Two new species of benthopelagic Stephos (Copepoda, Calanoida, Stephidae) from Korea

Abstract Two new species of benthopelagic copepods of the genus Stephos T. Scott, 1892, belonging to the family Stephidae G.O. Sars, 1902, are described based on specimens collected in the stagnant water flooding the burrows excavated by ocypodid crabs in two intertidal mud-flats, and from near-bottom shallow waters in Korea, respectively. They can be easily diagnosed based on the ornamentation of both the female genital double-somite and genital operculum; the morphology of the distal segment of the male right P5; the presence/absence of a tiny pointed process on the distomedial angle of second segment of female P5; and the condition (seta or spine) of the lateral armature element on the distal segment of female fifth legs, among other features. This is one of the few cases reported of calanoid copepods living as commensals of other invertebrates, and raises to six the number of members of the genus reported from Asia. This is also the first record of the family Stephidae in Korea.

In Korean waters, only three species of benthopelagic calanoids have so far been reported, namely: Sarsarietellus orientalis Soh et al., 2013 (Arietellidae), collected from the shallow water near-bottom of the Jeju Island, southern Korea (Soh et al. 2013), and Paramisophria sinjinensis Lim &Min, 2014 andP. koreana Lim &Min, 2014 (Arietellidae), both collected also from near-bottom shallow waters in southern Korea (Lim and Min 2014). The recent advances in the knowledge of species diversity of benthopelagic calanoids in the region is the result of intensive investigations using a diverse array of sampling methods.
During the general field surveys carried out recently to collect calanoid copepods from two inter-tidal mud flats and near-bottom shallow waters, two new species of the genus Stephos were recorded. This paper deals with their descriptions and presents the first record of the family in Korean waters.

Material and methods
Copepods were collected from the stagnant water retained in the burrows excavated by ocypodid crabs in two intertidal mud-flats using a hand net (0.2 mm mesh size) and also from near-bottom shallow waters using a light trap and a plankton net (0.2 mm mesh size) at high tide at dusk hours in eastern and southern Korea. For morphological examination, samples were fixed in 5% natural formalin-seawater solution. Specimens were later cleared in 70% lactic acid for 1 to 2 hours before dissecting under the dissection microscope (Nikon) in a drop of lactophenol on a wooden slide (Humes and Gooding 1964). The removed body parts and appendages were examined under a Olympus BX51 phase contrast microscope up to ×1,000. Drawings were made with the aid of a drawing tube attached to the microscope.
Body sizes of individuals were measured using a stage micrometer from the head to the tip of the caudal rami excluding caudal setae. The morphological terminology follows Huys and Boxshall (1991). Abbreviations used in the text and figures are as follows: ae, aesthetasc; P1-P5, first to fifth swimming legs. Specimens are deposited at the National Institute of Biological Resources (NIBR), Incheon, Korea. Stephos geojinensis sp. n. http://zoobank.org/57A20501-74CF-4E13-874D-4FC21A3C7ECE Figs 1-4 Material examined. Female holotype (NIBRIV0000304586) and male allotype (NI-BRIV0000304587) undissected and preserved in 70% ethanol; female paratype (NI-BRIV0000304738) and male paratype (NIBRIV0000304739) dissected on two glass slides; one female paratype and seven male paratypes (NIBRIV0000304293, 1 vial) preserved in 70% ethanol. All specimens were collected from the near-bottom using a light trap at high tide at dusk, on 28 August 2010 by the senior author (S. Y. Moon). The description below is based on the paratypes.
Antenna, mandible, maxillule, maxilla, maxilliped and P1-P4 similar to those of female. Fifth legs (Fig. 4D) strongly asymmetrical, uniramous and filiform. Left leg 5-segmented, shorter than right counterpart; second segment with blunt prominence medially; third and fourth segments elongated, about equal in length; distal segment reduced, with row of seven unequal long and 13 short hyaline lamellae disposed as figured. Right leg 4-segmented; third segment very elongated with short, curved proximolateral spurlike process; distal segment elongated and curved, bifid with short inner branch.
Etymology. The specific name geojinensis is taken after the type locality Geojin Port, Gosung-gun, Gangwon-do, Korea.
Remarks. Stephos geojinensis sp. n. is easily recognizable by the display of the following five diagnostic features: (1) female genital double-somite with protruding proximolateral margins in dorsal aspect; (2) genital double-somite with row of spinules anteriorly on ventral surface and patch of spinules at each side; (3) basis and distal segment of P5 elongated in female; (4) distal segment tapering with short spine implanted mid-laterally and coarsely serrated spine incorporated to segment distally in female P5; and (5) male right P5 distal segment elongated and curved, bifid with short inner branch.

