Nymphon dentatum

Staples, David A., 2019, Pycnogonids (Arthropoda, Pycnogonida) from the Southwest Indian Ridge, Zootaxa 4567 (3), pp. 401-449: 408-410

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Nymphon dentatum

sp. nov.

Nymphon dentatum  sp. nov.

Figure 4View FIGURE 4 A–L; Plate 1View PLATE 1 F–H

Material examined. Holotype, male ( NHMUK 2018.8View Materials), Southwest Indian Ocean , Middle of What Seamount, 37˚ 56. 795'S, 50˚ 27.24'E, ROV, 1414 m, specimen JC066-3538, amongst sortings, 2 December 2011. 

Paratype. One male (larvigerous) ( NHMUK 2018.9View Materials), Southwest Indian Ocean, Middle of What Seamount, 37˚ 57. 91'S, 50˚ 24.42'E, ROV, 1148 m, specimen JC066-3398, amongst sediment, 1 December 2011  .

Other material. Two subadults (NHMUK 2018.10), Southwest Indian Ocean, Middle of What Seamount, 37 ° 57.41´S, 50° 26.34´E, ROV, 1148 m, specimen JC066 2740, stn 6.4, coral framework and sediment, 1 December 2011.

Description. Male holotype. Trunk ( Fig. 4A, BView FIGURE 4. Plate 1F, GView PLATE 1) elongate, smooth, glabrous, segmentation distinct, neck forward of oviger bases long (greater than five times median diameter), very slender, neck proximal to oviger bases much shorter, broader than trunk; lateral processes separated by about three times basal width, length greater than twice basal width. Ocular tubercle very low, placed midway between first lateral processes and bases of ovigers, posterior margin sloping, transitioning from apex into cephalon; four eyes, anterior pair larger and well pigmented, lateral sense organs present, two dorsal papillae.

Proboscis narrowest at base, gently inflated in mid-region then narrowing slightly to a rounded tip, oral surface truncate, concave.

Abdomen ( Fig. 4JView FIGURE 4) reaching close to distal margins of fourth lateral processes, slightly inflated in mid-region, tapering from about two-thirds length to strongly cleft tip, length about three times maximum width, probably articulated at base but unclear, basal suture line preceded by what appears to be wide membrane.

Chelifore scape arched, over-reaching proboscis, chela palm slender, fingers longer than palm, curved inwards, finger tips sharply curved, crossing, not touching, teeth fine, mostly arranged one tall, one short, immoveable finger with 45 teeth, moveable finger with 33 teeth ( Fig. 4LView FIGURE 4).

Palp ( Fig. 4IView FIGURE 4) five-segmented, segment 2 marginally longer than segment 3, segment 4 longer than segment 5, segments 4 and 5 with long ventral setae.

Ovigers ( Fig. 4G, HView FIGURE 4) ten-segmented, attached to distinct lateral extensions placed at about one-third the distance between first lateral processes and the anterior margin of the cephalon. Segment 4 with posterior gland opening, segment 5 longest, claviform, without distinct apophysis, segment 6 with long, fine setae on distal half, distal three segments subequal in length. Compound spine formula 13:9:7:8, distal-most spine on each segment off set from remainder, each spine with 3 to 4 lateral teeth. Terminal claw pointed, eleven sharp teeth on inner margin.

Leg (detached from body) ( Fig. 4CView FIGURE 4) long, slender, second coxa ( Fig. 4EView FIGURE 4) about six times as long as coxa 3, tibiae longer than femur, tibia 2 longest, length of distal setae about four times width of segment, femur with two ventral, acutely conical cement gland ducts, height about one-half segment width ( Fig. 4F, CView FIGURE 4. Plate 1HView PLATE 1); tarsus about two thirds length of propodus ( Fig. 4DView FIGURE 4), tarsus and propodus each with 15 to 16 strong, needle-like spines, propodus without heel, spines about equal to width of propodus, terminal claw robust, little more than one-third propodus length, auxiliary claws absent. Gonopore ventrodistal, coxa 2, legs 3 and 4.

Paratype. Ocular tubercle more prominent than in holotype ( Fig. 4KView FIGURE 4).

Measurements of holotype (mm). Trunk length (frontal margin of cephalic segment to tip of 4 th lateral process), 4.480; length cephalon, 2.392; width across 2 nd lateral processes, 1.096; proboscis length (ventral), 2.509; greatest diameter proboscis, 0.400; length abdomen (lateral) 0.400. Palp: seg. 1, 0.096; seg. 2, 0.968; seg. 3, 0.928; seg. 4, 0.464; seg. 5, 0.408. Oviger seg.1, 0.136; seg. 2, 0.320; seg. 3, 0.368; seg. 4, 1.600; seg. 5, 2.400; seg. 6, 0.840; seg 7, 0. 408; seg.8, 0.288; seg. 9, 0.248; seg. 10, 0.224; claw, 0.240. *Leg: coxa 1, 0.304; coxa 2, 1.800; coxa 3, 0.360; femur, 2.960; tibia 1, 4.424; tibia 2, 6.672; tarsus, 0.360; propodus, 0.552; claw, 0.200.

*A complete leg previously detached from the body (either first or second leg) was selected for measurement in the absence of a complete third leg.

Etymology. The proposed specific name, Dentatum (derived from the Latin, Dens -tooth), alludes to the prominent teeth on the chela fingers.

Remarks. No other uniunguiculate species of Nymphon  has in combination the oviger bases distinctly separated from the first lateral processes and such a long slender neck. This species shares the elongate second coxa with N. longicoxa Hoek, 1881  but amongst other things it is readily distinguished from that species by the length of the neck.

Many proximal teeth on the chela fingers of the paratype are broken. A long flagella-like epibiont; perhaps a leech, is attached to the dorsal surface of trunk segment 3 of the holotype.

Adhesive threads with single egg and one protonymphon entangled remain attached to segment 5 of one oviger. It is likely that many eggs and larvae were dislodged in the collection process. The protonymphon larva has three pairs of developing appendages representing the chelifores, palps and ovigers of the adult.

Detail of the base of the holotype abdomen is unclear but in the paratype there appears to be an area of wide membrane representing a point of articulation.