Pycnogonum tenue Slater, 1879

Takahashi, Yoshie, Dick, Matthew H. & Mawatari, Shunsuke F., 2007, Sea spiders (Arthropoda: Pycnogonida) from waters adjacent to the Nansei Islands of Japan, Journal of Natural History 41 (1 - 4), pp. 61-79 : 75-77

Pycnogonum tenue Slater, 1879

( Figure 6 )

Pycnogonum littorale Ström var. tenue Slater 1879, p 281 .

Pycnogonum littorale Ström var. tenue: Ortmann 1890, p. 167 ; Shishido 1899, p. 200; Fukui 1919, p. 99.

Pycnogonum tenue Kishida 1927, p. 989 , Figure 1905; Ohshima 1936, p. 867; Ohshima and Kishida 1947, p. 1010, Figure 2865; Hedgpeth 1949, p. 303, Figures 48b, 50c; Stock 1954, p. 162, Figure 80; Utinomi 1955, p. 36, Figures 22 and 23, 1959, p. 220, 1965, p. 338, Figure 3 ; Stock 1966, p. 401 [key]; Utinomi 1971, p. 338; Nakamura and Child 1983, p. 64; Nakamura 1987, p. 37, Plate 34; Nakamura and Child 1991, p. 63.

Material examined. One male with eggs, one female, 26 ° 15.80 9 N, 127 ° 21.90 9 E, off Tokashiki Island, 31 May 1999, 95– 115 m depth; collected by sorinet, M. Shimomura collector . One male, one female, 30 ° 09.18 9 N, 130 ° 36.03 9 E, southeast of Yaku Island, 23 May 2002, 210– 214 m depth; collected by beam trawl, Y. Takahashi collector . One male, 30 ° 09.08 9 N, 130 ° 38.18 9 E, south of Yaku Island, 207– 197 m depth; collected by beam trawl, Y. Takahashi collector .

Description. Trunk ( Figure 6A, B ) robust, very crowded, oval in outline, completely segmented, segments one to four each with a tall median tubercular projection dorsally, projection taller than their basal width; just posterior to ocular tubercle is an additional single low median tubercle. Lateral processes as long as wide, slightly separated from each other; first lateral processes armed with a single posterolateral tubercle at distal end; second and third lateral processes each with a pair of prominent distal tubercles; second lateral processes with an additional single, small tubercle dorsodistally; fourth lateral processes with no distinct tubercles. Ocular tubercle large, slightly lower than median projections, rounded, with four slightly pigmented eyes; anterior pair of eyes smaller than posterior. Proboscis very slender, widest proximally, tapering to middle, distal half cylindrical and almost half the basal width, with cylindrical lips and with several tiny ventral setae. Abdomen large, clavate, rounded distally, reaching middle of second coxae of fourth legs, armed with four short distal setae.

Ovigers ( Figure 6C ) in males short, robust, nine-segmented, with terminal claw; first, second, and fifth segments equal in length; third segment almost twice the length of fourth segment; proximal three segments with a few setae ectally; fifth segment with single seta endally; distal four segments armed with one to four simple spines endally; sixth segment twice as long as seventh; seventh and ninth segments subequal and longer than eighth segment; terminal claw strongly curved, longer than subterminal segment.

Legs ( Figure 6D ) moderately short, robust, armed with several short setae; all coxae subequal in length; first coxa 1.5 times as wide as other coxae; second and third coxae as wide as long; femur and first tibia subequal in length; second tibia almost two-thirds the length of first tibia; tarsus short, rounded ventrally; propodus curved proximally, with several short setae dorsally; second tibia, tarsus, and propodus armed with numerous bifurcate spines ventrally ( Figure 6E ), the rami twisted; claw about one-quarter as long as propodus. Genital pores of male situated ventrally on tubercle on second coxae of fourth pair of legs, those of female placed dorsally on second coxae of fourth legs.

Remarks. Our specimens agree well with the original description of Pycnogonum tenue Slater, 1879 and with the redescriptions and figures by Stock (1954) and Nakamura (1987), in the shape of trunk, proboscis, and dorsal median projections. Among congeners, Pycnogonum tenue can easily be recognized by its long, slender, tapering proboscis, five dorsal median projections, and the short leg segments. Some variation has been reported in this species. The size of the proboscis has been reported as being from slightly less than 2 mm to slightly more than 4 mm ( Stock 1954); in our specimens it ranges from 2.38 to 2.87 mm. Some authors have reported prominent reticulation on the surface of the whole body, but others have not (e.g. Child 1991); our material has the first coxae faintly reticulated, but other parts not reticulated. Our material has clearly bifurcate setae on the propodus, tarsus, and second tibia. Such bifurcate setae have not been reported before in P. tenue , though some Pycnogonum species (e.g. P. elephas Stock, 1966 ; P. pustulatum Stock, 1994 ) are known to have such setae only on the propodus. Presence of the bifurcate setae may prove to be a useful taxonomic character to discriminate species of Pycnogonum in the future, after the setae have been examined in various species.

Distribution. Pycnogonum tenue is known from many localities in Japan: Sagami Bay, Suruga Bay, Kii Channel, and western Kyushu, ranging in depth from 7 to 416 m. This study extends the distribution of Pycnogonum tenue to Okinawa. The species is presently known only from Japan.

Figures

Figure 3. Rhopalorhynchus tenuissimum (Haswell, 1884), male, except for C: (A) trunk, dorsal view; (B) trunk, lateral view; (C) proboscis, lateral view, female; (D) palp; (E) oviger; (F) strigilis, with enlargements of two types of spines; (G) third leg. Scale bars: 1 mm (A–E, G); 0.5 mm (F).

Figure 6. Pycnogonum tenue Slater, 1879, male: (A) trunk, dorsal view; (B) trunk, lateral view; (C) oviger; (D) third leg; (E) endal setae of third leg, by SEM. Scale bars: 1 mm (A–D); 0.01 mm (E).