Nymphon plectrum, Takahashi & Kajihara & Mawatari, 2012

Takahashi, Yoshie, Kajihara, Hiroshi & Mawatari, Shunsuke F., 2012, Sea spiders of the genus Nymphon (Arthropoda: Pycnogonida) from waters around the Nansei Islands, Japan, Journal of Natural History 46 (21 - 22), pp. 1337-1358 : 1352-1357

Nymphon plectrum sp. nov.

( Figure 7 )

Material examined

Holotype. ZIHU 3331 , male, 26 ◦ 14.75 ′ N, 127 ◦ 31.90 ′ E, south of Nagannu Island , Okinawa, 22 May 2004, 55 m depth, dredge, Y. Takahashi.

Paratype. ZIHU 3332 , male, 26 ◦ 14.64 ′ N, 127 ◦ 31.01 ′ E, Nagannu Island , Okinawa, 22 May 2005, 57 m depth, dredge, Y. Takahashi .

Measurements of holotype (millimetres)

Trunk length, 1.79; body width, 0.64; proboscis length, 0.60; length of abdomen, 0.16; third leg (L3) coxa 1, 0.17; L3 coxa 2, 0.98; L3 coxa 3, 0.24; L3 femur, 1.56; L3 tibia 1, 1.65; L3 tibia 2, 2.28; L3 tarsus, 0.18; L3 propodus, 0.49; oviger first article (O1), 0.11; O2, 0.14; O3, 0.31; O4, 0.48; O5, 0.77; O6, 0.31; O7, 0.19; O8, 0.14; O9, 0.14; O10, 0.15; oviger terminal claw, 0.16.

Etymology

The specific epithet plectrum is a Latin noun in the nominative singular standing in apposition to the generic name, meaning stringed-instrument pick, here referring to the expanding fan shape of the proximal endal spines on outer segments of the oviger.

Description

Size very small for genus. Trunk ( Figure 7A,B ) slender, completely segmented, without tubercles or setae. Lateral processes about as long as their maximum diameters, separated distally by about their diameters, each with three or four dorsodistal setae. Ovigers inserted just anterior to bases of first lateral processes. Ocular tubercle is in midline between anterior margin of first lateral processes, its height equal to its basal diameter, dome-shaped with a small knob at apex flanked by a small tubercle on each side; eyes pigmented; anterior two eyes larger than posterior ones. Abdomen about 1.5 times as long as wide, angled dorsally, thinner than ocular tubercle, cylindrical proximally, tapering and blunt distally, without setae.

Proboscis ( Figure 7A–C ) barrel-shaped, one-third as long as trunk, proximal half slightly swollen, widest part 1.5 times basal width, constricted in middle and at one-quarter of length from distal end, without setae, narrowest distally and at base, rounded at tip.

Chelifore scape ( Figure 7A–C ) slightly shorter than proboscis, five times as long as wide, with a few lateral and several dorsodistal setae. Palm ( Figure 7D ) fully inflated, 1.5 times as long as wide, with some setae dorsally and laterodistally. Fingers without setae; movable finger almost equal in length to immovable finger; both fingers with bifurcate teeth endally, numbering 15–17 per finger.

Palps ( Figure 7E ) five-segmented; fifth segment longest, about six times as long as wide; first segment shorter than wide; second segment slightly shorter than fifth segment, about seven times as long as wide, narrower than other segments, with single setae dorsodistally and ventrodistally, each as long as first segment width; third segment 0.8 times length of second segment, gradually thickening toward distal end, six times as long as basal width, with ventrodistal setae each much longer than segment width; fourth segment short, slightly longer than its width; terminal two segments with many long setae ventrally, laterally, and distally, almost all of them more than twice as long as segment width.

Ovigers ( Figure 7F ) 10-segmented; fifth segment longest, 2.5 times as long as third, swollen distally to about twice the proximal width, with some ventral short setae on proximal half and longer setae on distal half, each seta up to twice as long as segment width; sixth segment as long as third segment, with fairly long setae ventrally, shorter setae dorsally and laterally; proximal three segments glabrous; fourth segment slightly shorter than proximal three segments combined, with several ventrodistal setae longer than segment width. Strigilis ( Figure 7G ) with trimorphic compound spines; proximal three to five spines on each segment plectrum-shaped, distal one or two spines on each segment oval, with narrower, pointed spines in between; all spines with fine denticles along margins; spine formula 13: 10: 10: 10; distal four segments with several long setae ectally. Terminal claw long, slightly longer than terminal segment, without serration.

Legs ( Figure 7H,I ) slender, all segments setose, especially tibiae; first coxa shorter than third, with a single seta ventrodistally; second coxa about six times as long as first, with several dorsal setae longer than segment width and a few lateral and ventral setae as long as segment width; third coxa with four setae ventrally; femur longer than all coxae combined, with several sparse setae, including a single fairly long seta distally, and with 17–20 cement gland pores ventrally; first tibia longer than femur; second tibia longest; both tibiae with fine setae longer than segment width over entire surface; propodus ( Figure 7I ) about three times as long as tarsus, with fairly long setae dorsally and shorter setae ventrally, and with five or six slender sole spines ventrally; main claw length 0.2 times that of propodus, with tiny endal serrations ventrally, auxiliary claws 1.25 times as long as main claw; both auxiliaries with endal serrated rugosities.

