Stephos concavus, 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 concavus sp. nov. Figures 6 , 7 , 8

Material examined.

Holotype ♀ (NIBRIV0000293109) dissected on two glass slides collected by D. H. Cho, 9 May 2012.

Type locality.

Near the bottom (ca. 4 m depth), Wimi port, Jeju Island (approximately 33°16'13"N, 126°39'43"E), Korea.

Description of female.

Body (Fig. 6A, B ) robust, length 0.93 mm. Prosome 5-segmented; cephalosome and first pedigerous somites completely separated; fourth and fifth pedigerous somites incompletely fused (Fig. 6A ), posterior corners of prosome slightly asymmetric. Rostrum represented by a rounded knob. Prosome-urosome ratio 2.25:1. 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 43.0: 18.9:18.1:9.4:10.7 = 100. Genital double-somite (Fig. 6C-E ) slightly asymmetric, with protruding lobe on the anterior to medial part of both sides and with a row of spinules in lateral view (Fig. 6C, D ); common operculum located ventromedially slightly round (Fig. 6E ) and with spermatophore and coupler in dorsal view (Fig. 6C ). First and second abdominal somites (Fig. 6C ), with transverse hyaline frill dorsally and ventrally. Anal somite shortest. Caudal rami with six setae, symmetric, 1.75 times longer than wide (49 × 28 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. 7A ) symmetric, extending beyond 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-2+ae, XXII-1, XXIII-1+ae, XXIV-1+1+ae, XXV-1+1, XXVI-1+1, XXVII-XXVIII-5+ae. Ancestral segments I-XIV and XVI-XXV with row of spinules on posterior surface.

Antenna (Fig. 7B ) 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 a small serrated process subdistally on 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. 7C ): well-developed coxal gnathobase, with a straight row of moderately incised teeth and patched spinules on the anterior and posterior corners. Mandibular palp biramous; basis with four setae on inner margin. Exopod 5-segmented, with setal formula of 1, 1, 1, 1, 2; endopod 2-segmented, proximal with 4 setae and distal segments with 10 setae.

Maxillule (Fig. 8A ): 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; with 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. 8B ): apparently 6-segmented, comprising coalesced praecoxa and coxa, allobasis, and 3-segmented endopod. Armature of praecoxal and coxal endites 5, 3, 3, 3, respectively. 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 a patch of spinules on the posterior margin. Praecoxal and coxal endites with a cluster of long spinules subdistally on the lateral surface; distal coxal endite with an additional row of spinules proximally on the medial surface.

Maxilliped (Fig. 8C ): syncoxa robust, with setal formula 1, 2, 2, 3 and an oblique row of tiny spinules on the anterior distal part; basis with three setae and patched setules on the mediolateral margin; endopod 6-segmented, with setal formula 2, 4, 4, 3, 3+1, 4.

Legs 1-4 (Fig. 8D-G ) progressively larger towards the posterior, each comprising coxa, basis, and 3-segmented exopod; endopod of leg 1 (Fig. 8D ) 1-segmented, that of leg 2 (Fig. 8E ) 2-segmented; endopods of leg 3 (Fig. 8F ) and P4 (Fig. 8G ) 3-segmented. Armature formula of legs 1-4 as follows in S. jejuensis sp. nov.

Leg 1 (Fig. 8D ) 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 dorsal surface.

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 ( Zagami et al. 2000), 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) in the following features of the female: the genital double-somite with protruding lobe on the anterior to medial part of both sides (vs. triangular lobe on the medial part of both sides in S. longipes ); the presence of a row of spinules on the left side of the genital double-somite (vs. absence in S. longipes ); the absence of a row of minute spinules on the dorsodistal surface of the genital double-somite (vs. presence in S. longipes ); the leg 5 distal segment is tapering and stout (vs. tapering and not stout in S. longipes ); and the teeth on the outer margin of both sides (vs. finely serrated fringe on the outer margin in S. longipes ).

Figures

Figure 6. Stephos concavus sp. nov. Female holotype A habitus, dorsal view B habitus, lateral view C urosome, dorsal view D urosome, lateral view E genital double-somite, ventral F leg 5. Scale bars in µm.

Figure 7. Stephos concavus sp. nov. Female holotype A antennule B antenna C mandible. Scale bars in µm.

Figure 8. Stephos concavus sp. nov. Female holotype A maxillule B maxilla C maxilliped D leg 1 E leg 2 F leg 3 G leg 4. Scale bars in µm.