Parartotrogus arcticus

Kim, Il-Hoi, 2016, Siphonostomatoid copepods (Crustacea) mainly associated with marine invertebrates from Korean waters, Journal of Species Research 5 (3), pp. 393-442 : 437-440

Parartotrogus arcticus T. and A. Scott, 1901

( Figs. 30 , 31 )

Parartotrogus richardi var. arcticus T. & A. Scott, 1901, p. 352, pl. 6.

Parartotrogus arcticus : G.O. Sars, 1915, p. 136, pl. 79.

Material examined. One $ from washings of more than 200 individuals of the ophiuroid Ophiopholis aculeata (Linnaeus) caught as a fisheries bycatch, in the depth of about 100 m, off Sacheon Port (37°50′14″N, 128°52′32″ E) in Gangneung, 20 August 2010.

Female. Body ( Fig. 30A ) orange in color when alive and 935 μm long, with broad prosome and small urosome. Prosome consisting of cephalothorax and 3 metasomites (second to fourth pedigerous somites). Cepahothorax 473 × 435 μm, much expanded laterally in anterior 2/3 and gradually narrowed posteriorly in posterior 1/3, with large, broad, apically truncated rostral prominence. Second and third pedigerous somites 77 × 257 and 46 × 200 μm, respectively. Fourth pedigerous somite much small­ er than preceding somites, 20 × 96 μm. Urosome ( Fig. 30B ) 5­segmented. Fifth pedigerous somite short and narrower than genital double­somite. Genital double­somite 112 × 113 μm, angularly expanded laterally in anterior part, widest across anterior 1/3, and gradually narrowed in posterior 2/3; genital aperture located dorsolaterally. Three abdominal somites 40 × 62, 32 × 62, and 46× 63 μm, respectively. Caudal ramus ( Fig. 30C ) 52 × 22 μm (length/width ratio 2.27: 1) and covered by hairy setules, with 6 setae; dorsodistal seta naked, other setae plumose.

Rostrum as large anterior prominence of cephalothorax. Antennule ( Fig. 30D ) 263 μm long and 9-segment­ ed; armature formula 1, 12, 4, 2, 2, 3, 2, 2, 12 + 2 aesthetascs; first 2 segments broader than other segments; first segment with setules on anterior margin; one of 2 aesthetascs on terminal segment large, but remaining one small, originated from proximal part of a distal seta; all setae naked. Antenna ( Fig. 30E ) 4­segmented; coxa (first segment) short and unarmed; basis (second segment) also unarmed. Exopod ( Fig. 30F ) 1­segmented, slender, about 42 × 8 μm, with 1 apical and 1 subapical setae and about 10 large setules. Endopod 2­segmented; proximal segment more than twice as long as wide and unarmed; distal segment slightly longer than wide, distally with massive hook and small claw, and with inner subdistal, claw­like spine and 4 minute distal setae( Fig. 30G, H ).

Oral cone short and directed ventrally. Mandible ( Fig. 30I ) tapering, with several denticles distally. Maxillule ( Fig. 30J ) bilobed; inner lobe 35 × 32 μm, gradually broadened distally, with 5 pinnate seta, 2 of which distinctly longer than other 3; outer lobe much smaller than inner lobe, 22 × 11 μm, gradually broadened distally, with 3 distal, plumose setae. Maxilla ( Fig. 31A ) 2­segmented; syncoxa unarmed and widest at proximal third; basis much narrower but longer than syncoxa, distally recurved, with 1 spiniform seta at place slightly distal to midlength and minute spinules subdistally. Maxilliped ( Fig. 31B ) 4­segmented; syncoxa narrow, with 1 inner distal seta; basis 128 × 49 μm, widest in middle, with 1 seta near middle of inner margin. Endopod 2­segment­ ed; first segment unarmed; second segment 51 μm long, with 1 seta distally; terminal claw 88 μm long, weakly curved, with several minute spinules on concave margin.

