Discorsotheres subquadratus ( Sakai, 1939 ), 2018

Ahyong, Shane T., 2018, Revision of Ostracotheres H. Milne Edwards, 1853 (Crustacea: Brachyura: Pinnotheridae), Raffles Bulletin of Zoology 66, pp. 538-571 : 561-563

Discorsotheres subquadratus ( Sakai, 1939)

( Figs. 8 , 12 )

Ostracotheres spondyli . — Sakai, 1933: 981, fig. 5a, b. (Not O. spondyli Nobili, 1905 ).

Ostracotheres subquadrata Sakai, 1939: 596–597 , fig. 82. — Sakai, 1940: 57. — Cheng, 1967: 333. — Schmitt et al., 1973: 6, 8, 29.

Ostracotheres subquadratus . — Sakai, 1965: 179, pl. 87 fig. 4. — Sakai, 1976: 577, fig. 316, pl. 201 fig. 4. — Pregenzer, 1988: 24, 27. — Takeda & Konishi, 1989: 1222. — Marumura & Kosaka, 2003: 67. — Ng et al., 2008: 250.

Ostracotheres subglobosus . — Konishi, 1996: 15. — Yang et al., 2008: 808. (Not O. subglobosus [ Baker, 1907]).

Type material. Neotype: NSMT Cr 10028, female (cl 7.1 mm, cw 8.6 mm), Osezaki, Suruga Bay, Japan, 35°00′N, 138°47′E, coll. I. Soyama, 25 October 1988.

Other material examined (all Japan). NSMT Cr 1036, 1 female (cl 7.2 mm, cw 8.2 mm), Ohseto-cho , Nagasaki, coll . T. Habe; CBM ZC5989 , 1 ovigerous female (cl 7.5 mm, cw 9.2 mm), off Hota, Boso Peninsula, 20–30 m, from Spondylus cruentus Lischke, 1868 , gill net, coll . T. Komai, 19 August 1998 .

Description. Female: Carapace ( Fig. 12A–C ) soft, thin, rounded-subquadrate, slightly wider than long, glabrous, surface smooth, appearing polished; strongly vaulted longitudinally, evenly rounded in lateral view; front broadly rounded to straight, transverse, slightly produced, with or without shallow transverse groove on dorsal surface immediately behind front ( Fig. 12A, E, J ); anterolateral margins, unarmed, forming bluntly rounded rim; lateral surface almost vertical; dorsal surface smooth, gently raised medially forming broad, low, rounded ridge.

Epistome ( Fig. 12D ) with narrow interantennular septum; median buccal margin broadly obtuse. Antennular sinus size subequal to orbit, aligned distinctly obliquely in anterior view; antennules folded obliquely. Antenna short, free antennal articles extending to about one-third height of eye; antennal articles 1 and 2 fused to epistome. Eyes visible in dorsal view, filling orbit, cornea pigmented.

Maxilliped 3 ( Fig. 12F ) ischiomerus surface glabrous, length about twice width; inner proximal two-thirds sinuous to weakly concave, distomesial angle obtuse, blunt; distal margin not produced beyond palp articulation; outer margin convex. Carpus slightly shorter than half propodus length. Propodus spatulate, length about twice width, distally widened, apex blunt, subtruncate. Exopod margins gently convex.

Cheliped (pereopod 1) ( Fig. 12A, G, H ) dactylus gently curved to straight, pollex relatively straight, apices crossing distally, without gape, irregularly, setose. Dactylus 0.8–0.9 × length of dorsal margin of propodus palm, outer occlusal margin with triangular proximal tooth, and row of short corneous denticles and short setae extending to about distal three-fourths of dactylus, otherwise smooth; inner occlusal margin with row of short corneous denticles and short setae along proximal half to two-thirds. Pollex ( Fig. 12H ) outer occlusal margin weakly crenulate, with small, 2 blunt triangular proximal teeth; inner occlusal margin setose, with row of small denticles along proximal three-fourths; inner ventral margin with row of setae. Propodus palm dorsal margin length 1.7–2.1 × height; ventral margin gently sinuous, slightly concave at base of pollex. Carpus mesial margin with setal tuft, unarmed. Merus unarmed, about as long as propodal palm.

Walking legs (pereopod 2–5) slender, smooth, similar in form ( Fig. 12A, J–M ); relative lengths: pereopod 3(both)>pereopod 4>pereopod 2>pereopod 5. Pereopods 2, 4, 5 merus to dactylus unarmed, glabrous, propodus occasionally with few scattered setae distally; propodus flexor and extensor margins subparallel, not widening distally; dactyli stout, length subequal, half propodus length, strongly falcate, apices spiniform, those of pereopods 4–5 turning perpendicular to main axis. Pereopod 3 asymmetrical in length and dactylpropodal form; “normal” pereopod 3 with merus to dactylus glabrous; dactylus apex spiniform, strongly falcate, similar to but slightly longer than Pereopod 2 dactylus. Longer pereopod 3 1.1–1.2 × length of “normal” pereopod 3; merus 1.4 × length of pereopod 4 merus; propodus usually slight expanded distally, distoflexor margin irregularly setose; dactylus longer, stouter but with slightly shorter apex than dactylus of opposite side, setose.

