Arcotheres obesus ( Dana, 1852 )

Ng, Peter K. L. & Ahyong, Shane T., 2022, The pea crab genus Arcotheres Manning, 1993 (Crustacea: Brachyura: Pinnotheridae) from Singapore and Peninsular Malaysia, with a reappraisal of diagnostic characters and descriptions of two new genera, Raffles Bulletin of Zoology 70, pp. 134-248 : 204-207

Arcotheres obesus ( Dana, 1852)

( Figs. 55 , 56 )

Pinnothera obesa Dana, 1852: 380 [type locality: Kumi Village, Viti Levu, Fiji, by neotype designation ( Komai et al., 2020)]; 1855: pl. 24 fig. 3; Woodward, 1886: 177 (list).

Pinnotheres obesus –– Silas & Alagarswami, 1967: 1204 (list); Serène, 1968: 94; Schmitt et al., 1973: 60 (part) (list and discussion).

Pinnotheres obesus –– Ng et al., 2008: 250 (list).

Arcotheres obesus — Komai et al., 2020: 223 , figs. 1–6; De Gier & Becker, 2020: tab. 1.

Type material. Neotype: female (5.5 × 4.4 mm) ( CBM-ZC 15945 ), Kumi Village , Viti Levu, Fiji, intertidal, associated with Gafrarium pectinatum , coll. K. Kawai, 22 February 2019.

Other material examined. Fiji: 3 ovigerous females (6.7 × 5.4 mm, 6.6 × 5.6 mm, 5.9 × 4.6 mm) ( CBM-ZC 15949–15951 ), same data as neotype, in G. tumidum ; 3 females (5.3 × 4.2–6.1 × 5.1 mm), 1 ovigerous female (5.4 × 4.8 mm), 2 males (4.2 × 3.6 mm, one damaged) ( CBM-ZC 15946 ), Kumi Village, Viti Levu , intertidal, in Gafrarium pectinatum , coll. K. Kawai, 16 August 2010 ; 2 females (5.3 × 4.0 mm, 9.2 × 7.2 mm) ( CBM-ZC 15947 , 15948 ), 2 soft females (8.8 × 6.2 mm, 8.9 × 6.9 mm), 2 males (1.9 × 1.7 mm, 2.6 × 2.2 mm) ( ZRC 2019.1876 ), Waiganake, Viti Levu , intertidal, associated with Gafrarium pectinatum (Linnaeus, 1758) , coll. K. Kawai, 22 August 2011 ; 1 ovigerous female (6.2 × 4.9 mm) ( ZRC 2020.6 ), associated with Gafrarium pectinatum, Telau , Viti Levu , coll. K. Kawai, 22 February 2019 . Peninsular Malaysia: 2 soft females (7.2 × 5.4 mm, 5.7 × 4.7 mm) ( ZRC 2000.2281 ), Pulau Tinggi , Johor, in Gafrarium dispar , 7 May 1969 .

Description. Carapace and pereopods very poorly chitinised. Female: Carapace subovate, wider than long; dorsal and lateral surfaces smooth, glabrous; appearing gently domed in frontal view; front not clearly projecting anteriorly beyond orbits, margin gently sinuous to gently convex, appearing almost contiguous with convex anterolateral margin, forming rounded angle with posterolateral margin ( Komai et al., 2020: fig. 55A–C). Eyes small, not visible in dorsal view in adults; filling orbit ( Komai et al., 2020: fig. 55A–C). Epistome with median part triangular, lateral margins concave ( Komai et al., 2020: fig. 3B).

MXP3 outer surface with scattered short setae; ischiomerus completely fused, subrhomboidal, inner margin rounded at widest point; carpus short; propodus about 3 times as long as high, spatulate, distinctly longer than carpus, tip subtruncate; dactylus slender, inserted at midlength of propodus, tip not reaching propodal apex; exopod relatively slender, about two-thirds length of ischiomerus, flagellum 2-segmented ( Fig. 56K, L ).

Chela not prominently elongate, dactylus about two-thirds palm length; palm relatively slender, proximally narrower than distally; outer surfaces of palm, fingers (except for distal part) almost glabrous, with only scattered short setae; ventral margin of palm gently concave; dactylus occlusal margin with distinct subproximal tooth; pollex occlusal margin with 1 low proximal tooth, 1 sharp submedian tooth, and denticles; tips of fingers sharp, hooked ( Figs. 55D , 56A ).

