Ostracotheres tridacnae ( Rüppell, 1830 )

Ahyong, Shane T., 2018, Revision of Ostracotheres H. Milne Edwards, 1853 (Crustacea: Brachyura: Pinnotheridae), Raffles Bulletin of Zoology 66, pp. 538-571 : 540-544

Ostracotheres tridacnae ( Rüppell, 1830)

( Figs. 1–3 )

Crabes. — Savigny, 1826: pl. 7, fig. 1.1–5, a–e, m–o, i, j, u, v.

Pinnotheres veterum . — Audouin, 1826: 88. (Not P. veterum Bosc, 1802 ).

Pinnotheres Tridacnae Rüppell, 1830: 22–24 , pl. 5 fig. 2 (type locality: Red Sea)

Ostracotheres Savignyi H. Milne Edwards, 1853: 219 , pl. 11 fig. 10 (type locality: Red Sea). — Nobili, 1906b: 300.

Ostracotheres Tridacnae. — H. Milne Edwards, 1853: 219. — Heller, 1861a: 20, 32. — Heller, 1861b: 371. — Paulson, 1875: 70. — Kossmann, 1877: 62. — Schmeltz, 1881: 14.

Ostracotheres tridacnae. — Adensamer, 1897: 109. — Laurie, 1915: 415. — Tesch, 1918: 262, 287. — Balss, 1924: 14. — Calman, 1927: 215. — Ramadan, 1936: 36–37. — Gohar & Al-Kholy, 1957: 146, 153–160, 175, 176, pl. IV–VII. — Guinot, 1967: 279. — Schmitt et al., 1973: 6, 29–30. — Rablais & Gore, 1985: tab. 1. — Takeda & Konishi, 1989: 1222. — Guinot & Bouchard, 1998: 653. — Guinot et al., 2013: 134, 292, fig. 33B. — Ng et al., 2008: 250.

Pinnotheres tridacnae. — Doflein, 1904: 124, 210, 121, 226, 229, fig. 37.

Ostracoteres tridacnae . — Nobili, 1906b: 299–300. — Guinot, 1964: 13; 1979: 148–149, fig. 24D.

Ostracoteres savignyi . — Takeda & Konishi, 1989: 1222.

Not Pinnotheres Tridacnae. — Kraus, 1843: 21, 47. (= Afropinnotheres dofleini [Lenz in Lenz & Strunck, 1914]).

Not Pinnotheres (Ostracotheres) tridacnae . — Doflein, 1904: 282. (= Afropinnotheres dofleini [Lenz in Lenz & Strunck, 1914]).

Not Ostracotheres tridacnae. — Stebbing, 1910: 331. — Lenz & Strunck, 1914: 268, 282, 283 (Simons Bay, South Africa). — Barnard, 1950: 82. — Kensley, 1981: 48. — Emmerson, 2016: 414. (= Afropinnotheres dofleini [Lenz in Lenz & Strunck, 1914]).

Type material. Lectotype: SMF 2719 , male (cl 10.8 mm, cw 10.7 mm), Red Sea, from Tridacna elongata , coll. E. Rüppell, January–July 1826 . Paralectotypes: SMF 2720 , 2 males (cl 6.9 mm, cw 6.9 mm; cl 8.0 mm, cw 7.6 mm), 4 females (cl 9.4 mm cw 9.4 mm to cl 12.5 mm cw 13.0 mm), collected with lectotype .

