Arctopontius minutus, Kim, 2016

Kim, Il-Hoi, 2016, Siphonostomatoid copepods (Crustacea) mainly associated with marine invertebrates from Korean waters, Journal of Species Research 5 (3), pp. 393-442 : 394-397

Arctopontius minutus n. sp. ( Figs. 1 , 2 )

Material examined. Two$$ from washings of a colony of bryozoan Heteropora sp. , off Sacheon Port (37° 50′14″N, 128°52′32″E) in Gangneung, 12 August 2011. Holotype ($, dissected and mounted on a glass slide, NIBRIV0000680686 ) has been deposited in NIBR, Incheon, Korea. Paratype ($, dissected and mounted on a glass slide) is retained in the collection of the author .

Female. Body ( Fig. 1A ) flattened, 1.58 mm long. Prosome 4­segmented and 1.10 mm long. Tergites of somites ornamented with minute setules (or sensillae) and pores ( Fig. 1D ). Cepahlothorax large, 848 × 976 μm, laterally expanded, wider than long, with roundly pronounced rostral area, and blunt, only slightly posteriorly produced posterolateral corners, but without keel or crest on dorsal midline. Three metasomites much narrower than cephalothorax. Posterolateral corners of second and fourth pedigerous somites angular, but that of third pedigerous somite blunt. Urosome ( Fig. 1B ) 5­segmented. Fifth pedigerous somite 195 μm wide, tapering laterally. Genital double­somite 175 × 295 μm, consisting of laterally greatly expanded anterior part and narrower posteri­ or part (152 μm wide across this narrower region). Three abdominal somites 68 × 117, 57 × 107, and 80 × 100 μm, respectively. Anal somite with parallel lateral margins. Caudal ramus ( Fig. 1C ) almost rectangular, 85 × 46 μm (length/width ratio 1.85: 1), armed with 6 setae including 2 dorsal setae; outer distal and inner distal setae plumose, other setae naked; inner margin with several setules proximally.

Rostrum ( Fig. 1E ) small, tapering and highly sclerotized, with nipple­shaped posterior apex. Antennule ( Fig. 1F ) 8­segmented, 360 μm long; second segment longest and first segment second longest; armature formula 1, 14, 8, 2, 2, 2, 2, and 13 + aesthetasc; one of distal setae on terminal segment inserted on proximal part of longest terminal seta; aesthetasc on terminal segment 182 μm long, half as long as antennule. Antenna ( Fig. 1G ) consisting of syncoxa, basis, 1­segmented small exopod and 2­segmented endopod; syncoxa unarmed; basis longest among segments and unarmed; exopod 1­segmented, longer than wide, gradually broadened distally, with 2 proximally annulated, distal setae; first endopodal segment slightly longer than wide, unarmed; second endopodal segment about 2.2 times as long as wide and armed with 1 spiniform seta laterally, 2 spiniform setae and 1 long spine distally and ornamented with patches of setules in distal and lateral regions.

Oral siphon ( Fig. 1H ) 606 μm long, extending beyond base of maxilliped, distally with paired membranous extensions bearing marginal setules. Mandible represented by thin, thread­like stylet. Maxillule ( Fig. 1I ) bilobed; inner lobe 132 μm long, setulose along inner margin, and tipped with 2 unequal setae, one of them spiniform; out­ er lobe 71 μm long, setulose on distal part of outer margin, and tipped with 2 heavily spinulose, spiniform setae and 1 small seta. Maxilla ( Fig. 2A ) 2­segmented; syncoxa (proximal segment) unarmed; basis (distal segment) slightly curved, as long as syncoxa, with terminal claw, 1 small, stout spine at distal 1/5 and several spinules distally; terminal claw distinctly articulated from basis, with row of minute spinules laterally. Maxilliped ( Fig. 2B ) consisting of 6 segments and terminal claw; syncoxa with 1 inner seta distally and tuft of setules at outer distal corner; basis with 1 seta at midway of inner margin, numerous minute spinules on inner surface distal to midway seta, and stiff setules on outer margin; first to fourth endopodal segments armed with 2, 0, 1, and 1 setae, respectively (second endopodal segment apparently unarm­ ed); terminal claw about 1.7 times as long as terminal endopodal segment, with minute spinules on concave inner margin.