P2–P5 dorsally, ventrally unarmed; outer surface covered with scattered, very short setae or glabrous; ventral margins of propodus and dactylus more setose; merus long, slender, relative lengths of meri P4>P3>P2>P5; right (sometimes left) P4 distinctly longer; P2 and P3 dactyli relatively short, subequal, tip slightly hooked, half or slightly longer than half propodus length; P4 dactylus about half propodus length, longer than P2 and P3 dactyli, shorter than P5 dactylus; P5 merus 4.9–5.0 times longer than wide; P5 dactylus longest, margins lined with short and long setae, denser on ventral margin, distoflexor margin without rows of graded spinules, at most with scattered, very low, minute spinules or acute granules of similar size ( Fig. 56B–J ).

Pleon extending to buccal region, covering bases of P2–P5; telson gently recessed into concave distal margin of somite 6 ( Komai et al., 2020: fig. 3D).

Male: Carapace almost circular, as wide as long; dorsal surface almost smooth, not prominently inflated, lateral surfaces with setae; front projecting anteriorly, margin gently sinuous ( Komai et al., 2020: fig. 5A). Eyes distinctly visible in dorsal view ( Komai et al., 2020: fig. 5A). MXP3 as in female ( Komai et al., 2020: fig. 5B). Anterior thoracic sternum wide, sternites 1, 2 fused, partially sunken into buccal cavity; suture between sternites 2 and 3 shallow; sternites 3, 4 completely fused, separated only by shallow grooves ( Komai et al., 2020: fig. 10B). Chela relatively stout, shorter than in female ( Komai et al., 2020: fig. 6A, B). P2– P5 dorsally, ventrally unarmed; outer surface covered with short setae; P3 and P4 carpus and propodus without long natatory setae; left and right meri equal, relative lengths of meri P4>P3>P2>P5; dactyli of P2–P4 progressively longer; left P4 dactylus longer, more slender than right side; P4 and P5 dactylus longer to that of P3, covered with short setae ( Komai et al., 2020: figs. 5A, 6E–N). Pleon slender, triangular, widest at somite 3, lateral margins of somite 4 gently concave; somite 6 trapezoidal or subquadrate; telson semicircular, slightly wider than long ( Komai et al., 2020: fig. 5D). G1 relatively stout, arcuate, curved outwards, distal part subtruncate, without elongate tip ( Komai et al., 2020: fig. 5E, F). G2 short, with spatuliform tip; exopod about half endopod length ( Komai et al., 2020: fig. 5G).

Variation. This is the most poorly chitinised and calcified Arcotheres treated here, with the carapace appearing almost membranaceous, even in the relatively fresh specimens recently collected from Fiji ( Komai et al., 2020). Nevertheless, the carapace shape is relatively invariate, being always transversely ovate (see Komai et al., 2020).

Colour. Not known.

Host. Known for certain from Gafrarium pectinatum (Linnaeus, 1758) and G. dispar (Holten, 1802) ( Veneridae Rafinesque, 1815 ).

Remarks. The identity of Pinnotheres obesus was long uncertain until Komai et al. (2020) redescribed and transferred it to Arcotheres based on a neotype from Fiji (type locality). Arcotheres obesus appears to be an obligate commensal of clams of the genus Gafrarium . The specimens from Peninsular Malaysia, while soft and in poor condition, agree very well with the material from Fiji. A male specimen will need to be obtained to confirm the identity of these Malaysian specimens, especially considering the geographical distance separating the two locations.

Distribution. Western Pacific from Fiji and Peninsular Malaysia.

Figures

Fig. 55. Arcotheres obesus (Dana, 1852), female (8.9 × 6.9 mm) (ZRC 2019.1876), Fiji. A, overall dorsal view; B, dorsal view of carapace; C, frontal view of cephalothorax; D, outer view of right chela.

Fig. 56. Arcotheres obesus (Dana, 1852), female (8.3 × 5.9 mm) (ZRC 2019.1876), Fiji. A, outer view of right chela; B–E, left P2–P5, respectively; F–I, right P2–P5, respectively; J, right P5 dactylus (long setae omitted); K, inner view of left MXP3; L, outer view of left MXP3. Scales = A–I, 1.0 mm; J–L, 0.5 mm.