Discorsotheres, Ahyong, 2018

Ahyong, Shane T., 2018, Revision of Ostracotheres H. Milne Edwards, 1853 (Crustacea: Brachyura: Pinnotheridae), Raffles Bulletin of Zoology 66, pp. 538-571 : 553

Discorsotheres new genus

Diagnosis. Female: Carapace weakly sclerotised, glabrous, cuticle thin, soft to firm; subquadrate in dorsal view, slightly wider than long, widest posteriorly, glabrous; longitudinally vaulted; front usually slightly produced anteriorly, subtruncate to broadly rounded in dorsal view; anterolateral margins defined, slightly produced anteriorly, forming bluntly rounded rim; dorsal surface smooth to slightly rugose, regions undefined, gently raised medially, forming low to distinct broad, blunt median prominence. Eyes small, usually visible in dorsal view. Antennal flagellum not reaching beyond mid-height of eye. Maxilliped 3 ischiomerus completely fused, without trace of suture; palp 2-segmented; carpus shorter than propodus; propodus spatulate; exopod flagellum 1-segmented, distally setose. Chelipeds equal, slender, smooth, largely glabrous; carpus length about twice width; dactylus and pollex occlusal surfaces broadened in distal half, forming flattened, slightly hollowed occlusal surfaces, margined by low, thin, corneous rim and/or minute denticles. Pereopod 2 and pereopods 4–5 symmetrical from left to right, similar, subcylindrical to ovate in cross-section; dactyli falcate, with simple spiniform apices. Pereopods 3 asymmetrical in length and dactyl-propodal form, usually left side the longer; shorter pereopod 3 similar to pereopod 2; longer pereopod 3 with propodus usually slightly widened distally, dactylus less curved and more elongate than on other side. Pereopod 5 articulating with body at level of pereopod 4. Abdomen with 6 free somites and telson; in adults, widest at somites 4 and 5, covering bases of walking legs. Gonopore simple, ovate, on sternite 5 near mesial end of sternite 5/6 suture. Male: unknown.

Type species. Pinnotheres subglobosus Baker, 1907 , by present designation.

Etymology. An arbitrary combination of of the Latin, discors, at variance with, and the suffix - theres, alluding to left-right asymmetry of pereopod 3. Gender: masculine.

Composition. Discorsotheres camposi new species, D. subglobosus ( Baker, 1907) new combination, D. spondyli ( Nobili, 1905) new combination, D. subquadratus ( Sakai, 1939) new combination.

Remarks. Species herein referred to Discorsotheres new genus are removed from Ostracotheres on the basis of the subquadrate (versus rounded) carapace, the slender (versus robust) chelipeds with broadened occlusal surfaces, and asymmetrical pereopod 3 (versus symmetrical in length and structure). An asymmetrical pereopod 3 is also known from Amussiotheres Ng & Ho, 2016 (Indo-West Pacific), Enigmatheres Campos, 2009 (eastern Pacific), some American species currently assigned to Fabia Dana, 1851 (see Campos, 1996), Solenotheres Ng & Ngo, 2010 ( Vietnam) , and Zaops Rathbun, 1900 (western Atlantic) – all genera with a 3-segmented maxilliped 3 palp. Among pinnotherids with a 2-segmented maxilliped 3 palp, however, pereopod 3 asymmetry is unique to Discorsotheres . In Discorsotheres , the pereopod 3 of one side (usually the right) is similar in form to pereopods 2 and 4. The opposite pereopod 3, however, is typically more elongated, with the propodus usually slightly expanded distally and with the dactylus more elongated and with a less curved, sometimes reduced tip. The flexor margin of the propodus and dactylus of the longer pereopod 3 is distinctly more setose than on the opposite pereopod 3 (e.g., Fig. 9H–M , 10H, L, P, Q ). The function of such asymmetry is not known, although a similar asymmetry is also exhibited by Solenotheres . Aside from the 2- rather than 3-segmented maxilliped 3 palp, the proportionally narrower carapace, and broadened rather than narrow cheliped finger tips, Discorsotheres is very similar to Solenotheres . Although Zaops (host Crassostrea virginica ( Gmelin, 1791) : Ostreidae ) and Amussiotheres (host Amusium spp. , Ylistrum japonicum ( Gmelin, 1791) : Pectinidae ) also have an asymmetrical pereopod 3, the asymmetry is expressed principally in significant elongation of the leg of one side, with a straight, largely glabrous dactylus; both of these genera have a 3-segmented maxilliped 3 palp and show little similarity to either Discorsotheres or Solenotheres . All species of Discorsotheres occur in the Indo-West Pacific and have discrete distributions in the Persian Gulf ( D. spondyli ), southern Australia ( D. subglobosus ), northeastern Asia ( D. subquadratus ) and the tropical western Pacific ( D. camposi ) ( Fig. 8 ).

Identification of species of Discorsotheres relies in part on the dorsal carapace ornamentation. Owing to the soft carapace of most species of the genus, poor preservation can overaccentuate the prominence of the median swelling, so care should be taken in identifying such specimens.

Figures

Fig. 9. Discorsotheres subglobosus (Baker, 1907) new combination. A–G, female lectotype, cl 10.6 mm, cw 12.4 mm, Gulf St. Vincent, AM P151; H–I, ovigerous female paralectotype, cl 8.7 mm, cw 9.9 mm, Gulf St. Vincent, AM P102300; J–K, ovigerous female, cl 8.6 mm, cw 10.5 mm, Edithburgh, AM P64673; L–M, ovigerous female, cl 7.3 mm, cw 8.9 mm, Edithburgh, AM P64673; N–O, females, cl 7.8 cw 8.4, cl 7.8 cw 9.5 mm, Seacliff reef, SAM C12812. A, dorsal habitus; B, carapace, anterior view; C, carapace, right lateral view; D, cephalothorax, anterior view; E, right maxilliped 3; F, right chela, anterior view; G, right cheliped pollex, oblique anterior view; H, J, L, left pereopod 3; I, K, M, right pereopod 3; N, O, carapace frontal margin, dorsal view. Scale: A–C, H–M = 4.0 mm; D–F, N, O = 2.0 mm; G = 1.0.

Fig. 10. Discorsotheres camposi new species. A–M, female holotype, cl 10.8 mm, cw 10.3 mm, Tonga, ZRC 2003.0299; N–Q, ovigerous female paratype, cl 12.9 mm, cw 14.0 mm, Lifou, MNHN-IU-2017-11712; R–T, ovigerous female paratype, cl 15.3 mm, cw 16.4 mm, Caroline Islands, USNM 104989; U–W, ovigerous female paratype, cl 14.9 mm, cw 17.5 mm, Caroline Islands, USNM 104989. A, dorsal habitus; B, carapace, anterior view; C, carapace, right lateral view; D, cephalothorax, anterior view; E, right maxilliped 3; F, right chela, anterior view; G–I, left pereopods 2–4; J–M, right pereopods 2–5; N, left cheliped pollex, oblique anterior view; O, R, U, carapace anterior margin, dorsal view; P, S, V, left pereopod 3 dactylus & propodus; Q, T, W, right pereopod 3 dactylus & propodus. Scale: A–C, G–M, O–W = 2.0 mm; D–F = 1.0 mm; N = 0.5 mm.

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.