Synurus ctenocheirus, Harris, 2014

Harris, Vernon A., 2014, Porcellidiidae of Australia (Harpacticoida, Copepoda). II. The Importance of the Male Antennule in Taxonomy, Records of the Australian Museum 66 (2), pp. 111-166 : 123-128

Synurus ctenocheirus sp. nov.

Figs 9–12 , Plate 1G, H

Type material. HOLOTYPE adult male, length 0.55 mm, P 81213 ; ALLOTYPE adult female, length 0.68 mm, P 81214 ; PARATYPE specimens, 6 ♀♀, 4 ♂♂, P 81215, deposited atAM, Sydney. Additional paratypes deposited at NHM, London. All washed from seagrass Zostera sp., collected at Green Island , Cairns, Queensland, Australia (16°41' S 145°56' E), V. A. Harris, 1973 .

Diagnosis. First (proximal) claw on P1 endopod ctenoid (comb-like) with recurrent teeth, second claw lamelliform; female caudal ramus fused to anal segment; male antenna with seta on basis (absent from females, Fig. 10A ); male P5 exopod with long lateral seta, terminal setae absent except for two small spine-like atrophied setae; female rostrum extremely broad (1/3 cephalosome width).

Biometric data. Females (N = 10): maximum length (Lmax) mean 0.68 mm, range 0.67–0.70 mm, body length (Lurs) mean 0.66 mm, range 0.65–0.68 mm; cephalosome width 0.44 mm, range 0.43–0.45 mm; rostrum 0.14 mm; genital double-somite width 0.18 mm, length 0.2 mm, arch 0.08 mm; caudal ramus length 0.078 mm, width 0.022 mm [length of caudal ramus from point of fusion with the anal segment (indicated by a slight notch, see Fig. 9 )].

Ratios: Lurs / W 1.5; W/R 3.0, cephalosome length 60% of Lmax; genital double-somite w/l 0.9, height of arch 40% of length; caudal ramus l/ w 3.8, 12% of Lurs, Hicks’ index for β 60%, distance between α and β setae 25% of ramus length.

Males (N = 10): maximum length (measured from anterior edge of shoulder Lmax) mean 0.55 mm, body length (from rostrum Lurs) mean 0.48 mm, range 0.47–0.50 mm; cephalosome width 0.37 mm; caudal ramus length 0.047 mm, width 0.028; antennule length (fully extended) 0.10 mm; spermatophore 0.09 × 0.02 mm.

Ratios: Lurs / W 1.3, Lmax / W 1.5; cephalosome length 67% of Lmax; caudal ramus l/ w 1.7; antennule 18% of body length (Lurs), antennule segments 3+4 57% and dactylus 18% of antennule length; spermatophore 18% of body length (Lurs).

Description. Adult females ( Fig. 9A ; Plate 1G, p. 163): colour lemon yellow. Body outline an ellipse (e = 0.814) strongly truncated anteriorly with slight bulge in midline. Cephalosome length greater than half maximum length of animal. Rostrum very wide (1/3 body width). Dorsal pits inconspicuous (1–1.5 µm), a few dorsal sensilla present. Hyaline border clear, 7 µm wide ( Fig. 9D ), surrounds cephalosome, ducts of marginal glands open dorsally. Genital double-somite cordate ( Fig. 9B ), length greater than width, no notch or scar to indicate boundary between anterior and posterior lobes, no anterolateral ridges (rugosities), border setules very small, posterior extremity pointed, terminated by four or five small setules. Posterior arch deep, nearly all of rami enclosed in arch. Caudal ramus long, narrow, rhomboid with parallel sides ( Fig. 9F ), fused to anal segment. Dorsal surface with very small dorsal pits (<1 µm) and parabolic ridge on each ramus that fits the edge of genital doublesomite arch. Seta T1 absent, γ seta at lateral corner, posterior border oblique, T4 at rounded apex, T2 and T3 very close mid-way along oblique edge (they tend to lie parallel to edge), terminal fringe of very fine setules between T3 and T4 (Fig.

