Arcotheres pollus, Ahyong & Ng, 2020

Ahyong, Shane T. & Ng, Peter K. L., 2020, New species of pinnotherid crabs from Southeast Asia and Papua New Guinea (Crustacea: Decapoda: Brachyura), Zootaxa 4816 (3), pp. 333-349 : 334-337

Arcotheres pollus sp. nov.

( Figs. 1 , 2 )

Type material. HOLOTYPE: ZRC 1965.11.24.49, male (cl 2.3 mm, cw 2.4 mm), Paye or Paway Island, Mergui Archipelago, Myanmar, 11°25’00”N, 98°31’00”E, coral reefs, shore collecting, from Malleus albus Lamarck , RIMSS Investigator , stn 593, 24 February–3 March 1914. PARATYPE: ZRC 1965.11.24.50, 1 ovigerous female (cl 3.6 mm, cw 4.6 mm), collected together with holotype.

Description. MALE: Carapace irregularly subcircular. Front produced, anterior margin straight in dorsal view. Anterolateral margins poorly defined. Dorsum smooth, very few, minute, scattered setae; dorsal midline convex in profile. Epistome with narrow, triangular, interantennular septum; median buccal margin with acute median point. Antennular sinus smaller than orbit; antennules folded slightly obliquely. Antennal articles 1 and 2 fused to epistome. Eyes visible in dorsal view, filling orbit, cornea pigmented.

Maxilliped 3 ischiomerus length about twice width; inner proximal margin convex; inner distal margin with blunt, obtuse angle; outer margin strongly convex. Carpus shorter than propodus. Propodus spatulate, gently tapering in distal half, apex rounded, dorsally and distal tally setose, length 4× dactylus length. Dactylus digitiform, distally setose, inserted slightly distal to propodal midlength, apex not reaching end of propodus. Exopod inner margin straight, outer margin convex; flagellum with 2 articles, distally setose.

Chelipeds stout, inflated, symmetrical from left to right, surfaces setose. Dactylus and pollex relatively straight, crossing distally, without gape. Dactylus occlusal margin with triangular tooth proximal to midlength, margin straight in distal half, minutely denticulate. Pollex occlusal margin with blunt triangular tooth proximal to midlength, straight in distal half, sparsely setose; with fringe of setae on inner ventral margin extending to proximal end of palm. Propodus palm dorsal margin 1.1× height, as long as dactylus; ventral margin almost straight. Carpus unarmed.

Pereopods 2–5 (walking legs 1–4) unarmed, symmetrical from left to right; covered in short, upright setae. Relative lengths in decreasing order: P4> P3> P2> P5. P4 merus 0.5× pcl. P2–3 dactylus length subequal, 0.6× propodus length, evenly arcuate, distally spiniform. P4 dactylus 0.7× propodus length, slightly curved, styliform. P5 dactylus 1.1× propodus length, styliform but thicker and straighter than P4 dactylus; surfaces covered in upright setae, of similar length on flexor and extensor margins; flexor margin unarmed. Relative dactylus lengths: P5> P4> P3 = P2.

Thoracic sternum anterior margin shallowly concave medially; sternites 1–3 indistinguishably fused; sternopleonal cavity reaching level of sternite ¾ suture.

Pleon slender, margins evenly converging distally; comprising 6 free somites and telson, widest at somites 1–3; telson wider than long, apex rounded.

Gonopod 1 elongate, slender, strongly arcuate, dorsoventrally compressed, about ¾ length of pleon, tapering to simple apex; margins setose, longest on outer margin. Gonopod 2 endopod simple, about ¼ length of gonopod 1; exopod short, slender, about ¼ length of endopod. Pleopods 3–5 absent.

FEMALE: Carapace ovate, slightly wider than long. Front weakly produced, anterior margin broadly rounded in dorsal view.Anterolateral margins poorly defined. Dorsum smooth, very few, minute, scattered setae; dorsal midline convex in profile. Epistome with narrow triangular interantennular septum; median buccal margin with obtuse median point. Antennular sinus of similar size to orbit; antennules folded slightly obliquely. Antennal articles 1 and 2 fused to epistome. Eyes not visible in dorsal view, filling orbit, cornea pigmented.

Maxilliped 3 ischiomerus length about twice width; inner proximal margin convex; inner distal margin with blunt, obtuse angle; outer margin strongly convex. Carpus shorter than propodus. Propodus spatulate, gently tapering in distal half, apex rounded, dorsally and distally setose, length 0.5× dactylus length. Dactylus digitiform, distally setose, inserted slightly distal to propodal midlength, apex reaching end of propodus. Exopod inner margin straight, outer margin convex; flagellum with 2 articles, distally setose.

Chelipeds symmetrical from left to right, surfaces setose. Dactylus and pollex relatively straight, crossing distally, without gape. Dactylus occlusal margin with blunt triangular tooth proximal to midlength, gently sinuous margin in distal half, sparsely setose. Pollex occlusal margin with blunt triangular tooth proximal to midlength, gently curved distal half, sparsely setose; with fringe of short setae on inner ventral margin. Propodus palm dorsal margin 1.3× height, 0.9× length of dactylus; ventral margin almost straight. Carpus unarmed.

