Nymphon natans, 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 : 2

Nymphon natans sp. nov.

Figure1a–j. Plate 1a–d u r n:l sid:z o o b a n k.o r g:a c t:814521A3-A4DF - 4B83 -89 77-

CBC04D39D440

Material examined. Holotype. Gravid female ( SAMA E9438 ). Edithburgh Jetty, free-swimming, flood (tide) current, at night, 2–5 m, K.L. Gowlett-Holmes, 23 May 1987.

Diagnosis. Neck long, about three times own width; lateral processes widely spaced, about 1.7 times longer than basal width, second lateral processes with anterior ventrolateral protuberances. Palps five-segmented. Chela fingers of about equal length, longer than palm, slender, strongly bowed, crossing at tip, each finger with 18–20 teeth. Oviger bases in contact with first lateral processes, claw with seven well-defined sharp teeth on inner margin. Auxiliary claws about two-thirds length of main claw.

Description. Holotype, female. Leg span about 46.0 mm. Cephalon slightly longer than remainder of trunk, length of neck about three times own width ( Plate1b). Lateral processes about 1.7 times longer than basal width, processes 1 and 2 separated by 2.4 times their basal width, separation between processes 2 and 3 about 2.7 times basal width, and processes 3 and 4 distanced by about their own diameter. Second lateral processes with anterior ventrolateral protuberance (plate 1b, fig. 1c).

Eye tubercle placed in line with oviger bases, height slightly less than basal width, two prominent dorsal papillae, dorsal surface between papillae almost level, four large eyes.

Proboscis cylindrical, about 2.5 times as long as wide, slightly constricted at about two-thirds length and inflated distally, distally rounded and fringed with tiny, sparse setae, lips slender, slightly projecting, jaws recessed.

Chelifore scape little shorter than proboscis; chela fingers of equal length (fig. 1d), slender, longer than palm, strongly bowed, crossing at tip, base of immoveable finger with pad on outer surface, bearing many long setae, each finger with 18–20 teeth separated by about own width, larger teeth much longer than width of finger, proximal teeth smaller, teeth on immoveable finger nearly erect, those on moveable finger curved.

Palp (fig. 1e) five segmented, segment 1 shortest, segment 2 longest, segment 4 next longest, segment 3 longer than segment 5, distal three segments with long setae particularly along inner margin, length of some setae greater than twice segment width.

Oviger (fig. 1f) ten segmented, bases in contact with first lateral processes, segment 5 longest, segment 4 next longest, with a probable glandular swelling, segments 5–10 decreasing in length distally; compound spine formula, segments 7–10, 13:10:10: 9, spine height little more than half segment width, proximal two spines smaller, distal-most spine on each segment offset, each spine with six pair of small lateral teeth, claw half-length of segment 10, with seven well-defined sharp teeth on inner margin.

Anal tubercle missing, broken off at base.

Legs (fig. 1h), setae and spination sparse, femur and tibia 1 with longer dorsodistal seta. Lateral line of thickened cuticle extending throughout length. Second coxa 2.7-times length of coxa 1, tibia 2 longest segment, tibia 1 longer than femur, femur filled with tiny oocytes, tibiae more slender than femur, tarsus (fig. 1i) about half length of propodus, propodus hardly curved, heel and primary spines absent, sole with about 15 slender main spines arranged in a single row, each interspaced by one or two lesser spines, distal spines longest and more robust; propodal claw short, slightly longer than one-fifth length of propodus, auxiliary claws about two-thirds length of main claw, without teeth or crenulations. Gonopores ventrodistal, second coxae, all legs.

Measurements of holotype (mm). Length trunk (frontal margin of cephalic segment to tip of fourth lateral processes), 4.53; width across second lateral processes, 1.72; scape 1.05; proboscis length (ventral); 1.12, diameter (at mid-length), 0.45. Palp: seg. 1, 0.18; seg 2, 0.75; seg. 3, 0.45; seg 4, 0.53; seg. 5, 0.31. Oviger seg. 1, 0.16; seg. 2, 0.31: seg. 3, 0.39; seg. 4, 1.12; seg. 5, 1.31; seg. 6, 0.66; seg. 7, 0.37; seg. 8, 0.29; seg. 9, 0.24; seg. 10, 0.23; claw, 0.12. Leg: coxa 1, 0.61; coxa 2, 1.65; coxa 3, 0.65; femur, 4.75; tibia 1, 5.28; tibia 2, 6.47; tarsus, 0.95; propodus, 1.83; claw, 0.35; aux claws 0.25.

Etymology. Latin: natans , (swimming, floating), alluding to the finding of the holotype free swimming in a flood tide.

