Rhopalorhynchus tenuissimum
Creators: name: Takahashi, Yoshie
name: Dick, Matthew H.
name: Mawatari, Shunsuke F.
Publisher: name: Zenodo
Year: 2007
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Pycnogonida
Pantopoda
Colossendeidae
Rhopalorhynchus
Rhopalorhynchus tenuissimum

description: Rhopalorhynchus tenuissimum (Haswell, 1884)   
(Figure 3)  
 Colossendeis tenuissima Haswell 1884, p. 1029, Plate 58, Figures 5 –8.   
 Rhopalorhynchus tenuissimus: Flynn 1919, p. 71, Plate 18, Figures 1–3.   
 Rhopalorhynchus tenuissimum: Stock 1958, p. 125; Staples 1982, p. 465, Figure 5A–J; Arango 2003, p. 2737, Figure 6.   
 Material examined. Two males, one female, 24 ° 14.64 9 N, 127 ° 31.01 9 E, Nagannu Island, 22 May 2005, 57 m depth, collected by dredge, S. Ohtsuka collector.   
 Measurements of proboscis. Using Stock’s (1958) conventions (in mm), for male and female illustrated in Figure 3A–C: OE, 3.63, 4.56; β, 1.48, 2.37; γ, 2.54, 3.99; δ, 2.34, 2.56; Ɛ, 0.60, 0.80; ζ, 0.20, 0.28.   
 Description. Size moderate for the genus, body slender. Trunk (Figure 3A, B) elongate, attenuate, completely segmented, posterior rims inflated; lateral processes glabrous, 1.5 times as long as their basal width; first and second, second and third lateral process separated by wide intervals that are more than eight times the width of the corresponding crurigers; third and fourth lateral process separated by four times cruriger width. Ocular tubercle dome-shaped, about as high as its basal width, with a low rounded apical point above four pigmented eyes, each eye consisting of two subtle bulges. Proboscis (Figure 3A– C) club-shaped, in male (female in parentheses) 0.7–0.8 (0.8) times as long as trunk, curved downward; proximal narrow stalk 0.81–0.92 (0.64) times length of inflated part, with a dorsal tooth at about 0.44–0.50 (0.43) of total length from posterior of inflated part; lips rounded. Abdomen tiny, located on ventral side, completely hidden from dorsal view.   
Palps (Figure 3D) very slender; 10-segmented, with two short basal segments; first segment glabrous, about twice as wide as other segments; second segment shortest; third segment longest, 2.1 times length of fifth segment, straight, with sparse short setae, with a few longer setae ventrodistally; fourth segment almost as long as first and second combined; sixth segment 0.68–0.69 the length of seventh segment (0.73 in female); seventh to ninth segments equal in length; tenth segment rather longer than sixth segment; distal five segments with many ventral setae and a few dorsal setae.  
Oviger (Figure 3E) very slender, 10-segmented, mounted at anterior edge of cephalon; fourth and sixth segments longest, subequal; first to third segments subequal, only second segment with a few tiny setae; fifth segment almost as long as second and third combined; strigilis (Figure 3F) curved endally, armed with sparse setae ectally, with rows of slender endal spines having serrated margins in distal half; terminal segment armed with a wider, denticulate leaf-like spine; terminal claw short, about one-fifth the length of terminal segment.  
 Legs (Figure 3G) slender; first to third coxae subequal; femur longest, slightly curved endally, swollen distally, with tiny setae scattered sparsely over nearly entire surface; tibiae straight; first tibia slightly shorter than femur; second tibia 0.8 times femur length; tarsus slightly shorter than propodus, with tiny setae dorsally and ventrally, propodus with dorsodistal brush of six or seven setae; tarsus ratio 77–88% (78–86% in female) (sensu Stock 1958), the length ratio of claw to propodus in all legs 0.69–0.85 (0.75–0.84 in female).   
 Remarks. Our male specimens agree well with the redescription of Rhopalorhynchus tenuissimum (Haswell, 1884) by Staples (1982) in the position of the dorsal tooth on the proboscis, in the length ratio of the stalk to the inflated part of the proboscis, in the tarsal ratio (sensu Stock 1958), and in the shape of the ocular tubercle and the distal spine of the strigilis. Our female is larger than the males (female trunk length, 8.0 mm; males, 5.9 mm and 6.7 mm), as was the case with Staples’ specimens.   
 Distribution. Rhopalorhynchus tenuissimum has previously been recorded from Port Denison (type locality), Calliope, and Geoffrey Bay, eastern Australia (Haswell 1884; Staples 1982; Arango 2003). Our record extends the known range of R. tenuissimum considerably to the north; it is the first record from Japan, not only for the species but also for the genus, and represents the northernmost extent of the genus in the Pacific Ocean.
description: Published as part of Takahashi, Yoshie, Dick, Matthew H. & Mawatari, Shunsuke F., 2007, Sea spiders (Arthropoda: Pycnogonida) from waters adjacent to the Nansei Islands of Japan, pp. 61-79 in Journal of Natural History 41 (1 - 4) on pages 67-69, DOI: 10.1080/00222930601121783, http://zenodo.org/record/5229875