Pycnogonum sentus

Staples, David A., 2019, Pycnogonids (Arthropoda, Pycnogonida) from the Southwest Indian Ridge, Zootaxa 4567 (3), pp. 401-449: 435

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Pycnogonum sentus

sp. nov.

Pycnogonum sentus  sp. nov.

Figure 17View FIGURE 17 A–H; Plate 6A, BView PLATE 6

Material examined. Holotype, male ( NHMUK 2018.45View Materials), Southwest Indian Ocean, Coral Seamount, 41°21.46'S, 42˚54.53'E, ROV, 702 m, specimen JC066-3447, amongst sortings, 20 November 2011. 

Description. Male holotype. Trunk ( Fig. 17A, BView FIGURE 17. Plate 6A, BView PLATE 6) segmented, dorsal surfaces granular, very coarse along trunk midline, lateral processes and legs, body surface interspaced with fine spinules, posterior margin of trunk segments 1, 2 and 3 form a raised transverse rim, dorso-median mounds varying from being rounded on segment 1 to being progressively more conical on segments 2 and 3, segment 4 with a single, small, conical median tubercle, cephalon with granulose mound immediately behind ocular tubercle, reaching to approximately half height of ocular tubercle ( Fig. 17AView FIGURE 17); lateral processes narrowly separated at bases, diverging distally, each lateral process shorter than width at base, well-developed dorsodistal, tuberculose mounds on lateral processes 2 to 4, first lateral processes granulose distally but without prominent mound. Ocular tubercle on anterior margin of cephalon, wider than tall, dorsal surface flat in lateral view with small anterior papillose tubercle; four eyes unpigmented in preserved specimen.

Proboscis almost half trunk length, evenly tapered from base to rounded tip, without prominences or tubercles, supposed oral glands (see Staples 2002) present, jaws tri-radiate, slightly recessed.

Abdomen ( Fig. 17FView FIGURE 17) unarticulated at base, reaching to about end of coxa 2, width about one-third length, carried almost horizontally, with prominent broad, granular, almost keel-like process over-reaching distal margin giving the impression that the abdomen tapers to a fine point when viewed dorsally.

Oviger ( Fig. 17EView FIGURE 17) segmentation incomplete and indistinct, possibly 2 to 4 segmented at this stage of development with perhaps a terminal claw, two spines present on distal margin of first segment.

Third leg ( Fig. 17GView FIGURE 17) robust, ecdysial line not evident, dorsal surfaces distinctly granular, coxa 2 longer than coxae 1 and 3, femur and tibiae each with single long, fine, dorsodistal seta, femur longer than either tibia, tibia 1 longer than tibia 2, ventral surfaces of tibia 2 and tarsus with dense field of spines; propodus gently curved, no hint of heel, propodal sole with about eighteen slender spines, length of claw about 40% of propodus length, auxiliary claws absent. Gonopores not evident. Coxal pellicula (vide Staples, 2007) not evident.

Measurements of holotype (mm).Trunk length (frontal margin of cephalic segment to tip of 4 th lateral process), 3.152; width across 2 nd lateral processes, 1.640; proboscis length (ventral), 1.680; proboscis width at base 0.784; proboscis width at tip, 0.320; abdomen length (dorsal), 0.944. Third leg: coxa 1, 0.320; coxa 2, 0.416; coxa 3, 0.320; femur,1.112; tibia 1, 0.976; tibia 2, 0.824; tarsus, 0.200; propodus, 0.800; claw, 0.312.

Etymology. From the Latin sentus  (rough), referring to the rough or granular surface.

Remarks. Although this specimen is not fully mature, the species-determinate characters are sufficiently distinct to identify the species subsequently. Using Stock’s (1966b) key to this genus, the new species can be followed to couplet 22b where it keys out with P. tenue Slater, 1879  but from which it differs significantly in the shape of the proboscis. The less compact P. guyanae Stock, 1974  shares some similarity with this species but differs principally in the shape of the femur and the absence of a post-ocular tubercle. The presence of a tubercle or process on the dorsodistal surface of the abdomen is certainly not unique in the genus but none appear to be as prominent as in this species.