Ostracotheres H. Milne Edwards, 1853

Ahyong, Shane T., 2018, Revision of Ostracotheres H. Milne Edwards, 1853 (Crustacea: Brachyura: Pinnotheridae), Raffles Bulletin of Zoology 66, pp. 538-571 : 539

Ostracotheres H. Milne Edwards, 1853

Ostracotheres H. Milne Edwards, 1853: 219 (type species Pinnotheres tridacnae Rüppell, 1830 , subsequent designation by Schmitt et al., 1973: 28; gender masculine).

Diagnosis. Female: Carapace subcircular, about as long as wide; weakly sclerotised, firm but not hard; longitudinally vaulted, without epigastric ridge; front slightly produced anteriorly, subtruncate; anterolateral margins well defined, lateral surfaces near vertical; dorsal surface smooth, regions poorly defined, indistinct, covered with fine, short tomentum. Eyes visible in dorsal view. Antennal flagellum reaching to or beyond mid-height of eye but not dorsal margin of orbit. Maxilliped 3 ischiomerus completely fused, without trace of suture; palp 2-segmented; carpus shorter than propodus; propodus spatuliform; exopod flagellum 1- or 2-segmented, distally setose. Chelipeds equal, stout, robust, dactylus and pollex with simple, pointed tips. Pereopods 2–5 symmetrical from left to right, slightly compressed, ovate in cross-section; dactyli with simple, spiniform apices, subequal or pereopod 5 dactylus longer. Pereopod 5 articulating with body at same level as 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: similar to female, though smaller, carapace less rounded, lateral margins often flatter; G1 dorsoventrally compressed, weakly curved, apex blunt, with short, spine-like papilla directed anterolaterally.

Composition. Ostracotheres tridacnae ( Rüppell, 1830) , O. cynthiae Nobili, 1906 .

Remarks. Prior to the present study, Ostracotheres comprised a group of Indo-West Pacific species having a spatulate (versus conical) maxilliped 3 propodus and a ‘normal’ carapace, without marginal lamellae or other elaborate structures ( Campos, 1996). Reconsideration of all species in Ostracotheres revealed that the genus comprised several disparate groups. Ostracotheres is herein restricted to the type species, O. tridacnae , and O. cynthiae , both of which share a finely setose, relatively weakly sclerotised (firm but not hard) subcircular carapace that is about as long as wide with defined anterolateral margins and near vertical lateral surfaces, robust chelipeds with simple, pointed finger tips, walking legs with left–right symmetry, and a relatively straight G1 with a short, spine-like anterolaterally directed papilla. The remaining species formerly placed in Ostracotheres are referred to three new genera: Austrotheres , Discorsotheres and Latatheres . Given that the maxilliped 3 dactylus has been independently lost several times within Pinnotheridae , the new genera recognised for former members of Ostracotheres cannot a priori be assumed closely related. Of these, Ostracotheres s.s. most closely resembles Austrotheres from southern Australia in carapace form and pereopod morphology, but differs chiefly in more weakly sclerotised and finely setose carapace (versus hard and glabrous), and the G1 form, in which the apex is blunt, with a short, spine-like, anterolaterally directed papilla, rather than a long lanceolate tip aligned with the main axis of the G1. The G2 exopod is absent in O. tridacnae and both species of Austrotheres , but remains to be determined in O. cynthiae . Austrotheres may prove closely related to Ostracotheres , but Discorsotheres and Latatheres probably have their affinities elsewhere. Discorsotheres is readily separated from Ostracotheres by the subquadrate carapace, slender chelipeds with spatulate finger tips, asymmetrical pereopods 3, and Latatheres by its transversely ovate to subrectangular carapace. Unfortunately, males of neither Discorsotheres nor Latatheres are known.

Species of Ostracotheres are unusual in Pinnotheridae in the similarity in form and maximum body size between the sexes, in particular O. tridacnae . This contrasts strongly with the significant size and structural disparity between sexes usually seen in Pinnotheridae . Why this should be the case in Ostracotheres , especially O. tridacnae , is not known, although it may owe to the seemingly minimal constraints on male crab size within the large host Tridacna relative to smaller host bivalves of most other pinnotherids. Although few specimens of O. cynthiae are available for comparison, the sexes are similar and also live in “spacious” host ascidians (relative to the small body size of the crabs).

Among species of Ostracotheres or those formerly placed there, the larvae of only O. tridacnae are known. Gohar & Al-Kholy (1957) reported four zoeal and one megalopal stages in O. tridacnae ; notably, the maxilliped 3 palp and exopod flagellum are already 2-segmented in the megalopa. Species of Ostracotheres occur in the tropical western Indian Ocean and Indonesia ( Fig. 1 ).

Figures

Fig. 1. Distribution of species of Ostracotheres H. Milne Edwards, 1853. (▲) O. cynthiae Nobili, 1906; (●) O. tridacnae (Rüppell, 1830).