Other material examined. UF37203–37204 , 1 male (cl 8.8 mm, cw 8.7 mm; BDJRS-3214), 1 spent female (cl 12.2 mm, cw 12.0 mm; BDJRS-3215), Tiger Head Island, Farasan Islands , Red Sea, 16°47.46′N, 42°11.92′E, 1–6 m, karstic shore to fringing reef, from Tridacna squamosina , SAFA- 023, coll. A. Anker , 10 March 2013; MNHN-IU-2017-8381 (B10583), 1 female (cl 8.0 mm, cw 8.1 mm), Abulat Island , Red Sea, 19°58′N, 40°07′E, from Tridacna , 0.5–1.0 m, Calypso, stn 9, 1952 ; NHMW 10066 , 3 males (cl 8.1 mm, cw 8.2 mm; cl 9.4 mm, cw 9.2 mm; cl 8.9 mm, cw 8.8 mm), 2 females (cl 10.7 mm, cw 10.7 mm, cl 11.0 mm, 11.0 mm), 1 ovigerous female (cl 10.1 mm, cw 10.0 mm), Noman Island , Red Sea, [27.11°N, 35.76°E], from Tridacna elongata , Pola Expedition ; NHMD-229248, 2 males (cl 12.1 mm, cw 12.1 mm; cl 12.5 mm, cw 12.6 mm), 2 ovigerous females (cl 15.1 mm, cw 14.7 mm; cl 15.4 mm, cw 15.2 mm), Ghardaqa , Red Sea, from Tridacna , low water springs, 4 August 1937 ; ZMB 20024, 3 females (cl 8.7 mm, cw 8.6 mm; cl 11.9 mm, cw 12.3 mm; cl 10.2 mm, cw 10.0 mm), Tor , Gulf of Suez, Red Sea, coll. Hartmeyer ; NHMW 10064 , 1 ovigerous female (cl 14.0 mm, cw 13.5 mm), Tor , Red Sea, AN: 1862.I.10, coll. Ransonnet ; NHMW 10065 , 1 male (cl 10.3 mm, cw 10.3 mm), 1 female (cl 12.2 mm, cw 11.9 mm), Tor , Red Sea, from Tridacna , Pola Expedition , 9 March 1896 ; MNHN- IU-2016-10946 (B10578), 6 males (cl 5.5 mm, cw 5.5 mm to cl 8.6 mm, cw 8.6 mm), 7 females (cl 9.2 mm, cw 9.0 mm to cl 14.6 mm, cw 14.4 mm), Suez, from branchial cavity of Tridacna , coll. Vaillant , 1864; MNHN-IU-2016-10947 (B10578), 1 female (cl 12.7 mm, cw 13.2 mm), same ; MNHN-IU-2016-10948 (B10578), 1 female (damaged), same ; MNHN-IU-2016-10949 (B10578), 1 female (cl 11.8 mm, cw 11.7 mm), same ; MNHN-IU-2016-10950 (B10578), 1 male (cl 8.0 mm, cw 7.6 mm), same ; MNHN-IU-2017-8382, B10560, 1 male (cl 10.3 mm, cw 10.3 mm), 1 female (cl 11.8 mm, cw 12.5 mm), Red Sea, from Tridacna elongata , coll. Jousseaume , 1903; MNHN-IU-2000-3069 (B3069), 3 females (not measured; dry), Red Sea, coll. M. Beaudouin , 864.66; MNHN-IU-2000-3066, 1 female (cl 12.1 mm, cw 11.3 mm; dry; lectotype of O. savignyi ), Red Sea, coll. M. Clot Bey , 864.66; MNHN-IU-2000-1120–1123, 4 males (cl 7.6 mm cw 8.1 mm to cl 9.5 mm cw 9.7 mm; dry; paralectotypes of O. savignyi ), Red Sea, coll. M. Clot Bey , 864.66.

Description. Female: Carapace ( Fig. 2A, H ) subcircular, about as long as wide, entire surface finely but densely tomentose; strongly vaulted longitudinally, rounded in lateral view; front slightly produced, subtruncate to weakly concave; anterolateral margins defined; dorsal surface smooth, regions weakly indicated, near absent.

Epistome ( Fig. 2I ) with broad triangular interantennular septum; median buccal margin with obtuse median point. Antennular sinus larger than orbit; antennules folded slightly obliquely. Antenna short, free antennal articles not extending dorsally beyond eye; antennal articles 1 and 2 fused to epistome. Eyes filling orbit, cornea pigmented.