Pereopods 2–5 (walking legs 1–4) unarmed, covered in short, upright setae, most pronounced on flexor and extensor margins of merus, carpus, propodus, and dactylus. P2, 3 and 5 similar, symmetrical from left to right; right P4 length 1.2× longer than left; relative lengths in decreasing order: P4> P5> P3> P2. Longer P4 merus 0.5× pcl. P2–3 dactylus 0.5× propodus length, evenly arcuate, distally spiniform, P3 dactylus slightly longer that of P2. P4 dactylus styliform, unarmed, 0.9× propodus length (right), 0.8× propodus length (left). P5 dactylus 1.1× propodus length, styliform but thicker than P4 dactylus; surfaces covered in upright setae, slightly longer on flexor margin; flexor margin unarmed; apex a short slender spine. Relative dactylus lengths: P5> P4> P3> P2.

Thoracic sternum anterior margin shallowly concave medially; sternites 1–3 indistinguishably fused.

Pleon of 6 free somites and telson, extending to buccal region, covering bases of walking legs.

Host. Malleus albus Lamarck, 1819 (Malleidae) .

Etymology. Named pollus , small, alluding to the diminutive adult size of the species.

Remarks. Arcotheres pollus sp. nov. is readily separated from all known congeners by the finely setose carapace (glabrous in congeners), distinctly setose pereopods in females with setose cheliped (P1) surfaces and setose flexor and extensor margins of the merus, carpus, propodus and dactylus of P2–5. The cheliped setation in females of other species of Arcotheres is limited to the fringe on the lower margin of the palm, a small brush of setae sometimes on the inner margin of the carpus, and scattered setae on the occlusal margins and tips of the fingers ( Campos 2001; Ahyong & Ng 2007a; Ng 2018b; Trivedi et al. 2018). On the walking legs of females of other species of Arcotheres , the P5 dactylus is always setose, but P2–5 are otherwise almost glabrous, at most with a few setae on the flexor margin of the dactylus and P2–4, and sometimes with setae on the flexor margin of P2–5 propodi. The male of A. pollus is setose as in the female, but too few males of other species of Arcotheres are known to permit a comprehensive comparison.As with the female, the carapace and cheliped setation will separate male A. pollus from known males of other species of Arcotheres , which lack dorsal carapace setae and or setae on the outer surface of the palm. As in other known males of Arcotheres , the G2 exopod is present in A. pollus .

Arcotheres pollus is morphologically nearest to those species in the genus with females having a transversely ovate carapace ( Fig. 2A ) and long, slender, almost styliform dactyli of P4 and 5 that are about twice the length of the P2 and 3 dactyli ( Fig. 2H, L, M ): A. boninensis ( Stimpson, 1858) , A. pernicola ( Bürger, 1895) and A. purpureus ( Alcock, 1900) . Aside from the pereopodal setation, A. pollus is readily separated from each of the aforementioned species by the shape of the propodus of maxilliped 3, which is distally thicker and less tapering ( Fig. 2B versus Alcock & McArdle 1902: pl. 57 fig. 6b; Bürger 1895: pl 10 fig. 16; Dai & Yang 1991: fig. 210.2), and the absence of one or more rows of denticles on the distoflexor margin of the dactylus of P5 ( Fig. 2M ). Arcotheres boninensis and A. pernicola have two short rows of distoflexor spines on the P5 dactylus, and A. purpureus , one row ( Trivedi et al. 2019).

With males and females already mature by cl 2.3 mm and 3.6 mm, respectively, A. pollus is the smallest known species of the genus. The next smallest species of Arcotheres appears to be A. exiguus , being mature (at least in females) by cl 4.6 mm (cf. Ahyong & Ng 2007). The type material of A. pollus was taken from the hammer oyster ( Malleus albus ) during shore collecting on coral reef. Other invertebrates taken from the same station, include a sipunculan worm ( Sipunculus robustus Keferstein, 1865 ; Prashad 1936), polychaete worms ( Sabellaria spinulosa Leuckart, 1927 , Branchiomma intermedium Beddard, 1887 , Sabellastarte indica Savigny, 1822 ; Fauvel 1932), and a mud shrimp ( Upogebia kempi Sankolli, 1972 ; Sankolli 1972).

Distribution. Known only from the type locality, Paway Island, Myanmar.

Figures

FIGURE 1. Arcotheres pollus sp. nov., male holotype, cl 2.3 mm, cw 2.4 mm, ZRC 1965.11.24.50. A, dorsal habitus; B, right maxilliped 3; C, anterior cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, left P2–5; J–M, right P2–5; N, pleon; O, left G1, sternal view; P, left G1, pleonal view; Q, left G2, pleonal view. Scales: A = 1.0 mm; B = 0.2 mm; C–N = 1.0 mm; O–Q = 0.4 mm.

FIGURE 2. Arcotheres pollus sp. nov., ovigerous female paratype, cl 3.6 mm, cw 4.6 mm, ZRC 1965.11.24.49. A, dorsal habitus; B, right maxilliped 3; C, anterior cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, left P2–5; J–M, right P2–5. Scales: A = 2.0 mm; B = 0.5 mm; C–M = 1.0 mm.