Rhynchothorax coralensis

Staples, David A., 2019, Pycnogonids (Arthropoda, Pycnogonida) from the Southwest Indian Ridge, Zootaxa 4567 (3), pp. 401-449: 428-430

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Rhynchothorax coralensis

sp. nov.

Rhynchothorax coralensis  sp. nov.

Figure 14A–F; Plate 4G, HView PLATE 4

Material examined. Holotype, sex undetermined ( NHMUK 2018.34View Materials), Southwest Indian Ocean, Coral Seamount, 37˚ 56.79'S, 50˚ 27.24'E, ROV, 732 m, specimen JC066-1296D, stn 4.38, mooring site, on Mango wood net, 20 November 2011. Right palp and right oviger mounted in Euparal on glass slides. 

Description. Holotype. Trunk robust (Fig.14A, B. Plate 4G, HView PLATE 4), surface finely papillose except proboscis, first two segmentation lines complete, third lacking, segments 1 to 3 with rounded mid-dorsal processes, those on segments 1and 2 placed on low transverse ridge; lateral cephalic extensions compact, bases partially in contact with first lateral processes, strong transverse constriction free of papillae at about mid-length, suture line absent; lateral processes touching at bases, length shorter than width, thickened transverse distal rim slightly raised in mid region. Ocular tubercle conical, two granulose tubercles at base of posterior surface; four eyes, pigmented, lateral sense organs raised.

Proboscis surface irregular, two proximal dorsolateral bulges and two smaller dorsodistal bulges, distal part narrowing strongly down-curved, mouth bilateral, mouth opening consisting of inner and outer antimeres.

Abdomen unarticulated at base, with slight mid-swelling, truncate distally, anal slit not cleft, placed vertically on distal surface, not seen in dorsal or ventral view.

Chelifores absent.

Palp five-segmented (Fig. 14D), papillose, first segment obscure in dorsal view, with few papillae, segment 2 longest, length about 3.5 times width, prominent inward-leaning dorsodistal digitiform process, height equal to width of segment, segment 3 little more than half length segment 2, with raised dorsodistal rugose thickening, segment 4 about one-quarter length segment 3, length segment 5 tiny, less than half segment 4.

Oviger ten-segmented (Fig. 14F), segment 4 longest, segment 5 slightly curved, widening distally, segment 6 longer than segment 5, segments 8–10 subequal, segment 10 with strong lamina on anterior surface, claw with broad cutting edge, length greater than half segment 10. Spine formula segments 7 to10 in the order of 3:2:3:2 but inconclusive due to small size and orientation of oviger mounted on glass slide, spine shape variable, some simple, others irregular or tridentate but mostly seen in lateral view and actual shape difficult to interpret.

Legs; coxae of similar length (Fig. 14E), first coxae legs 1 to 3 with distal tubercle, absent on fourth legs, femur longer than coxae 1 to 3 combined, femur third leg 3.3 times longer than greatest width, femur fourth leg narrower, 4.6 times longer than greatest width, tibia 1 slightly shorter than femur, tibia 2 shorter than tibia 1, tarsus about one-sixth length of propodus, propodus gently curved, heel absent, sole spines short varying between legs from 7 to13, claw slightly less than one-third length of propodus, auxiliary claws one-third length of main claw. Cement glands and gonopores not evident.

Measurements of holotype (mm). Trunk length (frontal margin of cephalic segment to tip of 4 th lateral process), 0.960; length of cephalon, 0.376; length cephalic lateral extension, 0.200; width across 2 nd lateral processes, 0.664; proboscis length (ventral), 0.600; greatest diameter proboscis, 0.264; length abdomen (dorsal), 0.296. Palp: seg. 1, 0.024; seg. 2, 0.344: seg. 3, 0.184; seg. 4, 0.064; seg. 5, 0.024. Oviger: seg. 1, 0.064; seg. 2, 0.088: seg. 3, 0.080; seg. 4, 0.160; seg. 5, 0.104; seg. 6, 0.152; seg. 7, 0.096; seg. 8, 0.080; seg. 9, 0.080; seg. 10, 0.080; claw, 0.040. Third leg: coxa 1, 0.104; coxa 2, 0.104; coxa 3, 0.104; femur, 0.392; tibia 1, 0.360; tibia 2, 0.304; tarsus, 0.064; propodus, 0.320; claw, 0.096; aux claws 0.048.

Etymology. Named for its type locality, the Coral Seamount.

Remarks. Although occurring sympatrically with R. swir  sp. nov. the trunk of this species is far more robust and compact. It differs conspicuously by the absence of prominent digitiform processes on the lateral processes, in the wider and more sculptured proboscis, more compact cephalic lateral processes, rounded mid-dorsal trunk processes and in having a low, conical ocular tubercle. The only other species of Rhynchothorax  that shares rounded mid-dorsal processes on the trunk is R. tiahurensis Müller, 1989  but these species share little else in common.

Using Clark’s 1976 key, this specimen can be followed to couplet 7 where it keys out to R. australis  (type

As with R. swir  the proboscis originates from within a wide collar on the anterior margin of the cephalon (Fig.

14C) but it appears to be withdrawn in this specimen and the arthrodial membrane is not visible.