Remarks. The legs are mostly detached and fragmented. This species belongs to a group of Nymphon defined by widely-spaced lateral processes and a conspicuously long, slender neck. Other shared characters are auxiliary claws shorter than the main claw, bases of ovigers in contact with the first lateral processes, the oviger claw with well-defined teeth and a tarsus shorter than the propodus. In this group, only N. spiniventris Stock, 1953 from the Basilan Straits shares the presence of ventrolateral protuberances on the anterior margin of the second lateral processes. Although quite evident in ventral view, these protuberances are mostly obscured in dorsal view and easily overlooked. Based on Stock’s figure ( Stock, 1953, Fig. 5), they are more conspicuous in N. spiniventris and have a proximal tooth-like process which he described as a “spiniform process”. A similar process is seen only on the left-side protuberance of this specimen, but it is only evident when viewed from a particular perspective. Nymphon spiniventris shares a similar long neck and wide spacing of the third and fourth lateral processes with this species, but otherwise differs conspicuously in the following respects: the first and second lateral processes of N. natans are more widely spaced, the tarsus is much longer relative to the propodus, the propodus is less curved and both chela fingers of N. natans bear many more teeth ( N. natans , 18–20 teeth versus N. spiniventris 9–10 teeth). The oviger compound spines of N. natans are of uniform shape, whereas those of N. spiniventris show a characteristic dimorphism, some with five pairs of lateral teeth and others with 15–22 pairs of fine denticulations ( Stock 1953, Fig. 6e, f). The new species agrees most closely with the holotype of N. andamanense Calman, 1923 from the Anderman Sea with which it shares the proportions of the neck and the wide separation of trunk lateral processes 3 and 4. The principal characters that distinguish N. natans sp. nov. from N. andamanense are the presence of ventro-anterior protuberances on the second lateral processes (absent in N. andamanense ), the much wider interval between first and second lateral processes, the proportionately shorter coxa 2 and longer tarsus. It is worth noting however, that the differences between N. andamanense and N. spiniventris appear to be few, and apart from the “spiniform processes”, differences appear to be limited to minor variations in proportions. I am grateful to Jan Beccaloni of the Museum of Natural History London who re-examined Calman’s type materialandwasabletoconfirmtheabsenceoftheprotuberances in N. andamanense adding support to the independent status of that species. Nymphon setimanus Barnard, 1946 is another species that could be confused with this specimen. Barnard’s (1946) brief description of N. setimanus was unaccompanied by figures and his later (1954) more detailed description was only accompanied by a single illustration of part of a chela ( Barnard, 1954, Fig. 8). Stock (1965) later provided detailed figures based on a specimen from Madagascar which generally reaffirmed Barnard’s (1954) description ( Stock, 1965, Figs 13–17). At that time, N. setimanus was recorded from several South African localities and perhaps from the South Arabian coast by Calman (1938) under the name N. andamanense although, he noted that his specimens were intermediate between N. andamanense and N. maculatum Carpenter, 1910 from the Red Sea. Müller (1989) later reported on several specimens from the Gulf of Aden which he assigned to N. setimanus ( Müller 1989) , and he agreed with Stock (1965) that Calman’s specimens probably belong to N. setimanus . Contrary to Müller’s statement that his specimens agree well with the material described in the literature, there appears to be little resemblance to the earlier descriptions by Barnard and Stock. Barnard (1954) described N. setimanus as having the lateral processes separated by “rather less than their own width” and the bases of the ovigers as “occupying one-third to nearly one-half of the neck” but Müller (1989 Figs 35-41) illustrated specimens with lateral processes separated by 2 to 3 times their diameters and oviger bases which clearly occupy much less than one-third to nearly one-half of a much longer neck. Except for the number of oviger spines and teeth on the claw, Stock’s (1965) Madagascar specimen agreed perfectly with Barnard’s description. It is quite evident that Muller’s Gulf of Aden specimens are not N. setimanus . Müller’s specimens share the wide spacing of the first and second lateral processes with this species but differ notably in the absence of a protuberance on the second lateral process, in having a proportionately shorter tibia 2 and a much shorter tarsus. Müller’s figures of N. setimanus agree more closely with Calman’s (1923) description of N. andamanense from which they seem to be distinguished only by the distance between the first and second lateral processes. Preserved specimens of both N. setimanus and N. maculatum retained pigmentation of the trunk and appendages ( Stock, 1965). There is no evidence of pigmentation in this specimen. The geographically closest species which could be confused with the new species, is N. molleri Clark, 1963 recorded from Spencer Gulf, South Australia to Cape Byron, New South Wales and northern Tasmania. Nymphon molleri is distinguished by a much longer cephalon relative to the remainder of the trunk, a greater number of oviger claw teeth, a distinctly shorter tarsus, and oviger compound spines that have up to ten pairs of lateral teeth.