Orthotheres Sakai 1969

Orthotheres Sakai, 1969: 275.

Type species. Orthotheres turboe Sakai, 1969, by original designation.

Diagnosis. Female carapace transversely ovate, distinctly wider than long, frontal margin slightly extending beyond orbits, just visible from dorsal view; male carapace rounded, slightly wider than long, with distinct front; MXP3 positioned obliquely in buccal cavity, ischium and merus completely fused, elongate, carpus of palp subovate, articulating on subventral margin, propodus elongate, subcylindrical, dorsodistal margin extends beyond articulation with dactylus, dactylus inserted subterminally on propodus; P2–P5 short, bilaterally symmetrical, P5 shortest, dactylus short, distinctly hooked, P3 and P4 dactylus with ventral surface prominently excavated; male abdomen with 6 free somites and telson.

1 Lee Kong Chian Natural History Museum, National University of Singapore, 2 Conservatory Drive, Singapore 117377, Republic of Singapore; Email: nhmngkl@ nus.edu.sg (* corresponding author)

2 Department of Environmental Biology and Fisheries Science,National Taiwan Ocean University, 2 Pei-Ning Rd., Keelung 202, Taiwan, R.O.C.

© National University of Singapore

ISSN 2345-7600 (electronic) | ISSN 0217-2445 (print)

Remarks. Ng et al. (2008: 250) recognised eight species in Orthotheres Sakai, 1969 (see also Ahyong & Ng, 2007). Five of these species occur in the western Pacific: O. glaber (Bürger, 1895) (= Pinnotheres impressus Bürger, 1895) [Philippines]; O. haliotidis Geiger & Martin, 1999 [Australia]; O. laevis (Bürger, 1895) [Philippines]; O. longipes (Bürger, 1895) [Philippines]; and O. turboe Sakai, 1969 [Japan and Palau]; with four others in the Americas: O. barbatus (Desbonne, in Desbonne & Schramm, 1867) [West Indies]; O. serrei (Rathbun, 1909) [Puerto Rico]; O. strombi (Rathbun, 1905) [Florida]; and O. unguifalcula (Glassell, 1936) [Mexico]. Campos (1989), however, had earlier removed O. laevis and O. longipes from the genus and referred them back to Pinnotheres, commenting that “… unlike Orthotheres, the female’s carapace of these species are not appreciably broader than long, and the dactylus of the walking legs are not uniformly very short and sharply hooked.” (Campos, 1989: 1125). He also suggested that Pinnotheres glaber (Bürger, 1895) and Pinnotheres impressus Bürger, 1895, were close to O. laevis and O. longipes. Ahyong & Ng (2007) argued that because of the unusual structure of the MXP3 palp, it seemed best to retain Pinnotheres laevis and Pinnotheres longipes, as well as Pinnotheres glaber (which they regarded as a senior synonym of Pinnotheres impressus) in Orthotheres for the time being. Campos (1989) is certainly correct in his assertion that Orthotheres is not monophyletic.

Orthotheres turboe (type species of the genus), O. haliotidis and O. baoyu n. sp. form a tight group that should be recognised as Orthotheres s. str. Members of this genus are characterised by possessing a carapace which is transversely ovate and much wider than long, the females have a distinct front that is not prominently deflexed and is still visible from dorsal view; the MXP3 ischiomerus is subrectangular to subovate with the mesial margin gently concave; the MXP3 has a subcylindrical and elongate propodus with its dorso-distal margin extending beyond the articulation with the dactylus which is inserted subterminally on it; the ambulatory dactylus is strongly curved (almost at right angles) and prominently hook-like (Figs. 3A, B, D–H, J–M; 6A–K; 9); and the dactylus of P3 and P4 is relatively shorter and broader, with the ventral surface prominently concave (Figs. 3E, G; 6J; 9E). The concave ventral surfaces of the P3 and P4 dactyli are more pronounced in female specimens. The dactylus on the other legs is flattened laterally, but there is not prominent concavity on the ventral surface. In O. haliotidis and O. baoyu n. sp., the MXP3 dactylus insertion on the cylindrical propodus is clearly subterminal (Figs. 2B, C, G; 5 B–E; 8D–F). The condition in O. turboe is the same (Sakai, 1969: fig. 19a) although his figure is not very precise. The problem is that in these species, the carpus of the palp is not inserted at the margin of the ischiomerus but actually submarginally on the inner surface (Sakai, 1969: fig. 19a). As such, it is not easy to draw the palp accurately with the surfaces flat.

A second group contains the American species: O. barbatus, O. serrei, O. strombi and O. unguifalcula, all of which have female carapaces that are also distinctly wider than long and the front strongly deflexed such that it is not visible from dorsal view (cf. Rathbun, 1918: pl. 19 figs. 1–4, 9, 11, pl. 20 figs. 1, 2; Campos, 1989: fig. 2). The mesial margin of the MXP3 ischiomerus of these species is also deeply concave, and the MXP3 propodus is clearly short and subovate or subconical rather than elongate, with the dactylus inserted terminally (cf. Rathbun, 1905: unnumbered second figure; Rathbun, 1909: 69, unnumbered figure; Rathbun, 1918: figs. 41, 44a, 45; Campos, 1989: figs. 1b, 3a). The MXP3 palp of O. unguifalcula figured by Glassell (1936: pl. 21 fig. 2) is inaccurate and it actually has three rather than two articles (see Campos, 1989: fig. 3a). The MXP3 ischiomerus of O. barbatus figured by Rathbun (1918: fig. 44a) is rather short but appears to be damaged. The ambulatory dactyli of all four species are also gently curved and not strongly hooked (cf. Rathbun, 1918: pl. 19 figs. 1–4, 8–11, pl. 20 figs. 1, 2; Campos, 1989: fig. 2).

There is a third group for the remaining West Pacific species. As re-described and figured by Ahyong & Ng (2007: fig. 24), O. glaber (= O. impressus) differs markedly from other Orthotheres in having a rounded carapace, the female frontal margin is visible from dorsal view, and the ambulatory legs (notably the dactyli) are asymmetrical, and while curved, are not prominently hooked. The carapace shape and P2–P5 characters are more typical of species of Arcotheres Manning, 1993, but the form of the palp is clearly different. Orthotheres laevis and O. longipes also share the same set of characters as O. glaber; and all three species probably belong to their own group (cf. Bürger, 1895: pl. 9 figs. 23–25, 31; pl. 10 figs. 21–24; Ahyong & Ng, 2007: fig. 3E, F). The MXP3 dactylus is inserted subterminally in O. glaber (Ahyong & Ng, 2007: fig. 24C, F); but appear to be more terminal in position for O. laevis and O. longipes (cf. Bürger, 1895: pl. 10 figs. 22, 24).

While Orthotheres s. str. can be defined relatively easily; the other two groups of species previously referred to this genus will need to referred to new genera in the future. Specimens of these species are now being examined, and new genera will need to be established for them once this study is complete.