Egg diameter 0.2–0.3 mm.

Hosts. Bivalve molluscs: Crassostrea nippona ( Seki, 1934) (Ostreidae) and Mytilus edulis Linnaeus, 1758 (Mytilidae) ( Sakai, 1976), and Spondylus cruentus Lischke, 1868 (Spondylidae) (this study).

Remarks. Sakai (1933) reported material from Momotori, Ise Bay, Japan, as Ostracotheres spondyli Nobili, 1905 , recognising it as new species in 1939. Sakai (1939) described D. subquadratus based on the holotype and two other females, all from Momotori, distinguishing his new species from the Australian D. subglobosus by the supposedly shorter carpus and distally broader propodus of maxilliped 3, and elongate dactylus of pereopod 3. Pregenzer (1988), however, observed Sakai’s (1939) diagnostic features to be invalid and synonymised the two species because the pereopod 3 dactylus is elongated in both species and the maxilliped 3 indistinguishable. Present comparison of Australian and Japanese material, however, revealed additional distinguishing features separating two, albeit similar species. The Japanese form is here referred to D. subquadratus on the basis of geography. Unfortunately, the two species are indistinguishable based on Sakai’s (1939) account and brief figures of the holotype and his original specimens may have been lost from early on. Sakai (1965) lists specimens of D. subquadratus , but none from Ise Bay, and his 1976 monograph cites specimens from Ise Bay, but only as “reported in 1939” suggesting that he did not have had access to the material. Sakai’s carcinological collections are distributed among several institutions/collections ( Ng et al. 2017), but searches of Japanese and overseas museum collections over the past 15 years, including the Natur- Museum und Forschungsinstitut Senckenberg, Frankfurt am Main, which now holds much of Sakai’s material, failed to locate any of his original specimens of D. subquadratus ; they are considered lost. Given the strong similarity between D. subquadratus and D. subglobosus , and inadequacy of the type description to distinguish the two species, a neotype is herein designated to fix the identity of D. subquadratus . In the absence of specimens from the original type locality (Ise Bay, Japan), an ovigerous female from the adjacent Suruga Bay (cl 7.1 mm, cw 8.6 mm, NSMT Cr10028), approximately 170 km east of the original type locality, is selected as the neotype. Features distinguishing Ostracotheres subquadratus from O. subglobosus are discussed under the account of the latter.

The present specimens of D. subquadratus agree well in most respects. Pereopod 3 is longer on the left side in two specimens, and longer on the right in one (CBM ZC5989; Fig. 12J, L ); the specimens otherwise differ only in the presence of a shallow transverse groove or crease on the carapace immediately behind the frontal margin in two specimens (NSMT Cr10028, CBM ZC5989; Fig. 12A, E, I ), absent in the third (NSMT Cr1036). All examined specimens of D. subquadratus are mature but the largest (cl 7.5 mm, cw 9.2 mm; CBM ZC5989) is notably smaller than the largest recorded D. subglobosus (cl 10.6 mm, cw 12.4 mm; AM P151). Comparatively few records of D. subquadratus are known, so further sampling is required to determine whether these size differences are artefacts or actual.

Yang et al. (2008) listed Ostracotheres subglobosus from the Nansha Islands, South China Sea, presumably following Pregenzer (1988) in treating O. subquadratus as its synonym. The record requires verification but given that both nominal species are herein regarded as valid, the Nansha Islands record is here referred to D. subquadratus .

Distribution. Presently known from localities around Japan (Sagami Bay, Suruga Bay, Ise Bay, Onomichi, Nagasaki), and the Nansha Islands, South China Sea; 10–30 m (Marumara & Kosaka, 2003; present study).

Figures

Fig. 8. Distribution of species of Discorsotheres new genus. (■) D. camposi new species; (●) D. spondyli (Nobili, 1905) new combination; (▲) D. subglobosus (Baker, 1907) new combination; (▼) D. subquadratus (Sakai, 1939) new combination.

Fig. 12. Discorsotheres subquadratus (Sakai, 1939) new combination. A–H, female neotype, cl 7.1 mm, cw 8.6 mm, Suruga Bay, NSMT Cr10028; I–M, ovigerous female, cl 7.5 mm, cw 9.2 mm, Boso Peninsula, CBM ZC5989. A, dorsal habitus; B, carapace, anterior view; C, carapace, right lateral view; D, cephalothorax, anterior view; E, I, carapace frontal margin, dorsal view; F, right maxilliped 3; G, right chela, anterior view; H, right cheliped pollex, oblique anterior view; J–K, left pereopods 3–4; L–M, right pereopods 3–4. Scale: A–C, J–M = 2.0 mm; D–I = 1.0 mm.