Nymphon gravidus

Staples, David A., 2019, Pycnogonids (Arthropoda, Pycnogonida) from the Southwest Indian Ridge, Zootaxa 4567 (3), pp. 401-449: 410-412

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Nymphon gravidus

sp. nov.

Nymphon gravidus  sp. nov.

Figure 5View FIGURE 5 A–I; Plate 2View PLATE 2 A–D

Material examined. Holotype, female ( NHMUK 2018.11View Materials), Southwest Indian Ocean , Middle of What Seamount, 

37˚ 57. 91'S, 50˚ 24.42'E, ROV, 1135 m, specimen JC066-3415, parent 2584, amongst sediment sample, 1 December 2011.

Description. Female holotype. Trunk ( Fig. 5A, BView FIGURE 5. Plate 2A, CView PLATE 2) smooth, segmented, ovigers attached to lateral extensions of the neck well-forward of first lateral processes, neck short, strongly constricted immediately forward of oviger implants; lateral processes longer than width at bases, processes 1 and 2 separated by 1.3 times basal width, distance between remaining processes progressively decreasing. Ocular tubercle placed midway between first lateral processes and bases of ovigers, two tiny dorsal papillae; four eyes well developed but unpigmented in preserved specimen.

Proboscis tubular, distally swollen ( Fig. 5IView FIGURE 5).

Abdomen with obscure suture line near base, slightly swollen dorsally at base, cleft distally, length about twice width.

Chelifore scape slightly longer than proboscis, chela fingers much longer than palm, moveable finger with 25 teeth, immoveable finger with 21 teeth ( Fig. 5EView FIGURE 5. Plate 2DView PLATE 2).

Palp ( Fig. 5HView FIGURE 5) five-segmented, segment 2 longest, segment 4 longer than segment 5, setae on distal segments fine, long, mostly longer than width of segment.

Oviger ( Fig. 5CView FIGURE 5) ten-segmented, segment 5 longest, segment 4 with prominent gland opening on posterior surface, claw with four or five teeth on inner margin, length of claw about equal to segment 10, compound spines segments 7 to 10 well developed, pointed, with three pair of upwardly-curved lateral teeth, mid-pair longest, compound spine formula 8:6:5:6.

Third leg ( Fig. 5FView FIGURE 5) with scattered fine setae, coxa 2 almost three times length of coxa 1, femur shorter than tibia 1, proximal two-thirds strongly swollen with oocytes; tibia 2 longest, almost 1.5 times length of tibia1, distal seta about three times width of segment, tarsus less than half propodus length, sole spines 10 to 20 varying between legs, heel absent, auxiliary claws absent. Gonopore large, ventrodistal surface coxa 2, all legs.

Measurements of holotype (mm). Length trunk (frontal margin of cephalic segment to tip of 4 th lateral process), 2.432; length cephalon, 1.224; width across 2 nd lateral processes, 0.864; proboscis length (ventral), 1.080; greatest diameter proboscis, 0.400; length abdomen (lateral) 0.376. Palp: seg. 1, 0.112; seg. 2, 0.576; seg. 3, 0.424; seg. 4, 0.240; seg. 5, 0.216. Oviger: seg. 1, 0.136; seg. 2, 0.104; seg. 3, 0.232; seg. 4, 0.680; seg. 5, 0.800; seg. 6, 0.304. seg. 7. 0.240; seg. 8, 0.280; seg. 9, 0.160; seg. 10, 0.160; claw, 0.168. Third leg: coxa 1, 0.224; coxa 2, 0.560; coxa 3, 0.264; femur, 1.624; tibia 1, 2.040; tibia 2, 0.928; tarsus, 0.256; propodus, 0.496; claw, 0.200.

Etymology. Gravid, from the Latin gravidus  -pregnant, references the extraordinary number of oocytes carried in the legs and body cavity.

Remarks. There are four uniunguiculate species of Nymphon  that have the oviger implantation well forward of the first lateral processes and well-developed eyes on an ocular tubercle placed between the ovigers and the first lateral processes. These species are N. dentatum  sp. nov. N. rogersi  sp. nov., N. longicoxa Hoek, 1881  and N. prolatum Fage, 1942  . This species it is most like N. rogersi  but from which it differs most notably in the shape of the neck region, by having much more elongate palp segments (particularly the fourth segment) and in the shape of the chela fingers. From N. dentatum  it differs conspicuously in the length of the neck. From the other two species that make up this group this species is most like N. longicoxa  but from which it differs in the much shorter second coxa. Nymphon prolatum  is distinguished by its long ocular tubercle and much longer abdomen.

Considering the low fecundity of some species which may carry as few as one oocyte in each femur ( Bamber and Thurston 1993), the estimated many hundreds of oocytes carried by this female is remarkable. The oocytes have a diameter of about 0.040 µm and are tightly packed into the femora and tibiae and in lesser numbers, in almost the entire body cavity. They are present in the coxae, trunk, abdomen scapes and one oviger. Those in oviger segment 5 are lined up in a single row ( Plate 2CView PLATE 2). The presence of oocytes throughout the body cavity is common in the genera Austrodecus ( Bamber & Thurston, 1993)  and Rhynchothorax  (see R. swir  sp. nov.) but this occurrence is unique in the genus Nymphon  .