Figure 11. Synurus ctenocheirus sp. nov. Female: (A) P3; (D) P4; (E) P5 (dorsal); (F) border setules of P5; (G) P2. Male: (B) P2; (C) P3 endopod. Scale bar: A, C, D, G = 0.14 mm. B = 0.1 mm. E = 0.17 mm.

9E). Structure and setation of mouthparts and ambulatory limbs typical of family except that segment 3 of P2, P3 and P4 exopod has only two spine-like external setae (Figs 11A, B, D, G). Antenna basis without seta (cf. male), exopod with five plumulose setae and one finely serrulate spine-like seta, geniculate setae on endopod with plain terminal segment, terminal claw with fine comb ( Fig. 10A ). Mandible ( Fig. 10B ), maxillule endopod with six setae ( Fig. 10E ), maxilla ( Fig. 10F ), coxae of maxillipeds touch in midline, basis with fimbriate process ( Fig. 10C ). First segment of P1 exopod with crescent of denticulate setules ( Fig. 10D , left side), first segment of endopod broad (l/w = 1.1), area of denticulate setules at lateral end of fimbriate crescent, first claw on terminal segment comb-like with recurved teeth, second claw lamelliform ( Fig. 10D ). Serrulate spinous seta on segment 3 of P2 endopod more than 1/2 length of endopod (0.6:1, Fig. 11G). Serrate spinous seta on segment 2 of P3 endopod 2/3 length of endopod (Fig. 11A), large serrate spinous seta on segment 3 longer than endopod (1.38:1). P4 (Fig. 11D) seta on endopod segment 2 and first internal seta of segment 3 plain spinous. P5 exopod ( Fig. 9C , 11E) broad, lanceolate with blunt apex, ventral falciform ridge terminates in a deep indentation or notch that appears to fit round the end of the caudal ramus on some specimens ( Fig. 9C ), medial edge of P5’s dorsal expansion overlaps genital double-somite extensively, ventral expansion absent, only one sub-terminal dorsal seta present, dorsal pits absent from P5, lateral border with double row of setules (ventral row of triangular setules and dorsal row of short filiform setules (Fig. 11F). Females carry four large eggs.

Adult males ( Fig. 12F ; Plate 1H, p. 163). Colour lemon yellow. Outline of cephalosome a truncated hemi-ellipse, deeply concave anteriorly with small medial bulge. Cephalosome unusually long (2/3 body length). Posterior region of body (metasome plus genital double-somite with P5s) semicircular. Epipleural lobes of third metasome segments very long, they wrap round P5s plus caudal rami and extend back as far as the posterior limit of the body ( Fig. 12F, H ). Rostrum not keeled ventrally, dorsal pits and hyaline border ( Fig. 12E ) as for female. Caudal rami ( Fig. 12A ) not fused to anal segment, more or less rectangular, medial edge concave leaving an opening between the rami, lateral edge slightly convex, posterior border straight. T1 absent, γ seta recessed at lateral corner, T2 and T3 small, very close together, T4 at medial corner, fringe of short setules along posterior border between T3 and T4. Limbs as described for female except for the following differences. First segment of antennule with pinnate seta ( Fig. 12B ), second segment short, very narrow posteriorly ( Fig. 12D ), segment 3+4 unusually long (> 1/2 total length of antennule), ventral process or blade present, two small tooth-like and one rounded coupling denticle present on segment 4 ( Fig. 12G ), no denticulate pad or brush-pad present, aesthetasc and σ seta short (about length of segment 3+4), dactylus broad, very short (1/4 length of segment 3+4). Antenna with seta at distal end of basis ( Fig. 10A , (absent in female). P1 as for female, P2 with two plumose terminal setae on endopod (Fig. 11B), P3 similar to female but terminal serrate seta on endopod is S-shaped and equal to length of endopod (Fig. 11C). P5 baseoendopod fused with metasome segment 4 and genital double-somite. Exopod of P5 appears to articulate with fused genital somite ( Fig. 12H ), exopod ovate with one plain lateral seta half way down lateral edge ( Fig. 12C ) and row of 15+ ventral setules, two microscopic spinules (representing atrophied terminal setae) near apex ( Fig. 12C ). Spermatophore very small ( Fig. 12F ).

