardo (Korea Institute of Ocean Science & Technology (KIOST)). Sediments were taken with a Smith-McIntyre grab (0.1 m ) and the upper surface (> 5 cm) of the sediments was subsampled using a trowel for qualitative analyses. A solution of 7.5% magnesium chloride (MgCl ) was added to the subsample and fixed after 30 minutes with a 10% formalin/seawater solution. Meiofauna was separated from the coarser grains of sediment through the centrifugation method with LUDOX® HS-40 (Burgess 2001). Copepods were sorted under a stereomicroscope (M165 C; Leica, Germany) and preserved in 5% formalin. Specimens were dissected with tungsten needles, mounted in lactophenol:glycerine (1:3) or Fluoromount-G (SouthernBiotech, USA) on glass slides or H-S slides (Double slide plate, BSDS-011R; Biosolution, Republic of Korea) (cf. Shirayama et al. 1993) and sealed with transparent nail varnish. A differential interference contrast (DIC) light microscope (DM2500; Leica, Germany) with a drawing tube was used to make morphometric measurements and illustrations. The mouth appendages were photographed with a CCD camera (DP26; Olympus, Japan) mounted on a differential interference contrast microscope (BX53; Olympus, Japan). The total body length of individuals was measured dorsally from the anterior tip of the rostrum to the posterior end of the caudal rami in the dorsal view and was sometimes calculated as the sum of the mid-dorsal lengths of each somite measured in the lateral view, not considering the various degrees of telescoping of somites. The length/width ratio (L/W) of the anal somite was calculated by measuring the length along the middle in ventral view and the largest width at the anterior part. The L/W of the caudal rami was calculated by measuring the length along the outer margin in the dorsal view and the widest part at the insertion point of lateral setae I and II. The L/W of the P5 exopod was calculated by measuring the length along the outer margin and width above the insertion of the proximal outer seta. Scale bars in the figures are indicated in micrometers (μm).
Type material was deposited in the Marine Biodiversity Institute of Korea (MABIK), Seocheon, Republic of Korea and additional materials were stored at the Marine Interstitial fauna Resources Bank (MInRB) in KIOST, Busan, Republic of Korea.
Caudal rami ( Fig. 2A-B and Fig. 3A-C) distinctly divergent, cylindrical, extremely elongate, as long as three abdominal urosomites combined, L/W ratio 12.7, as long as 1/4 of body length; with few spinules at posterior margin of ramus ventrally (Fig. 3C); with 7 setae: lateral setae I and II inserted in proximal 1/8 of ramus, seta II about twice as long as seta I; seta III shortest, arising from outer distal corner; terminal seta IV fused basally to well-developed seta V, as long as seta II; terminal seta V longest, Stylicletodes trifidus sp. nov. Female A urosome, dorsal view, with setae on caudal rami numbered using Roman numerals; B urosome, lateral view; C urosome, ventral view. slightly shorter than ramus; seta VI short, slightly longer than seta III, located at inner distal corner; tri-articulate seta VII arising from minute dorsal pedestal, located in middle of caudal ramus.
Maxilliped absent, with a small plate in the place where this appendage was supposed to be (see asterisk in Fig. 5A-B).
Caudal rami (Fig. 9C, D) as in female, except for seta V longer than caudal ramus.
Antennule (Fig. 9A, B) 6-segmented, chirocer, with geniculation between fifth and sixth segments; with aesthetasc on fifth and sixth segments; first segment with 4 rows of inner spinules; fourth segment very small (difficult to see); fifth segment markedly swollen, with 1 anterior patch of minute spinules; sixth segment conical, slightly curved inwards. Armature formula as follows: P3 (Fig. 10A). Exopod as in female; endopod 2-segmented, as in female, except for outer spine of enp-2 (indicated with an arrowhead in Fig. 10A) fused to segment forming an apophysis pinnated subdistally.
Variability: Both sexes exhibited some variability in the L/W ratio of the caudal ramus (12-14.6 in females; 13.6-17.3 in males) and the L/W ratio of the exopod of P5 (7.1-10.0 in females; 5.3-7.2 in males).