Maxilliped 3 ( Fig. 2G ) ischiomerus length about 2.5 × width; surface finely setose; inner margin sinuous, proximal two-thirds weakly concave, distomesial margin rounded, produced slightly beyond palp articulation; outer margin strongly convex. Carpus half propodus length. Propodus spatulate, length about twice width, gently tapering to blunt, round apex. Exopod margins convex; flagellum 2-segmented. Cheliped (pereopod 1) ( Fig. 2A, B ) surface finely, densely tomentose. Dactylus and pollex relatively straight, apices crossing distally, without gape, irregular, setose. Dactylus longer than dorsal margin of propodus palm, occlusal margin with 1 or 2 small teeth. Pollex occlusal margin weakly crenulate, with small, low distal tooth and large, triangular proximal tooth; inner ventral margin with row of setae. Propodus palm dorsal margin length about 1.1–1.2 × height; ventral margin distinctly sinuous, concave at base of pollex. Carpus mesial margin with setal tuft, unarmed; merus unarmed, stout, shorter than propodus.

Walking legs (pereopods 2–5) similar ( Fig. 2C–F ) stout; relative lengths: pereopod 2≈pereopod 3>pereopod 4>pereopod 5; surfaces finely but densely tomentose; pereopod 3–4 propodus with scattered setae on flexor margin and row of long natatory setae near extensor margin, extending onto dorsal surface of carpus; pereopod 5 carpus usually with row of setae on extensor margin. Meri unarmed, length 3.5–3.8 × height (pereopod 2–4), about 3.0 × height (pereopod 5). Propodi unarmed, 2.7–2.8 × height. Dactyli similar, stout, sparsely setose, strongly curved, falcate, evenly tapering, apices spiniform, corneous; flexor margin with 1 or 2 rows of minute, stiff, simple setae; pereopod 2–4 dactyli 0.6–0.7 × propodus length; pereopod 5 dactylus 0.9 × propodus length.

Egg diameter 0.6–0.7 mm (in preservative).

Male: Similar to female but smaller maximum size ( Fig. 2J ); carapace slightly more flattened laterally; chelipeds slightly more inflated; pereopods 2–5 articles stouter than in female. Pereopod 2–4 dactylus 0.6 × propodus length; propodus length 2.0 × height; merus length 2.8–3.0 × height. Pereopod 5 dactylus 0.9 × propodus length; propodus length 1.7 × height; merus length 4.4 × height. Abdomen narrow ( Fig. 2K ), distally tapering, widest at somite 3; somite 6 trapezoid, slightly wider than long, longer than telson; telson as long as or slightly wider than long, apex bluntly rounded. G1 apex with short, conical, spine-like papilla, directed anterolaterally; distal surface lateral margin fully setose; mesial margin setose along distal half ( Fig. 2L, M ). G2 short, simple, about one-fifth G1 length; exopod absent ( Fig. 2N ).

Hosts. Giant clams, Cardiidae : Tridacna maxima ( Röding, 1798) (see Rüppell, 1830, as T. elongata Lamarck, 1819 ) and T. squamosina Sturany, 1899 . Ramadan (1936) and Gohar & Al-Kholy (1957) consistently observed single male-female pairs in the host clams.

Remarks. Rüppell (1830) hesitantly named Pinnotheres tridacnae as a new species from the Red Sea, noting the strong similarity to Savigny’s (1826: pl. 7, fig. 1.1–5) figures (incorrectly attributed by Audouin [1826] to Pinnotheres veterum Bosc, 1802 ), but apparently differing in having marginal setae on pereopod 5. Kraus (1843) went further and identified Savigny’s (1826) form as P. tridacnae . Subsequently, Henri Milne Edwards (1853) erected Ostracotheres and formally named O. savignyi , again based on Savigny’s (1826) figures, separating his new species from O. tridacnae on the setation of the pereopod 5 extensor margin (said to be absent in the former, present in the latter). Paulson (1875), however, synonymised O. savignyi with O. tridacnae , citing variable pereopod 5 setation in his Red Sea material. Most subsequent authors followed Paulson (see Schmitt et al., 1973) although Pregenzer (1988) resurrected O. savignyi , erroneously treating O. tridacnae as glabrous rather than tomentose. The present specimens, including the type series of O. tridacnae and O. savignyi , have a dorsal tomentum and a setose pereopod 5, corroborating Paulson’s (1875) synonymisation of the two species. The pereopod 5 setation within the wider series of specimens examined is variable, though usually present but sometimes easily overlooked.