Etymology. Synurus ctenocheirus has been named from the unique condition of the first terminal comb-like seta on P1 endopod. In all other members of the Porcellidiidae both claws are lamelliform, Harris and Robertson (1994), but in adult and late stage copepodids of this species the first claw is comb-like (G. ctenos = a comb + cheiros = hand).

Remarks. Synurus ctenocheirus is remarkable for four unique sexually dimorphic features: presence of a seta on basis of male antenna, epipleural lobes of male metasomal segment 3 that extend back to the same level as the P5 limbs, fusion of male M4 with genital double-somite and P5 baseoendopod, and fusion of female caudal rami with the anal segment.

Animals named Porcellidium unicus by Ummerkutty (1970), although not fully described, clearly belong to the genus Synurus by virtue of the oval male P5 with atrophied setae, only two external setae on P2, P3 and P4 exopod segment 3, and the fusion of male genital double-somite with P5 baseoendopod and M4, but these features exclude the species from Porcellidium . It should be renamed Synurus unicus (Ummerkutty, 1970) comb. nov.

The following characters distinguish S. unicus from S. ctenocheirus . It is larger (female 0.75 mm, male 0.63 mm), Hicks’ index for β is 50%, the distance between α and β setae is 1/2 ramus length and the female caudal ramus is not fused to the anal segment. The male P5 is described as pentagonal bordered with setules on lateral edge as far as the lateral seta and there are three atrophied terminal setae. Synurus unicus was collected from algae off Vedalai in the Gulf of Mannar, Sri Lanka.

Distribution. The type series GI2,9/73 (comprising 44 ♀♀, 30 ♂♂ + 23 juveniles) was collected from Zostera capricornia growing on coral sand in the sublittoral fringe at Green Island, Great Barrier Reef, Cairns, but it has not been recorded from the southern coast of Queensland (25°S), V. A. Harris, 1973. Specimens of this species collected from Okinawa, Japan (approx. 27°N) in the collection of Yuka Sasaki have been examined by the author. They appear morphologically identical to the Australian material. Their size falls within the range given above (Yuka Sasaki, pers. comm.).

Figures

Figure 9. Synurus ctenocheirus sp. nov. Female: (A) adult; (B) genital double-somite; (C) P5 (ventral); (D) edge of cephalosome (d. pits, dorsal pits; hy.b. hyaline border); (E) detail of terminal setae on caudal rami; (F) caudal rami showing fusion of anal segments with rami (arrow marks point of fusion); (G) genital opening.Scale bar:A = 0.45 mm. B, C = 0.15 mm. D, F = 0.08 mm.E = 0.06 mm. G = 0.12 mm.

Figure 10.Synurus ctenocheirus sp. nov.Male:(A) antenna (* seta on basis);(B) mandible.Female:(C) maxilliped;(D) P1;(E) maxillule; (F) maxilla. Scale bar: A = 0.1 mm. B, C, E, F = 0.08 mm. D = 0.14 mm.

Figure 12.Synurus ctenocheirus sp. nov.Male:(A) caudal rami;(B) antennule, ventral (* ventral process);(C) P5 (ventral);(D) antennule, dorsal; (E) edge of cephalosome;(F) adult with spermatophore;(G) coupling denticles on antennule;(H) genital double-somite and caudal rami (showing fusion of genital double-somite with metasome segment 4 and P5 baseoendopod. Scale bar: A, B, D, E, H = 0.08 mm. C = 0.06 mm. F = 0.45 mm. G not to scale.