Leg 1 ( Fig. 31C ) with 2­segmented rami. Leg 2 ( Fig. 31D ) and leg 3 with 3­segmented rami. Third endopodal segment of leg 2 with expanded distal processes ( Fig. 31D ), but that of leg 3 with slender, pointed distal processes ( Fig. 31E ). Leg 4 ( Fig. 31F ) uniramous, consisting of protopod and 1­segmented exopod; protopod with 1 naked, outer distal seta; exopod 17 × 9 μm, with 2 distal spines of 34 (inner) and 27 μm (outer), respectively. Armature formula of legs 1­3 as follows:

Coxa Basis Exopod Endopod Leg 1: 0­0 1­0 I­0; III, 1, 4 0­1; 1, 2, 4

Leg 2: 0­0 1­0 I­1; I­1; III, I, 4 0­1; 0­2; 1, 2, 3 Leg 3: 0­0 1­0 I­1; I­1; III, I, 4 0­1; 0­1; 1, I, 2

Leg 5 ( Fig. 31G ) consisting of 1 dorsolateral pinnate seta on fifth pedigerous somite and free exopod; exopod wider than long, 15 × 20 μm, with 2 distal pinnate setae and 2 inner spiniform setae. Leg 6 represented by 1 small seta in genital aperture ( Fig. 30B ).

Male. Unknown.

Remarks. In the body form Parartotrogus arcticus is very similar to the sole conger P. richardi T. and A. Scott, 1893. Nevertheless, they can easily be distinguished from each other by the following differences: (1) the body length in the female of P. arcticus is 0.8 mm (G.O. Sars, 1915) or 0.94 mm (this study), whereas that of P. richardi is 0.5 mm (T. and A. Scott, 1893) or 0.47­0.52 mm ( Giesbrecht, 1899); (2) the second endopodal segment of leg 1 carries 7 setae in P. arcticus , but 5 setae in P. richardi ; (3) the third exopodal segment of leg 3 carries 3 outer spines (formula III, I, 4) in P. arcticus , but 2 outer spines (formula II, I, 4) in P. richardi ; (4) the endopod of leg 3 is 3­segmented, with armature formula 0­1; 0­1; 1, I, 2 in P. arcticus , whereas it is 2­segmented, with armature formula 0­1; 0, 1, 3 in P. richardi .

The hosts of the genus Parartotrogus have not been known until now. The present finding of P. arcticus as an associate of an ophiuroid in the Sea of Japan suggests that copepods of Parartotrogus are associates of ophiuroid echinoderms, as other genera in the Cancerillidae . Because P. arcticus and its host Ophiopholis aculeata (Linnaeus) are the circum­arctic species, it is presumable that P. arcticus had migrated along with the ophiuroid host from the North Pacific to the Atlantic across the Arctic Ocean during the geological time, as in most of circum­arctic species of marine invertebrates ( Briggs, 1995; Vermeij and Roopnarine, 2008). Similar examples of trans­Arctic migrations of copepods associated with echinoderms are Asterocheres lilljeborgi Boeck, 1859 which is redescribed in the present paper and Scottomyzon gibberum (T. and A. Scott, 1894) (see Kim, 1992).

Figures

Fig. 30. Parartotrogus arcticus T. and A. Scott, female.A, habitus, dorsal; B, urosome, dorsal; C, left caudal ramus, dorsal; D, antennule; E, antenna; F, exopod of antenna; G, distal part of antennary endopod; H, same, opposite side; I, mandible; J, maxillule. Scale bars:A, 0.1mm; B, D, E, 0.05 mm; C, F­J, 0.02mm.

Fig. 31. Parartotrogus arcticus T. and A. Scott, female. A, maxilla; B, maxilliped; C, leg 1; D, leg 2; E, leg 3 endopod; F, leg 4; G, leg 5. Scale bars:A­E, 0.05mm; F, G, 0.02mm.