Legs 1, 2 ( Fig. 2C, D ) and leg 3 with 3­segmented rami. Leg 4 ( Fig. 2E ) with 3­segmented exopod and 2­ segmented small endopod. Outer seta on basis of legs 1­3 plumose, that of leg 4 naked. Inner seta on coxa of leg 1 expanded proximally; inner distal setae on basis extending to middle of second endopodal segment; outer spines on exopod each tipped with setule. Inner distal part of leg 2 basis strongly projected. Outer distal corner of second endopodal segments of legs 1-3 bifid. Leg 3 with same armature formula as that of leg 2, but inner seta on coxa smaller and inner distal projection of basis less prominent. Leg 4 coxa with small, naked inner seta; endopod ( Fig. 2F ) half as long as first exopodal segment; first endopodal segment armed with 1 naked inner seta; second endopodal segment shorter than first segment and armed with plumose distal seta. Armature formula for legs 1­4 as follows:

Coxa Basis Exopod Endopod

Leg 1: 0­1 1­1 I­1; I­1; III, 1, 4 0­1; 0­2; 1, 2, 3 Legs 2 & 3: 0­1 1­0 I­1; I­1; III, I, 5 0­1; 0­2; 1, 1+I, 3 Leg 4: 0­1 1­0 I­1; I­1; III, I, 5 01; 1

Leg 5 ( Fig. 2G ) consisting of 1 ventrolateral seta on fifth pedigerous somite and small, free exopod; exopod with 2 larger distal and 1 small anterior setae. Leg 6 ( Fig. 2H ) represented by 1 short spiniform seta and 1 longer plumose seta on genital operculum.

Male. Unknown.

Etymology. The specific name is derived from the relatively small body of the new species ( minutus , the Latin, means “small”).

Remarks. The genus Arctopontius G.O. Sars, 1915 contains three known species: A. expansus G.O. Sars, 1915 from northern waters of Norway, A. hanseni Eiselt, 1986 from the Arctic Alaskan coast, and A. novenarius Johnsson in Johnsson and Rocha (2002) from the Antarctic Ocean. The type species A. expansus has 2 spines and 5 setae on the third exopodal segment of leg 1 (G.O. Sars, 1915), but other two species have 3 spines and 5 setae on the same segment. The most characteristic feature of this genus may be the 2­segmented endopod of leg 4. It is armed with 1 seta on the first and second segment each, except for A. novenarius in which the distal segment is armed with 5 setae. Arctopontius minutus n. sp. differs from A. expansus in having a different armature condition on the third exopodal segment of leg 1 and from A. novenarius in having only 1 seta on the second endopodal segment of leg 4. According to Eiselt (1986), in A. hanseni the body length of the female is 2.1­2.3 mm (vs. 1.58 mm in A. minutus n. sp.), the caudal rami are slightly longer than wide (vs. distinctly longer than wide, 1.85 times as long as wide), and the second endopodal segment of leg 3 in the female is armed with only 1 seta (vs. 2 setae). Therefore, A. minutus n. sp. cannot be confused with A. hanseni .

Figures

Fig. 1. Arctopontius minutus n. sp., female. A, habitus, dorsal; B, urosome, dosal; C, left caudal ramus, dorsal; D, tergal ornamentation of cephalothorax; E, rostral area, ventral; F, antennule; G, antenna; H, oral siphon; I, maxillule. Scale bars:A, 0.2 mm; B, E, H, 0.1 mm; C, D, F, G, I, 0.05 mm.

Fig. 2. Arctopontius minutus n. sp., female.A, maxilla; B, maxilliped; C, leg 1; D, leg 2; E, leg 4; F, leg 4 endopod; G, leg 5; H, left genital aperture, ventral. Scale bars:A­E, H, 0.05mm; F, G, 0.02mm.