Rhopalorhynchus sibogae Stock 1958

Arango, Claudia P., 2009, New species and new records of sea spiders (Arthropoda: Pycnogonida) from deep waters in Western Australia, Zootaxa 1977, pp. 1-20 : 3-5

Rhopalorhynchus sibogae Stock 1958

( Figure 2 )

Rhopalorhynchus sibogae — Stock 1958: 114, 117, 124–125. — Child 1975: 7.

Material: AM P72804, 1female. 4 December 2005, Kalbarri, Western Australia, CSIRO station 096-025, collected by Sherman sled from 27.8126°S 113.2990°E to 27.8080°S 113.2970°E, soft bottom, 123m. AM P72807, 1 male. 7 December 2005, Carnarvon, Western Australia, CSIRO station 120-049 collected by Sherman sled from 112.6660°S 24.6194°E to 112.6660°E 24.6228°S, hard bottom, 100 m.

Remarks: Rhopalorhynchus sibogae had been reported from Australia only once ( Child 1975). I have compared these two specimens to the two juveniles in Child (1975) deposited at Western Australian Museum and, based on the proboscis, palps and leg configurations, they can be all attributed to the same species. The four specimens agree in the position of the proboscis tooth at 31–37% along the inflated part ( Figure 2 B–D), and the proportion of the sixth:seventh palp segments being about 50% ( Figure 2 E, F), in agreement to the type material ( Stock 1958). Strangely, these observations differ from the report in Child (1975), as he described the proboscis tooth on 24–26% the distance along the inflated part and the sixth-seventh palp segments proportion as 93%; the reason for this inconsistency is unclear. Tarsal ratio and length of the propodal claw are in agreement in the four specimens examined. Rhopalorhynchus sibogae and R. tenuissimum ( Haswell 1884) are very closely related species, if not the same, as there is an overlap in many of the diagnostic characters of the two species, at least based on the very few individuals available for measurements and according to the re-description in Staples (1982) and Takahashi et al. (2007). The main aspects that can be used to attempt differentiation of the two species are the position of the proboscis tooth, at near 44–50% the distance of the inflated part in R. tenuissimum ( Figure 2 A) and between 30–40% in R. sibogae ( Figures 2 B–D), the sixth and seventh palp segments ratio which is 0.65–0.70 in R. tenussimum and 0.5 in R. sibogae ( Figure 2 E, F), and the main claw: propodus ratio, 69–85% in R. tenuissimum , 100% in R. sibogae .

The species of this genus are very homogeneous in their morphology and only subtle variations have been used to recognise species, at the same time stating that there is clear intraspecific variation of most characters depending on age and gender ( Stock 1958). In Australia, R. sibogae is known from Point Cloates to Rottnest Island in WA and R. tenuissimum is known from localities in Queensland on the Eastern coast. The two specimens reported herein are within the range of distribution of R. sibogae .

Figures

FIGURE 2. Comparison of diagnostic characters in Rhopalorhynchus sibogae and R. tenuissimum. A. Proboscis of juvenile of R. tenuissimum, QM S 910 (dash line shows where it is broken) in Staples (1982). B. Proboscis of juvenile of R. sibogae WAM 70 - 3942 in Child (1975). C. Proboscis of female R. sibogae AM P 72804 in this study. D. Proboscis of male R. sibogae AM P 72807 in this study. E. Palp of female R. sibogae AM P 72804. F. Palp of juvenile R. sibogae WAM 70 - 3942.