Remarks

Nymphon plectrum is most similar to N. giraffa Loman, 1908 and N. aequidigitatum Haswell, 1885 in having main and auxiliary claws with teeth, and in the shapes of the chela palm, proboscis, and cement gland pores. However, N. plectrum is easily distinguished from these two species by the ratio of tarsus to propodus: N. plectrum has a tarsus about one-third the length of propodus, whereas the others have a tarsus more than half the length of propodus. Moreover, N. plectrum differs from the others in the number of male chela finger teeth (15–17 in N. plectrum compared with 23–45 in N. giraffa and 35–40 in N. aequidigitatum ). Nymphon plectrum is unique in having trimorphic spines on the strigilis and conspicuously setose appendages with fairly long setae, much longer than the segment width.

Stock (1975) proposed the N. aequidigitatum species group, members of which share certain characters, which was accepted by Child (1988). Nymphon plectrum is the 12th member of this species group, having in common with the other species a chela finger with usually bifurcate teeth; the oviger terminal claw without denticles; the propodal claws with auxiliary claws longer than main claw, both with slight endal rugosity; and the palp with the terminal segment the longest. Child (1988) presented a key to the N. aequidigitatum group, in which N. giraffa was incorrectly characterized in couplet 3 by “Chela teeth triangular, not bifurcate; oviger spine simple, not serrate; palp segment 5 shorter than segment 2; propodus only slightly longer than tarsus”. According to the published account and figures by Nakamura (1987), N. giraffa has bifurcate teeth on the chela finger and serrate oviger spines, and the propodus is twice as long as the tarsus. Largely based on Child (1988), we present here a revised key to the N. aequidigitatum group. In the following key, the terms “monomorphic”, “dimorphic” and “trimorphic”, applied to the oviger spines refer to the character states in which the oviger has one, two or three types of spines, respectively. Figure 7D shows a “bifurcate” tooth of the chela finger.

Key to Nymphon aequidigitatum group

1. Neck long, some part of its sides parallel................................. 2 Neck short, little or no part of its sides parallel........................... 8

2. Parallel sides of neck longer than twice neck diameter.................... 3 Parallel sides equal to or less than twice neck diameter.................... 6

3. Neck less than four times as long as wide................................. 4 Neck more than four times as long as wide............................... 5

4. Oviger spines monomorphic, with a pair of large proximal and small distal serrations; propodus 1.5 times as long as tarsus..... N. giraffa Loman, 1908 Oviger spines trimorphic, with mostly equal serrations; propodus about three times as long as tarsus................................. N. plectrum sp. nov.

5. Chela teeth equal in size on the two fingers; oviger spines with a few large proximal and many small distal serrations; palp segment 5 subequal to segment 2; propodus twice as long as tarsus...... N. aldabrense Child, 1988 Chela teeth small on movable, larger on immovable finger; oviger spines with mostly equal serrations; palp segment 5 only 0.6 times length of segment 2; propodus 2.5 times as long as tarsus............... N. setipedes Child, 1988

6. Main propodal claw almost as long as auxiliaries, all with setules or rugosity; movable finger shorter than palm, 11–19 bifurcate teeth per chela finger; oviger spines dimorphic................................................. 7 Main claw reduced to tiny hook, claws smooth; chela movable finger equal to palm in length, 21–22 teeth per finger; oviger spines trimorphic................................................................ N. kensleyi Child, 1988

7. Terminal palp segment longer than third, subequal to second; neck less than twice its diameter; propodus less than three times as long as tarsus........................................................ N. micronesicum Child, 1982 Terminal palp segment subequal to third, shorter than second; neck twice as long as its diameter; propodus more than three times as long as tarsus................................................... N. floridanum Hedgpeth, 1948

8. Most or all chela finger teeth unevenly bifurcate; tarsus a quarter to half as long as propodus; chela fingers shorter than palm........................ 9 Chela finger teeth simple, not bifurcate; tarsus about 0.2 times length of propodus; chela fingers as long as or longer than palm....................................................................... N. aemulum Stock, 1975

9. Proximal few chela finger teeth simple, others bifurcate; tarsus length half propodal length or longer; propodus with equal-sized spines or none; strigilis claw with endal setules................................................. 10 All chela finger teeth bifurcate; tarsus one-quarter as long as propodus; propodus heel spines twice as large as sole spines; strigilis claw with endal lamina......................................... N. megacheles Child, 1988

10. Chelae with 8–11 finger teeth; oviger claw with cilia or teeth endally...... 11 Chelae with 35–40 finger teeth; oviger claw without cilia or teeth endally................................... N. aequidigitatum Haswell, 1885

11. Chelae with 8–10 finger teeth; oviger claw with cilia; oviger spines monomorphic; both propodal claws with endal setules.... N. biformidens Stock, 1974 Chelae with 11 finger teeth; oviger claw with teeth; oviger spines dimorphic; propodal main claw smooth, auxiliary claws with setules...................................................................... N. draconis Child, 1990

Figures

Figure 7. Nymphon plectrum sp. nov., ZIHU 3331, holotype, male. (A) Trunk, dorsal view; (B) trunk, lateral view; (C) cephalic parts, lateral view; (D) chela; (E) palp; (F) oviger; (G) enlargement of distal segments of oviger; (H) third leg; (I) enlargement of distal segments of third leg. Scale bars: A–C, F, H, 1 mm; D, E, I, 0.25 mm; G, 0.1 mm.