Etymology:
The specific name is derived from the Latin adjective trifidus, meaning "cleft into three" and refers to the characteristic shape of the rostrum with trifurcated processes at the tip. It is in the nominative singular, gender masculine.
The new species discovered in Korean waters, S. trifidus sp. nov., can be placed in the genus Stylicletodes, based on the P1-P4 exp-3 longer than exp-1 and exp-2, respectively, the outer spines of P1-P4 exopods with long outer spinules subdistally, the exopod and endopodal lobe of female P5 conspicuously elongate, and elongate caudal rami (Lang 1936, Lang 1948, Fiers 1996, Ma et al. 2021. Following the subdivision of the genus by Ma et al. (2021), S. trifidus sp. nov. is assigned to 'Group I', which includes S. longicaudatus, S. oligochaeta, S. stylicaudatus and S. verisimilis, sharing an unmodified operculum (without a median linguiform extension) and the densely plumose outer elements on the P5 exopod. However, females of S. trifidus sp. nov. are distinguished from the other four species by the following characters (Table 3): (1) no inner seta on P3-P4 exp-2 (absent in S. stylicaudatus and S. oligochaeta; but present in S. longicaudatus and S. verisimilis); (2) P3-P4 exp-3 with four elements (vs. five in S. stylicaudatus and S. oligochaeta; six in S. longicaudatus and S. verisimilis); (3) the P1 endopod does not reach the tip of exp-3 (vs. reaching as far as or beyond the tip of exp-3 in the other four species); and (4) the anal somite is constricted at lateral mid-length (vs. straight in the other species; the female of S. stylicaudatus remains unknown); longer than the wide (vs. short in S. longicaudatus and S. verisimilis and subequal in length in S. oligochaeta; the condition is unknown for S. stylicaudatus) and with the anal operculum located at anterior 1/3 of the anal somite (vs. about 2/3 in the other four species). Moreover, the antennule in the male of S. trifidus sp. nov. is 6-segmented consisting of an additional small segment between the third and swollen segments (Fig. 9B); it is 5-segmented in the males of other species of Stylicletodes. This additional segment is very difficult to observe, but recent studies of harpacticoid copepods have revealed the small segment of the male antennule under highresolution microscopy Huys 1999, George 2018). Therefore, it is necessary to observe the male antennule carefully.
The tip of the rostrum is either rounded or bifid in Cletodidae, with two (sub)apical sensilla and its morphology is occasionally considered a good character for identifying cletodid taxa at the genus or species levels (Gee 1994. The rostral tip is minutely bifid in S. longicaudatus and S. verisimilis or not split in S. stylicaudatus and S. wellsi, while the rostrum of S. oligochaeta and S. reductus have not been described. Stylicletodes trifidus sp. nov. has a unique trifid rostrum with the sensilla between its furrows, as shown in Fig. 2C. The trifid rostrum of the new species is regarded here as apomorphic within Stylicletodes. This trait has been observed in a few cletodid harpacticoids, for example, Scintis variifurca Por, 1986 (see Por 1986: figs. 22 and33).
Within Cletodidae, sexual dimorphism occurs in the male P3 endopod (except for some species lacking sexual dimorphism); although Gee (1994) defined three states of the sexual dimorphism pattern in the P3 endopod, the origin of the apophysis in the males remains controversial (Gee 1994, Fiers 1996. Gee (1994) postulated that the outer element on the female P3 enp-2 is homologous to the apophysis in the male, while Fiers (1996) suggested it developed from the outer hyaline membrane of the male P3 enp-2 at the last moult from the copepodid V stage to adult.