The type series of O. tridacnae comprises three males and four females, from which Schmitt et al. (1973) selected a male lectotype (SMF 2719). The original type series of O. savignyi consisted of specimens figured by Savigny (1826) and additional material available to H. Milne Edwards (1853: pl.11 fig. 10). Savigny’s (1826) material is now lost (Guinot pers. com.). Two dry lots from the Red Sea labelled as O. savignyi , however, are present in the old collections of the MNHN, collected by Beaudouin and M. Clot Bey, respectively, and clearly referable to O. tridacnae . The Beaudouin specimens (MNHN-IU-2000-3069) postdate 1853 and cannot be type material ( Guinot & Cleva, 2009). Clot Bey (Antoine Barthélémy Clot, 1793–1868), however, was based in Egypt from 1825–1849 ( Aboul-Enein & Puddy, 2016) during which he provided many specimens to the MNHN. These include the series of four males and one female labelled as O. savignyi (MNHN-IU-2000-3066, 2000-1120–1123; Fig. 3 ), which were available to H. Milne Edwards, and can thus be considered syntypic. Of these, the largest syntype (female, cl 12.1 mm, cw 11.3 mm; MNHN- IU-2000-3066), selected on the advice of Danièle Guinot, is designated as the lectotype to fix the identity of the species. The sexes in O. tridacnae are largely similar, differing in primary sexual features and maximum body size (females to cl 15.4 mm, males to cl 12.1 mm).

Schmitt et al. (1973) (citing Rüppell, 1830) listed Pinna sp. as a host of O. tridacnae , but Rüppell (1830) was in fact referring to secondary observations on a Mediterranean pinnotherid in Pinna , and clearly stated that his new species was only from Tridacna . Adensamer (1897) recorded pinnotherids from the Red Sea in Tridacna and ascidians, which he identified as O. tridacnae and O. savignyi , respectively. H. Milne Edwards (1853), however, did not record the host of the types of O. savignyi (= O. tridacnae ); Adensamer (1897) presumably simply ‘matched’ the two species names to the specimens from the two hosts. Reexamination of Adensamer’s (1897) material revealed that the specimens reported as O. savignyi from ascidians are referable to O. cynthiae . Thus, O. tridacnae is confirmed only from Tridacna . In the Red Sea area, O. tridacnae is the only pinnotherid hosted by Tridacna , but elsewhere in the Indo-West Pacific, is apparently replaced by Xanthasia murigera White, 1846 or Tridacnatheres whitei ( De Man, 1888) ( Ahyong & Ng, 2005) . Xanthasia murigera and T. whitei differ considerably from O. tridacnae in carapace ornamentation and presence of a G2 exopod (absent in O. tridacnae ); T. whitei further differs by its 3- rather than 2-segmented maxilliped 3 palp ( Ahyong & Ng, 2005).

