Rhodinicola similis, Kim & Sikorski & O’Reilly & Boxshall, 2013

Kim, Il-Hoi, Sikorski, Andrey, O’Reilly, Myles & Boxshall, Geoff A., 2013, Copepods associated with polychaete worms in European seas, Zootaxa 3651 (1), pp. 1-62 : 26-32

Rhodinicola similis n. sp.

Type material: Holotype ♀ (a damaged specimen, with burst prosome and broken abdomen, dissected and mounted on a glass slide), from Rhodine gracilior (Tauber, 1879) ; Loch Creran , Scotland, Rubha Garbh (56 o 31.59’N, 05 o 21.25’W), depth 15 m, collected by M.O’Reilly, 11 January 1995; BMNH Reg. No. 2012.1383.

Allotype ♂ (allotype, dissected and mounted on a glass slide) from R. gracilior ; Irvine Bay, Scotland, Stn H (55 o 35.92’N, 04 o 47.40’W), depth 38 m; collected by M.O’Reilly, 08 September 1989; BMNH Reg. No. 2012.1384. Juvenile from R. gracilior ; Irvine Bay, Scotland, Stn H (55 o 35.92’N, 04 o 47.40’W), depth 38 m; collected by M.O’Reilly, 08 September 1989; BMNH Reg. No. 2012.1385.

Female. Body similar to that of preceding species in shape and size; slender, elongate and composed of cephalothorax, unsegmented trunk and 4-segmented abdomen. Cephalothorax ( Fig. 13A ) 508 µm long, consisting of narrower cephalic region (538 µm wide) and laterally expanded thoracic trunk region (608 µm wide). Abdomen clearly 4-segmented, without ornamentation on ventral surface. Caudal rami directed posteriorly ( Fig. 13B ); each ramus 119×56 µm (ratio 2.13:1), with 7 setae, including small outer proximal seta (seta I); outer lateral seta (seta II) positioned in middle of outer margin. Egg sac 1.10× 0.29 mm; containing of 3 rows of eggs; each egg about 190 µm in diameter.

Rostrum absent. Antennule ( Fig. 13C ) 242 µm long and 6-segmented, with indistinct suture line between second and third segments; armature formula 4, 5, 4, 3+aesthetasc, 2+aesthetasc, and 7+aesthetasc; first segment with large anteroventral patch of spinules; distalmost of 4 setae on first segment plumose; aesthetascs on 3 distal segments distally pointed, setiform. Antenna ( Fig. 13D ) 4-segmented; first segment with 1 distal pinnate seta; second segment with 1 small subdistal seta; third segment with 1 claw and 2 small setae distally and ornamented with longitudinal row of 8 foliaceous scales; terminal segment about twice as long as wide, bearing 4 setiform claws and 3 setae of different lengths, longest seta spinulate.

Labrum ( Fig. 13E ) with roundly convex posterior margin and ornamented with patches of minute spinules. Mandible ( Fig. 13F ) terminating in 1 distal cusp bearing bifurcate or trifurcate apex and carrying 1 large plumose seta subdistally. Maxillule ( Fig. 13G ) with 1 minute lateral and 5 (3 large and 2 small) distal setae. Maxilla ( Fig. 13H ) 2-segmented; proximal segment unarmed; distal segment with slightly recurved and pointed apex, armed with 1 small tubercle-like seta and 1 dentiform process on concave margin and spinulose pad on slightly curved distal margin. Maxilliped ( Fig. 13I ) 3-segmented; first segment with 1 distal seta on medial margin; second segment with 1 seta in distal quarter of medial margin and 1 small patch of spinules near base of seta; third segment tapering into spiniform distal process with 2 setae and 1 small spine; 2 setae on third segment extending to tip of distal process.

Legs 1–4 ( Fig. 14A–D ) biramous with 3-segmented rami; basis of leg 1 ornamented with patches of spinules. Inner setae on coxae of legs 1–3 small and naked, inner seta on leg 4 pinnate; outer setae on basis large and naked. Armature formula of legs 1–4 as follows:

Leg 1: coxa 0-1; basis 1-0; exp. I-0; I-1; II, I, 5; enp. 0-1; 0-1; I,4

Leg 2: coxa 0-1; basis 1-0; exp. I-0; I-1; I, I, 5; enp. 0-1; 0-2; I, II, 3

Leg 3: coxa 0-1; basis 1-0; exp. I-0; I-1; I, 7; enp. 0-1; 0-2; I, 3

Leg 4: coxa 0-1; basis 1-0; exp. I-0; I-1; I, 7; enp. 0-1; 0-2; I, 2

Leg 5 ( Fig. 14E ) 2-segmented; proximal segment fused with somite, with 1 outer seta; free distal segment (exopod) 69×30 µm (ratio 2.30:1), with 1 outer and 3 distal setae, innermost smallest. Leg 6 not seen.

Male. Body ( Fig. 15A ) narrow with clear articulations between somites. Body length 1.37 mm. Prosome 4- segmented, consisting of cephalothorax and 3-segmented metasome. Cephalothorax 423×369 µm. Second to fourth pedigerous somites 119×365, 104×335, and 92×273 µm, respectively. Urosome 6-segmented. Fifth pedigerous somite 73×227 µm. Genital somite 135×242 µm, much wider than long. Abdomen 4-segmented, 115×192, 96×177, 77×154, and 92×123 µm, respectively, from anterior to posterior. Caudal ramus 71×37 µm (ratio 1.92:1), with 6 setae.