2000, S. morii
The male fifth legs are diagnostic to distinguish Stephos goejinensis from other congeners in this group. Thus, S. angulatus is easily differentiated from the new species by the more developed inner branch of thebifid distal segment of the right P5, and by the distal segment of the left male P5 with only three elongate hyaline lamellae and a rounded cluster of short spinules (see Bradford-Grieve 1999). In S. marsalensis, the distal segment of right male P5 is not bifid whereas there are only 5 lamellate hyaline processes on the distal segment of left male P5 (see Costanzo et al. 2000).
Stephos morii differs from the new species in having the right P5 pseudochelate with a large inner branch on the distal segment,and the left leg carrying about 5 lamellate processes on the distal segment, which is produced into a long spinous process about 1.6 times longer than the segment (see Greenwood 1978 as S. tropicus). In S. pacificus, the distal segment of the right leg is not bifid and is bordered by a narrow lamella, whereas the left leg carries three terminal and two subterminal lamellate processes on the distal segment (see Ohtsuka and Hiromi 1987).
Stephos rustadi is easily separated from the new species by having the segment 3 of the right leg slightly shorter than segment 4, which terminates in a finely serrated claw-like structure, whereas the left leg carries two strong hook-like processes on the terminal segment, the larger one bifid (see Strömgren 1969).
Stephos pentacanthos and S. tsuyazakiensis share with the new species the same ornamentation on the male P5, but the new species has 7 unequal long and 13 short hyaline lamellae on the distal segment of left leg and a bifid distal segment with a short inner branch on right leg (Chen and Zhang 1965;Tanaka 1966).
Finally, Stephos vivesi can be differentiated from S. goejinensis based on the male right fifth leg distal segment, which is spatulate and displays two rounded outgrowths proximally on the anterior surface (vs. segment not spatulate, slender and bifid in S. goejinensis) (see Jaume et al. 2008).
Etymology. The specific name, projectus, is derived from the dorsolateral spiniform projections present on the female genital double-somite.
Antenna (Fig. 6B) similar to preceeding species except for presence of two transverse rows of spinules (instead of tiny serrated process plus spinule) on lateral margin of distal segment of endopod.
Mandible (Fig. 6C) similar to preceding species except for: (1) coxal gnathobase with straight row of moderately incised teeth; (2) outer margin of proximal segment of exopod with row of setules; and (3) distal segment of endopod with transverse row of spinules.
Maxillule (Fig. 6D) and maxilla (Fig. 6E) similar to preceding species except for presence of one additional seta on basal endite of maxilla.
Maxilliped (Fig. 6F) differing from S. geojinensis in presence of additional rows of tiny spinules on syncoxa.
P1 to P4 (Fig. 7A-D) with armature formula as in preceding species but with outer spine on second exopodal segment of P1 transformed into seta.
Fifth legs (Fig. 7E) symmetrical, uniramous, 3-segmented with proximal segment fused to intercoxal sclerite; second segment similar to S. geojinensis but shorter (1.72 times longer than wide; 31 × 18 μm); distal segment with a seta instead of spine on lateral margin, and with spinulation on terminal spine not so coarse.
Male. Body (Fig. 8A, B) robust, length 0.93 mm (mean 0.91±0.05, n=4) and similar to female in all major features except for last pedigerous somite, urosomal segmentation, armature of antennules and morphology of fifth legs. Fourth and fifth pedigerous somites, incompletely fused, latter asymmetrical with lateral lobe on left margin. Rostrum as in female. Prosome-urosome ratio 1.87:1. Urosome 5-segmented, comprising genital somite, three abdominal somites and anal somite; length ratio of urosomites as 23.7: 25.6: 22.6: 18.6: 9.5 = 100. Genital somite asymmetrical, with protruding lobe on left side and patch of tiny spinules proximally at each side. Abdominal somites with transverse hyaline frill both dorsally and ventrally. Anal somite shortest. Caudal rami similar to those of female.
Antennules (Fig. 8C) similar to preceding species except for not extending beyond distal margin of fifth pedigerous somite and for failure to express an aesthetasc on ancestral segments VI, VIII, composite X-XI, XX and XIII. In addition, the composite segment I-II displays 4 setae (vs. 3 in S. geojinensis).
Antenna, mandible, maxillule, maxilla, maxilliped and P1 to P4 similar to female. Fifth legs (Fig. 8D, E) strongly asymmetrical, uniramous and filiform. Left leg 5-segmented; segments 3 and 4 each with rounded outgrowth on medial margin, outgrowth on segment 3 more slender and crowned with hyaline frill, that on segment 4 with ridged plate terminally; fourth segment with additional short conical outgrowth and longitudinal row of spinules; distal segment short, rounded, with 4 long hyaline lamellae on distal margin and patch of short spinules on posterior surface. Right leg 4-segmented; segment 3 elongate, straight and slender except for blunt triangular process proximally on lateral margin; fourth segment sickle-shaped with rounded tip; 4 rounded outgrowths along inner margin and single outgrowth subterminally on outer margin of segment.
Stephos projectus sp. n. differs from S. hastatus in the following features: (1) presence of dorsolateral process at each side on the female genital double-somite (vs. processes absent, in S. hastatus); (2) fourth segment of male left P5 without strong spine (vs. strong spine present in S. hastatus); (3) fifth segment of male left P5 with 4 unequal long hyaline lamellae on distal margin (vs. two hyaline lamellae in S. hastatus); and (4) distal segment of male right P5 sickle-shaped (vs. segment bifid in S. hastatus). The new species can be easily differentiated from S. robustus based on the following features: (1) the presence of a dorsolateral pointed process at each side of the genital double-somite in female (vs. presence of a small mid-dorsal rounded process and not dorsolateral   The second new species, S. projectus sp. n. was collected in the stagnant water flooding the burrows excavated byocypodid crabs in two intertidal mud flats. Cases of calanoid copepods associated with invertebrates have rarely been reported (Fosshagen 1970;Humes and Smith 1974;Moon and Soh 2014), whereas two epibenthic calanoid genera, Placocalanus Fosshagen, 1970 andBoholina Fosshagen, 1989, are known to burrow into the sediment temporally (Fosshagen 1970;Ohtsuka et al. 1996;Moon and Soh 2014).