Outside the Red Sea area, O. tridacnae has been reported only from two South African localities: the Natal coast (Kraus, 1843, as P. tridacnae ) and Simons Bay ( Lenz & Strunck, 1914), with no new records since then. Subsequent literature records of O. tridacnae from southern Africa ( Stebbing, 1910; Barnard, 1950; Schmitt et al., 1973; Kensley, 1981; Emmerson, 2016) derive from Krauss (1843) and Lenz & Strunck (1914). The Simons Bay material, reexamined herein (ZMB 17923, 1 male, cl 2.6 mm, cw 2.7; 1 juvenile male, cl 2.3 mm, cw 2.5 mm and cl 2.6 mm, cw 2.7 mm), is referable to Afropinnotheres dofleini (Lenz in Lenz & Strunck, 1914) (see Ng, 2018). Ostracotheres tridacnae and juvenile A. dofleini have a superficially similar habitus and body setation, making small specimens easily confounded unless the maxilliped 3 palp is examined carefully. Unfortunately, Krauss’ (1843) material from Natal, perhaps the basis of Lenz & Strunck’s (1914) identification, could not be traced, but his record is probably also based on A. dofleini , which is common on the Natal coast ( Emmerson, 2016). South African records of O. tridacnae are herein referred to A. dofleini . Neither Lenz & Strunck nor Krauss indicated the South African host of their material, although the male paralectotype of A. dofleini from Simon’s Bay was reported from an ascidian ( Lenz & Strunck, 1914).

Ostracotheres tridacnae is distinguished from its congener, O. cynthiae [type locality: Djibouti], by its larger maximum size (females to cl 15.4 mm versus cl 7.8 mm), more densely tomentose carapace (versus sparse), and features of pereopods 2–5, as discussed further under the account of O. cynthiae .

In life, O. tridacnae has red corneas and is pale tan brown overall, with the carapace and upper surfaces of the pereopods darker reddish brown ( Fig. 3E, F ). The pale spots on the carapace of the figured specimens are artefacts of small sediment deposits and not the actual colour of the crabs.

Distribution. Western Indian Ocean: presently known only from the vicinity of the Red Sea, from Djibouti (Obock) to the Gulf of Suez; shore to shallow subtidal depths.

Figures

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

Fig. 2. Ostracotheres tridacnae (Rüppell, 1830). A–I, female, cl 11.8 mm, cw 11.7 mm, Red Sea, MNHN-IU-2016-10949; J–K, male, cl 8.0 mm, cw 7.6 mm, Red Sea, MNHN-IU-2016-10950; L–O, male, cl 12.1 mm, cw 12.1 mm, Red Sea, NHMD-229248. A, J, dorsal habitus; B, right cheliped, anterior view; C–F, right pereopods 2–5; G, right maxilliped 3; H, carapace, right lateral view; I, cephalothorax, anterior view; K, male abdomen; L, left G1, sternal view; M, left G1, tip, abdominal view; N, left G2, sternal view; O, right pereopod 5 dactylus. Scale: A–F, H, J, K = 4.0 mm; G, I, L, N, O = 1.0 mm; M = 0.5 mm.

Fig. 3. Ostracotheres tridacnae (Rüppell, 1830). A–D, type material of Ostracotheres savignyi H. Milne Edwards, 1853, Red Sea; E–F, colour in life, male (cl 8.8 mm, cw 8.7 mm) and female (cl 12.2 mm, cw 12.0 mm), Farasan Islands, Red Sea, UF37023–37024. A, from left to right and upper to lower, lectotype female (cl 12.1 mm, cw 11.3 mm, MNHN-IU-2000-3066), paralectotype male (cl 9.5 mm, cw 9.4 mm, MNHN-IU-2000-1120), paralectotype male (cl 9.5 mm, cw 9.7 mm, MNHN-IU-2000-1121), paralectotype male (cl 7.7 mm, cw 7.9 mm, MNHN-IU-2000-1122), paralectotype male (cl 7.9 mm, cw 7.6 mm, MNHN-IU-2000-1123); B, lectotype female, cl 12.1 mm, cw 11.3 mm, MNHN-IU-2000-3066; C, paralectotype male, cl 12.1 mm, cw 11.3 mm, MNHN-IU-2000-1120; D, same, right G1, scale = 1.0 mm. (Photos: A–D, Noémy Mollaret and Marie Hennion; E–F, Arthur Anker).