Although there are few descriptions of the males of the genus (only the males of S. longicaudatus, S. stylicaudatus and S. wellsi are known) ( Willey 1935, Petkovski 1955, Lang 1965, Gómez 2000, Ma et al. 2021, the morphology of the male P3 endopod, including S. trifidus sp. nov., appears to be variable. The males of three species have a 3segmented P3 endopod bearing a stout apophysis on the distal margin of enp-2, the third type reported by Gee (1994). In their females (except for S. stylicaudatus), the armature on P3 enp-2 is different, that is, S. longicaudatus has an outer element (spiniform) and two distal setae, whereas S. wellsi has only two distal setae (Table 3). Similar sexual dimorphism (it does not consider the ontogenesis) to S. longicaudatus was found in species of Limnocletodes (e.g. L. mucronatus Gee, 1998in Gee (1998: fig. 4D), Strongylacron [e.g. S. buchholzi (Boeck, 1873) in : fig. 5C] and Spinapecruris [e.g. S. curvirostris (Scott, 1894) in Gee (2001): fig. 5B]. The form of S. wellsi appears in some species of Cletodes (e.g. C. macrura Fiers, 1991in Fiers (1991: fig. 12a; C. confusum Gómez, 2000in Gómez (2000: fig. 7B) and Enhydrosoma (e.g. E. baruchi Coull, 1975in Coull (1975: fig. 4).   fig. 16), but other literature showed the same setal formula as that of P4 (Sars 1920, Monard 1935, Griga 1963, Lang 1965, Gómez 2000  In comparison, the female of S. trifidus sp. nov. has a 2-segmented P3 endopod with an outer apical spiniform seta plus a distal seta and the male P3 endopod is 2-segmented as in the female; the outer element is enlarged and fused to the segment forming an  3B) and Paracrenhydrosoma normani Gee, 1999(see Gee 1999: fig. 11A, G). Unfortunately, nothing is known about the development of species of Stylicletodes and a discussion of the sexual dimorphism of the P3 endopod will be possible only after phylogenetic studies have been conducted. Nevertheless, we suggest that the male P3 endopod morphology is a distinct apomorphy of the new Korean species in the genus.
In the description of the type species S. longicaudatus, Brady (1880) mentioned only the maxilliped, which (as "second pair of foot-jaws" in his text) is of '[moderate] size, with an ovate hand,' but he did not provide any information about other mouth appendages. Sars (1920) first illustrated the mandible and maxilla of S. leptostylis (= a junior synonym of S. longicaudatus), but omitted the written description of the maxilliped. Later, Lang (1948) described the mouth appendages very briefly in his diagnosis of the genus Stylicletodes and only mentioned that the maxilliped is relatively small. Gómez (2000) and Kornev and Chertoprud (2008) (in males) provided the most detailed descriptions and illustrations of S. longicaudatus, although Gómez (2000) reported that the maxilliped was lost during dissection and Kornev and Chertoprud (2008) fig. 2F). However, both sexes of S. trifidus sp. nov. lack the maxilliped as shown in Fig. 5A and B, remaining as a minute single plate where this appendage is situated in other harpacticoids. In his taxonomic work, Monard (1935) also noted that he did not see the maxilliped on his specimen of S. numidius (= a junior synonym of S. longicaudatus) from Tunisia. This observation might intimate a close relationship between Korean and European specimens and some species of the genus might not retain the maxilliped. To the best of our knowledge, there have been no reports of the absence of the maxilliped in species or genera within Cletodidae and this character (without maxilliped) is unique within the family. Within harpacticoids, however, the loss of the maxilliped has rarely been documented in the genus Leptocaris T. Scott, 1899 of the family Darcythompsoniidae Lang, 1936 and the extreme reduction of this ramus can be observed in the genera Cylindropsyllus Brady, 1880 and Cylinula Coull, 1971 of the family Cylindropsyllidae Sars, 1909 (Huys 1988, Huys andWillems 1993). In addition, the loss of mouth parts in the male, as sexual dimorphism, appears in few deep-sea harpacticoids, such as families Aegisthidae Giesbrecht, 1893, Argestidae Por, 1986and Pseudotachidiidae Lang, 1936(Gómez 2018. Although the existence or non-existence of the maxilliped in all Stylicletodes is uncertain, this character might be important for resolving the polyphyly of Stylicletodes as suggested by Ma et al. (2021).