Rostrum ( Fig. 15B ) present as weak anterior protuberance of cephalothorax. Antennule ( Fig. 15C ) 171 µm long and 6-segmented, with armature formula 5, 7, 7, 4+aesthetasc, 2+aesthetasc, and 7+aesthetasc; aesthetascs on 3 distal segments enlarged, with pointed tip. Antenna ( Fig. 15D ) similar to that of female, but third segment with row of 10 foliaceous scales, one seta on this segment transformed to spinule-bearing foliaceous element.

Labrum, mandible and maxillule as in female. Maxilla ( Fig. 15E ) also as in female, but proximal seta on concave margin of distal segment more distinct than that of female. Maxilliped ( Fig. 15F ) 4-segmented; first segment with 1 seta on medial margin; second segment with serrated protuberance and 1 seta in middle of medial margin; small third segment unarmed; terminal segment forming large claw bearing tapering tip, and bearing 1 minute seta proximally plus 2 large naked setae.

Legs 1–4 ( Fig. 16A–D ) biramous with 3-segmented rami, but segmentation incomplete between second and third endopodal segments of leg 1. Inner seta on coxa of legs 1–4 well-developed and pinnate. Armature formula as follows:

Leg 1: coxa 0-1; basis 1-0; exp. I-0; I-1; II, I, 5; enp. 0-1; 0-1; I,4

Leg 2: coxa 0-1; basis 1-0; exp. I-0; I-1; III, I, 5; enp. 0-1; 0-2; I, II, 3

Leg 3: coxa 0-1; basis 1-0; exp. I-0; I-1; I+1, I, 5; enp. 0-1; 0-2; I, 3

Leg 4: coxa 0-1; basis 1-0; exp. I-0; I-1; II, I, 5; enp. 0-1; 0-2; 2

Leg 5 ( Fig. 15G ) 2-segmented; proximal segment incompletely demarcated from somite; distal segment 60×28 µm (ratio 2.14:1), carrying 4 naked setae. Leg 6 ( Fig. 16E, F ) represented by 2 or 1 (abnormal?) setae on posterior corner of genital operculum.

Etymology. The specific name alludes to the close similarity between the new species and R. tenuis n. sp. described above.

Remarks. The male is identified as conspecific with the holotype female on the basis of the following evidence: 1) the labrum, mandible and maxillule are the same in both sexes; 2) both have the unusual number of seven setation elements, rather than six, on the terminal segment of the antenna; 3) both have similar ornamentation on the claws (spinulate distally) on the distal segments of the antenna; 4) both have similar proportional lengths of the setae on the exopod of leg 5; and 5) the aesthetascs on the three distal segments of the antennules taper to a pointed tip.

Rhodinicola similis n. sp. most closely resembles R. tenuis n. sp. in sharing the similar shape of the cephalothorax, labrum, maxilla and maxilliped, the completely 3-segmented legs 1–4 with well-developed setae, and in having caudal rami of similar size and shape. However, they differ in numerous details including: 1) the antennule of R. similis n. sp. has a patch of minute spinules on the first segment which is absent in R. tenuis n. sp.; 2) the second and third segments of the antennule of R. similis n. sp. have 5 and 4 setae, respectively, in the female compared to 9 and 5 setae, respectively, in R. tenuis n. sp.; 3) the spiniform distal element of the mandible of R. similis n. sp. is articulated at the base whereas it is not articulated in R. tenuis n. sp.; 4) the maxillule of R. similis n. sp. is armed with 6 setae compared to 5 setae in R. tenuis n. sp.); 5) the third endopodal segment of female leg 1 of R. similis n sp. is armed with 1 spine and 4 setae (formula I, 4) instead of 2 spines and 3 setae (formula I, I, 3) as in R. tenuis n. sp.; 6) the armature formula of the third exopodal segment of leg 2 of R. similis n. sp. is III I, 5 (9 in total), instead of II, II, 4 (or II, I, 5) (8 in total), as in R. tenuis n. sp.; and 7) the exopod of leg 5 of R. similis n. sp. is armed with 4 setae compared to only 3 setae in R. tenuis n. sp.

Figures

FIGURE 13. Rhodinicola similis n. sp., female. A, cephalothorax, dorsal; B, caudal rami, dorsal; C, antennule, ventral; D, antenna; E, labrum, ventral; F, mandible; G, maxillule; H, maxilla; I, maxilliped. Scale bars: A, 0.2 mm; B, 0.1 mm; C–F, H, I, 0.05 mm; G, 0.02 mm.

FIGURE 14. Rhodinicola similis n. sp., female. A, leg 1; B, leg 2; C, leg 3; D, leg 4; E, leg 5. Scale bars: A–D, 0.05 mm; E, 0.02 mm.

FIGURE 15. Rhodinicola similis n. sp., male. A, habitus, dorsal; B, rostrum, dorsal; C, antennule, ventral; D, antenna; E, maxilla; F, maxilliped; G, leg 5. Scale bars: A, 0.2 mm; B, D, E, 0.02 mm; C, F, G, 0.05 mm.

FIGURE 16. Rhodinicola similis n. sp., male. A, leg 1; B, leg 2; C, leg 3; D, leg 4; E, F, leg 6, dorsal. All scale bars: 0.05 mm.