Nannopus dimorphicus sp.nov.

(Figs. 9–15)

Type locality. Intertidal mudflat, Seocheon-gun, Chungcheongnam-do, South Korea, Yellow Sea, 36°01'45.2"N 126°39'56.0"E (Fig. 1A, C).

Type material. Holotype: 1 female adult in 70% ethanol (NIBRIV0000753984). Allotype: 1 male adult in 70% ethanol (NIBRIV0000810836). Paratypes: 2 females dissected on 5 and 2 slides (NIBRIV0000810833 – 34) 1 male dissected on 4 slides (NIBRIV0000810837); 2 females and 2 males on SEM stub (NIBRIV0000810835). All samples were collected by Vinod Vakati, 27 May 2015.

Etymology. The species epithet is derived from the Greek “ dimorphicus ” referring to the sexual dimorphism in shape of caudal seta V.

Description of female (based on holotype and paratypes). Body (Fig. 9A, B) fusiform and broad along posterior region of cephalothorax, total body length ranged from 400 to 420 µm (mean = 408 µm, n = 3; holotype: 400 µm, measured from tip of rostrum to posterior margin of caudal rami). Maximum width ranged from 132 to 136 µm (mean 134 µm, n = 3; holotype: 135 µm, measured at posterior margin of cephalothorax). Boundary between prosome and urosome clearly visible, urosome/prosome length ratio 0.62, and body length/width ratio about 3.1. Body ornamentation (Fig. 10A, B) as in N. minutus . Cephalothorax (Figs. 9A, 10A, B) anteriorly attenuated in dorsal view, 0.9 times as long as wide, comprising 38% of total body length, with several paired sensilla as figured, 1 row of sensilla along serrated posterior margin, and 1 medial integumental window close to posterior margin dorsally (arrowed in Fig. 10B).

Rostrum (Fig. 9C) as in N. minutus and with pair of sensilla anteriorly.

Prosome (Fig. 9A) 4-segmented, comprising cephalothorax and 3 subequal free pedigerous somites. P1- bearing somite fused to cephalothorax, posterior margin of free pedigerous somites serrated, and with row of sensilla as figured.

Urosome (Figs. 9A, 12A) tapering posteriorly, consisting of P5-bearing somite, genital double-somite, 2 abdominal somites, and anal somite. P5-bearing somite and first half of genital double-somite with serrated posterior margin dorsolaterally, and sensilla along posterior margin dorsally except for penultimate urosomite. Genital double-somite, following 2 urosomites with serrated posterior margin dorsally and ventrally, and with additional medial row of spinules ventrally.

Genital double-somite (Figs. 9A, 12A) almost 1.4 times as wide as long (ventral view), completely fused ventrally, with original segmentation indicated by serrated surface ridge dorsally, and copulatory pore located between both P6 with median depression.

Anal somite (Figs. 10A, 12A–C) almost as long as width with well-developed operculum, dense carpet of setules, pair of dorsal sensilla and pore, setules and denticles produced randomly, and 1 row of spinules on either side of anal segment ventrally.

Caudal rami (Figs. 10C, D, 12A–C) cylindrical, 1.8 times as long as wide in dorsal and ventral view, and ornamentation consisting of setules (Fig. 10C, D). Each ramus bearing 7 setae: seta I naked, shorter than ramus width, and located anterodorsally; seta II naked, as long as ramus width, and inserted anterodorsally midway along inner margin; seta III bipinnate, almost as long as ramus length, and anterolaterally midway along outer margin; seta IV bipinnate, 0.27 times as long as seta V, located at outer margin distally (arrowed in Fig 12A) but inserted at dorsal proximal area of seta V (visible in lateral view, arrowed in Fig. 12C), and with globular expansion at proximal region (arrowed in Fig. 12A, B); seta V strongest, bipinnate, located at distal inner margin, inflated, and extremely bulbous at proximal region; seta VI small, and naked; seta VII triarticulate, naked, and located midway along inner margin.

Antennule, antenna, and mouth parts as in N. minutus .

