Discorsotheres spondyli ( Nobili, 1905 ), 2018

Ahyong, Shane T., 2018, Revision of Ostracotheres H. Milne Edwards, 1853 (Crustacea: Brachyura: Pinnotheridae), Raffles Bulletin of Zoology 66, pp. 538-571 : 559-561

Discorsotheres spondyli ( Nobili, 1905)

( Figs. 8 , 11 )

Ostracoteres spondyli Nobili, 1905: 164 . — Nobili, 1906b: 300, 301; 1906c: 149–150, pl. 5 fig. 24, pl. 6 fig. 33.

Ostracotheres spondyli . — Tesch, 1918: 262, 263, 287. — Pérez, 1920: 1615. — Stephensen, 1945: 185–186, 198, 210. — Schmitt et al., 1973: 6, 29. — Page, 1985: 199. — Takeda & Konishi, 1989: 1222. — Springthorpe & Lowry, 1994: 113. — Ng et al., 2008: 250. — McDermott, 2009: tab. 1, 2. — An et al., 2014: tab. 1. — Naderloo, 2017: 423–424, fig. 38.3.

Type material (all Persian Gulf, Abu Dhabi). Lectotype: AM P5412 , ovigerous female (cl 7.5 mm, cw 8.8 mm), bank NE of Arzana Island , collections among corals, inside Spondylus , stn LIII, coll. J. Bonnier & Ch. Perez, 1901 . Paralectotypes: MNHN IU-2014-8621 (B10577), 32 females (cl 3.2 mm, cw 3.1 mm to cl 10.0 mm, cw 10.5 mm), 8 miles NNW of Arzana Island , 5 fms [9 m], stn LIV, in Spondylus , dredge, coll. J. Bonnier & Ch. Perez, 1901; MNHN IU-2014-11993 (B10576), 80 females (cl 2.0 mm, cw 2.4 mm to cl 9.5 mm, cw 10.5 mm), bank NE of Arzana Island , collections among corals, inside Spondylus , stn LIII, coll. J. Bonnier & Ch. Perez, 1901; RMNH D22681, 1 female (not measured), 8 miles NNW of Arzana Island , 5 fms [9 m], stn LIV, in Spondylus , dredge, coll. J. Bonnier & Ch. Perez, 1901.

Description. Female: Carapace ( Fig. 11A–C ) soft, thin, rounded-subquadrate, slightly wider than long, glabrous, surface smooth, appearing polished; strongly vaulted longitudinally, evenly rounded in lateral view; front weakly convex to straight, not produced; anterolateral margins, unarmed, forming bluntly rounded rim; lateral surface almost vertical; dorsal median surface without low, rounded longitudinal ridge, evenly outline evenly rounded in anterior view.

Epistome ( Fig. 11D ) with narrow interantennular septum; median buccal margin broadly obtuse. Antennular sinus slightly larger than orbit, aligned distinctly obliquely in anterior view; antennules folded obliquely. Antenna short, free antennal articles extending to about one-third to almost one-half height of eye; antennal articles 1 and 2 fused to epistome. Eyes visible in dorsal view, filling orbit, cornea pigmented.

Maxilliped 3 ( Fig. 11E ) ischiomerus surface glabrous, length about twice width; inner proximal two-thirds weakly concave, distomesial margin produced mesially, evenly rounded, not forming angle; 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 to slightly sinuous.

