Stephos jejuensis, Moon & Soh & Cho, 2020

Moon, Seong Yong, Soh, Ho Young & Cho, Dae Hyun, 2020, Three new species of the genus Stephos Scott, 1892 (Crustacea, Copepoda, Calanoida, Stephidae) from Jeju Island, Korea, ZooKeys 944, pp. 1-30 : 1

Stephos jejuensis sp. nov. Figures 2 , 3 , 4 , 5

Material examined.

Holotype ♀ (NIBRIV0000840220), allotype ♂ (NIBRIV0000840219) undissected in 70% ethanol, 11 November 2012. Dissected paratypes ♀ (NIBRIV0000840221), ♂ (NIBRIV0000840222) mounted on two glass slides, 11 November 2011. All specimens collected by D. H. Cho.

Type locality.

Near the bottom (ca. 5 m depth), Pyoseon port, Jeju Island (33°19'32"N, 126°50'42"E), 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 ), 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.

Antennule (Fig. 3A ) symmetric, extending beyond the distal area of genital double-somite; 24-segmented, apparently ancestral. Segments I-II, III-IV, X-XI, and XXVII-XXVIII are fused. Segmentation and setation pattern as follows (ancestral segment number-setae+aesthetasc): I-II-3+2ae, III-IV-4+3ae, V-2+ae, VI-2, VII-2+ae, VIII2+ae, IX-2, X-XI-4+ae, XII-1, XIII-1, XIV-2+ae, XV-1, XVI-2+ae, XVII-1, XVIII-1, XIX-1, XX-1, XXI-1+ae, XXII-1, XXIII-1+ae, XXIV-1+1, XXV-1+1, XXVI-1+1, XXVII-XXVIII-5+ae. Ancestral segments I to XIV and XVI to XXV with a row of spinules on the posterior surface.

Antenna (Fig. 3B ) biramous; coxa and basis separate, coxa with one and basis with two setae; endopod 2-segmented, proximal segment with two setae, compound distal segment bilobed with eight and seven plumose setae subterminally and terminally, respectively, outer margin ornamented with small serrated process subdistally on the medial margin; tiny spinule adjacent to serrated process; exopod 7-segmented, with intersegmental articulation between segments 2 and 3 not completely expressed, with setal formula of 1, 3, 1, 1, 1, 1, 3.

Mandible (Fig. 3C ): well-developed coxal gnathobase, with a straight row of moderately incised teeth. Mandibular palp biramous; basis with four setae on the inner margin. Exopod 5-segmented, with setal formula of 1, 1, 1, 1, 2; endopod 2-segmented, proximal with four setae and distal segments with ten setae.

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.

Maxilla (Fig. 3E ): apparently 6-segmented, comprising coalesced praecoxa and coxa, allobasis and 3-segmented endopod. Armature of praecoxal and coxal endites 5,3,3,3. Basal endite with four setae, one stouter than the rest; endopodal endite with one seta on tip. Free endopod setal formula 1, 1, 3 respectively. Integument of praecoxa ornamented with patch of spinules on the posterior margin. Praecoxal and coxal endites with cluster of long spinules subdistally on the lateral surface; distal coxal endite with additional row of spinules proximally on the medial surface.

Maxilliped (Fig. 4A ): syncoxa robust, with setal formula 1, 2, 2, 3 and oblique row of tiny spinules on the posterior distal part; basis with three setae and a row of setules on the mediolateral margin; endopod 6-segmented, with setal formula 2, 4, 4, 3, 3+1, 4.

Legs 1-4 (Fig. 4B-E ) progressively larger toward the posterior, each comprising coxa, basis, and 3-segmented exopod; endopod of leg 1 (Fig. 4B ) 1-segmented, that of leg 2 (Fig. 4C ) 2-segmented; endopods of P3 (Fig. 4D ) and leg 4 (Fig. 4E ) 3-segmented. Armature formula of legs 1-4 as follows (Roman numerals indicate spines, Arabic numeral indicates setae):

Leg 1 (Fig. 4B ) biramous, with long curved inner setae on the basis, and endopod with lobe on the outer margin, bearing a minute spinous process and a row of minute spinules on the anterior surface.

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.

Description of male.

Body (Fig. 5A, B ) robust, length 0.93 mm. Prosome 5-segmented; cephalosome and first pedigerous somites completely separated; fourth and fifth pedigerous somites incompletely fused (Fig. 5A ). Rostrum same as in female. Prosome-urosome ratio 2.18:1. Urosome 5-segmented, comprising genital somite, three free abdominal somites, and anal somite; length ratio of genital somite, first to fourth free abdominal somites, and anal somite as 27.5: 19.2: 16.3: 14.5: 10.9: 11.5 = 100. Genital somite with asymmetric and protruding lobe on the left side. First to third abdominal somites with transverse hyaline frill dorsally and ventrally. Anal somite shortest. Caudal rami similar to those of the female.

Antennule (Fig. 5C ) symmetric, extending beyond the distal area of the genital double-somite; 24-segmented, apparently ancestral; segments I-II, III-IV, X-XI, and XXVII-XXVIII are fused. Segmentation and setation pattern as follows (ancestral segment number-setae+aesthetasc): I-II-3+2ea, III-IV-4+3ae, V-2+ae, VI-2, VII-2+ae, VIII2+ae, IX-2, X-XI-4+ae, XII-1, XIII-1, XIV-2+ae, XV-1, XVI-2+ae, XVII-1, XVIII-1, XIX-1, XX-1, XXI-1+ae, XXII-1, XXIII-1+ae, XXIV-1+1, XXV-1+1, XXVI-1+1, XXVII-XXVIII-5+ae. Ancestral segments I-XIV and XVI-XXV with row of spinules on the posterior surface.

Antenna, mandible, maxillule, maxilla, maxilliped and legs 1-4 similar to those of the female.

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 ).

Figures

Figure 2. 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.

Figure 3. Stephos jejuensis sp. nov. Female paratype A antennule B antenna C mandible D maxillule E maxilla. Scale bars in µm.

Figure 4. Stephos jejuensis sp. nov. Female paratype A maxilliped B leg 1 C leg 2 D leg 3 E Leg 4 F Leg 5. Scale bars in µm.

Figure 5. Stephos jejuensis sp. nov. Male paratype A habitus, dorsal view B habitus, lateral view C antennule D-F leg 5. Scale bars in µm.