P1–P4 (Fig. 11A–E) with smooth and short concave intercoxal sclerite (not illustrated). Praecoxa somewhat triangular and shorter than coxa, and with 1 row of outer spinules (P1, P2 and P4). Coxa with 1 spinular row on anterior surface, and row of outer strong spinules (P1, P2, P3 and P4). Basis with 1 row of strong outer spinules near insertion of exopod, and 1 row of distal spinules near insertion of endopod (P1, P2, P3 and P4). Basal outer seta naked (P1 and P2) or bipinnate (P3 and P4), and P1 with inner strong pinnate spine. Exopod 3-segmented, all segments subequal in length, and with spinules and setules along outer and inner margins as figured. Endopod 2- segmented in P1, P2 and P3 or 1-segmented in P4. All setae on P1–P4 endopods pinnate. Armature formula as in N. minutus .

FIGURE 11. Nannopus dimorphicus sp. nov., line drawings, A–C & E–F, paratype ♂ 1; D, paratype ♂ 2: A, P1; B, P2; C, P3; D, left P3 endopod; E, P4; F, P5.

P5 (Fig. 11F) with separate baseoendopod and exopod. Baseoendopod transversely elongated, with 1 row of spinules along inner distal margin, and endopodal lobe with 2 pectinate and 2 pinnate setae. Exopod squarish, almost as long as width, and with 5 pinnate setae (innermost element longest and fused to segment).

P6 (Fig. 12A, D) with semi circular flap bearing 1 small pinnate distal seta.

Description of male (based on allotype and paratypes). Body (Fig. 13A) as in female, total body length ranged from 325 to 410 µm (mean = 377 µm, n = 2; allotype: 325 µm, measured from tip of rostrum to posterior margin of caudal rami). Maximum width ranged from 119 to 125 µm (mean 122 µm, n = 2; allotype: 120 µm, measured at posterior margin of cephalothorax). Urosome/prosome length ratio 0.5 in dorsal view and body length/ width ratio about 2.5 in dorsal view. Cephalothorax (Figs. 13A, 14B) 0.8 times as long as wide, comprising 29% of total body length, and with 1 integumental window as in female (arrowed in Fig. 14B). Body ornamentation (not illustrated), anal somite (Fig. 13A), rostrum (Fig.13A), and P1 (not illustrated) as in female. Antennule (not illustrated) as in male N. minutus . Antenna (not illustrated) and mouth parts (not illustrated) as in N. minutus .

Prosome 4-segmented (Fig. 13A), comprising cephalothorax (bearing first pedigerous somite), and 3 free somites. Posterior margin of prosomites serrated, and with sensilla as in female.

Urosome 6-segmented (Fig. 13A, B), comprising P5-bearing somite, genital somite, 3 free abdominal somites and anal somite. Posterior margin of urosomites serrated except for anal somite, and with sensilla as in female. P6- bearing somite, and urosomite 3 to 5 with longitudinal row of spinules along posterior margin ventrally.

Genital somite (Fig. 13A, D) homologous to anterior part of genital double-somite in female, and second and third urosomite well segmented in ventral view. Posterior margins of each somite serrated dorsally with few sensilla.

Caudal rami (Figs. 13A–D, 14C–D) almost as in female except for seta IV and V without modifications (normal type) at its base.

P2–P4 (Fig. 15A–D). P2 praecoxa without spinules. P4 basis without spinules near insertion of endopod, and with inner setules. Apical outer spine of exp- 3 in P2–P3 with inner setules, and 1 subdistal spinule on distal outer spine of exp- 3 in P3. Endopod 2-segmented in P2 and P3 or 1-segmented in P4. P2 enp-1 with distal spinules. P3 enp-2 with short inner pinnate seta (arrowed in Fig. 15B), and outer spine fused to enp-2 forming sharp apophysis (arrowed in Fig. 15C). P4 endopod with naked inner seta.

P5 (Fig. 13E) baseoendopod confluent with somite, and spinules along posterior margin. Endopodal lobe armed with 2 pectinate, and 2 naked setae. Exopod completely fused with baseoendopod, and with 5 elements (4 bipinnate, and 1 naked setae, and outer most one smallest).

P6 (Fig. 13F) asymmetrical, only right flap operational, and with 3 bipinnate setae (medial one longest).