Cheliped (pereopod 1) ( Fig. 11A, F, G ) dactylus gently curved to straight, pollex relatively straight, apices crossing distally, without gape, irregularly, setose. Dactylus about 0.8 × length of dorsal margin of propodus palm; outer occlusal margin with triangular proximal tooth and row of short corneous denticles and short setae extending length of margin; inner occlusal margin with row of short short setae, margin smooth. Pollex ( Fig. 11G ) outer occlusal margin weakly crenulate, with blunt triangular and rounded proximal teeth; inner occlusal margin setose, weakly dentate along proximal threefourths; inner ventral margin with row of setae. Propodus palm dorsal margin length 1.9–2.0 × 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 (pereopods 2–5) slender, smooth, similar in form ( Fig. 11A ); relative lengths: pereopod 3(both)>pereopod 4>pereopod 2>pereopod 5. Pereopods 2, 4, 5 merus to dactylus unarmed, glabrous; propodus flexor and extensor margins subparallel, not widening distally; dactyli stout, length subequal, half propodus length, apices spiniform, pereopod 2 dactylus moderately to strongly falcate, pereopod 4–5 dactyli strongly falcate, apices turning perpendicular to main axis. Pereopod 3 asymmetrical in length and dactyl form, merus and carpus glabrous; “normal” pereopod 3 propodus setose on distal one-third or less of flexor margin; dactylus apex spiniform, falcate, similar to but slightly longer than pereopod 2 dactylus, flexor margin setose. Longer pereopod 3 1.1 × length of “normal” pereopod 3; merus 1.4–1.5 × length of pereopod 4 merus; propodus at most slightly expanded distally, distoflexor margin irregularly setose along distal two-thirds; dactylus longer, stouter but with slightly shorter, often blunter apex than dactylus of opposite side, setose.

Egg diameter 0.2–0.3 mm (in preservative).

Hosts. Bivalve molluscs: Spondylus sp. ( Nobili, 1905; Naderloo, 2017). Schmitt et al. (1973) regard the host species as S. exilis Sowerby, 1895 , on the basis of geographic distribution.

Remarks. Discorsotheres spondyli , known only from the Persian Gulf, differs from all other species of the genus in the evenly rounded rather than obtusely angled inner distal margin of the maxilliped 3 ischiomerus. It also has the simplest carapace ornamentation, having a smooth surface without a median longitudinal swelling ( Fig. 11A–C ). Note that the thin carapace cuticle in D. spondyli , if poorly preserved, can partially collapse creating a slight dorsomedian prominence, artificially resembling D. subglobosus or D. subquadratus . In D. spondyli and D. subglobosus , the flexor margin of the pereopod 2 dactylus is sparsely setose rather than glabrous as in D. subquadratus and D. camposi . The left pereopod 3 is longer than the right in about two-thirds of specimens examined.

The type material of D. spondyli examined here consists of 114 specimens from the vicinity of Arzana Island , Persian Gulf , in the collections of the AM, MNHN, and RMNH. One female is also deposited in the Zoological Survey of India (C253/1, not examined; Schmitt et al., 1973). A female (cl 7.5 mm, cw 8.8. mm; AM P5412 ) from station LIII is herein selected as the lectotype to fix the identity of the species. A strikingly abnormality in one paralectotype (female, cl 9.0 mm, cw 10.9 mm; MNHN IU-2014-8621 ; Fig. 11H ) is the presence of a dactylus on the left maxilliped 3. The right maxilliped 3 is normal, but the left side propodus is distally tapered and slightly excavated on the ventral margin to which a digitiform dactylus inserts near the midlength. The present abnormality is extraordinary given the significance of the maxilliped 3 palp at all levels of the pinnotherid system and the stability observed in its segmentation. The loss of the maxilliped 3 dactylus is a derived condition in pinnotherids, indicating the unusual presence of the right dactylus in the specimen of D. spondyli is atavistic .

Pérez (1920) described the bopyrid isopod parasite, Rhopalione uromyzon , from D. spondyli .

Distribution. Presently known only from the Persian Gulf; shallow water to at least 9 m.

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. 11. Discorsotheres spondyli (Nobili, 1905) new combination. A–G, ovigerous female lectotype, cl 7.5 mm, cw 8.8 mm, Persian Gulf, AM P5412; H, female paralectotype, cl 9.0 mm, cw 10.9 mm, Persian Gulf, MNHN IU-2014-11993. A, dorsal habitus; B, carapace, anterior view; C, carapace, right lateral view; D, cephalothorax, anterior view; E, left maxilliped 3; F, right chela, anterior view; G, right cheliped pollex, oblique anterior view; H, right and left maxillipeds 3. Scale: A–C = 2.0 mm; D = 0.7 mm; E, G, H = 0.5; F = 1.0 mm; G = 0.5 mm.