Nymphon sigmoides, Staples, 2025

Staples, David A., 2025, Pycnogonids, ‘ sea-spiders’ (Arthropoda, Pycnogonida) selected from the collections of the South Australian Museum with descriptions of new species and review of the genus Pallenella., Memoirs of Museum Victoria 84, pp. 1-47 : 5

Nymphon sigmoides sp. nov.

Figure 2a–i. Plate 2a–d u r n:l sid:z o o b a n k.o rg:a c t:61C3E6 85 - 0 E B4 - 475 4 - 81FA-

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Material examined. Holotype. Male ( SAMA E9437 ). Sir Joseph Banks Group, between Lusby Rocks and Partney I., under rocks on bryozoan, 8 m, K. Gowlett, 1 Feb 1985.

Diagnosis. Length of neck about three times width. Lateral processes widely spaced, length of lateral processes about twice basal width. Eye tubercle low, rounded. Palps 5-segmented. Proboscis elongate, mid-region slightly swollen. Chelae fingers about as long as palm, distal one-third delicate, thin, strongly curved upwards and slightly recurved, teeth widely spaced. Oviger bases merged with the anterior surface of the first lateral processes, terminal claw with three teeth along inner margin. Tarsus slightly less than one-third length of propodus, auxiliary claws slightly less than half length of main claw.

Description. Holotype, male. Leg span about 12.0 mm. Cephalon shorter than remainder of trunk, 1 or 2 short setae at bases of chelifores; length of neck about three times width. Length of lateral processes about twice basal width, one or two short and fine distal setae may be present; processes 1 and 2 separated by about 3.5 times basal width, processes 2 and 3 separated by 2.3 times basal width processes 3 and 4 separated by about 1.5 times own diameter.

Eye tubercle low, rounded, two dorsal papillae, four eyes.

Proboscis elongate, attached obliquely to the anteroventral surface of the cephalon, vaguely upturned, mid-region slightly inflated with few inconspicuous spinules; length almost 3.0 times maximum width, jaws recessed (plate 2 c).

Chelifore scape lowly arched (plate 2b), arthrodial membrane at base broad, allowing near vertical orientation, two strong spines on outer surface, distal most spine longer than proximal spine, single dorsodistal seta accompanied by several shorter setae; chela palm elongate, with three slender spines on dorsal surface; fingers (measured to bases of distal upcurved parts) about as long as palm, distal one-third delicate, thin, sigmoid-shaped, strongly curved upwards and slightly recurved distally (fig. 2i); teeth widely-spaced, simple and of similar structure on both fingers, slightly curved inwards towards palm, moveable finger with nine teeth, immoveable finger with eight teeth, in both cases proximal two teeth small, distal two teeth tiny.

Palp (fig. 2h) 5-segmented, segment 1 short, segment 2 longest, with strong dorsodistal seta, segment 4 longer than either segment 3 or 5, setae sparse.

Oviger bases contiguous with the anterior surface of the first lateral processes (plate 2a), segment 1 attached ventrally, segment 5 longest, 1.5 times length segment 4, widest distally, several long setae throughout length, terminal claw with three teeth along inner margin; compound spine formula, segments 7–10, difficult to interpret but in the order of 9:7:6:7, spines with four or five pairs of small lateral teeth.

Anal tubercle obliquely erect, height about twice width, several inconspicuous spinules, no evidence of basal segmentation (fig. 2b).

Legs slender, (fig. 2d, plate 2d) first coxa with about six fine setae around dorsodistal surface, second coxa 1.6 times length coxa1, about twice as long as coxa 3, tibia 2 longest segment, tibia 1 longer than femur, femur and tibia 1 with single, long, dorsodistal spine; length tarsus slightly less than one-third length of propodus, with single strong ventrodistal spine and a less robust dorsodistal spine, propodus gently curved, sole with five widely-spaced spines, distal two spines smaller; main claw less than half length propodus, auxiliary claws slightly less than half length of main claw. Gonopores tiny, difficult to discern, ventrodistal, probably legs 3 and 4 only.

Measurements of holotype (mm). Trunk length (frontal margin of cephalic segment to tip of fourth lateral processes), 1.87; width across second lateral processes, 0.76; scape, 0.61; palm 0.34; fingers 0.33; proboscis length (lateral), 0.43: greatest diameter proboscis, 0.17; anal tubercle length, 0.16. Palp: seg. 1, 0.07; seg 2, 0.38: seg. 3, 0.31 seg. 4, 0.34; seg. 5, 0.23. Oviger: seg. 1, 0.07; seg. 2, 0.12: seg. 3, 0.16; seg. 4, 0.32; seg. 5, 0.51; seg. 6, 0.28; seg. 7, 0.18; seg. 8, 0.13; seg. 9, 0.09; seg. 10, 0.10; claw. 0.05. Leg: coxa 1, 0.27; coxa 2, 0.44; coxa 3, 0.18; femur, 1.12; tibia 1, 1.26; tibia 2, 1.80; tarsus, 0.18 propodus, 0.51; claw, 0.22; aux claws 0.12.

Etymology. Named for the likeness of the chela fingertips to a sigmoid curve.

Remarks. Unpublished records also place this species at nearby Upper Spencer Gulf, South Australia. Nymphon sigmoides sp. nov. belongs to a small group of species that share the presence of auxiliary claws, a long neck, a tarsus less than half length of propodus and slender sigmoid-shaped chelifore fingertips. Of those species, N. sigmoides is most-closely aligned with the larger N. fortunatum Stock, 1997 from New Caledonia. This species is however, distinguished by having chela teeth of equal length on both fingers (much shorter teeth on moveable finger in N. fortunatum ), the straight first tibia (contorted in N. fortunatum ) and a much shorter male coxa 2 (about 1.6-times as long as coxa 1 in N. sigmoides versus greater than three-times in N. fortunatum ). Based on Stock’s figure ( Stock, 1997, Fig 5a), the lateral processes of N. fortunatum appear to be more widely separated, particularly processes 3 and 4. Unfortunately, Stock did not comment on the contorted first tibia of N. fortunatum which is presumed here to be a characteristic of all legs. All setae are much shorter and far fewer than illustrated by Stock for N. fortunatum . This specimen shows evidence of a mid-dorsal spine having been broken off each chelifore scape.