Chiromantes silus Davie & Ng, 2013, sp. nov.
Creators: name: Davie, Peter J. F.
name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2013
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Sesarmidae
Chiromantes
Chiromantes silus

description: Chiromantes silus sp. nov.
(Figs 3 A–C, 4B, 5C, 6C, 7C, 8B, 9C, 13A–E)
Chiromantes obtusifrons —Paulay et al. 2003: 508.—Paulay & Starmer 2011: 2, 11.
Material Examined. HOLOTYPE: ZRC 2012.0787, male (16.3× 12.8 mm), Tanguisson, Guam, 10–20 m from shore, 5–8 m elevation, under limestone rocks, G. Paulay, 0 5.10.1997. PARATYPES: ZRC 2012.0788, data as for holotype, male (15.1× 11.3 mm), female (14.9× 11.2 mm).—UFUF-122, 3 males (16.3×13.2, 15.9×11.9, 13.6× 10.5 mm), Tanguisson, 10–20 m from shore, 1 m from forest, under rocks, G. Paulay, 05.10.1997.—ZRC 2001.0742, male (15.7× 11.7 mm), Haputo Beach, P.K.L. Ng, 3.08.2001.—ZRC 2001.0743, ovigerous female (15.2× 11.6 mm) (with zoea 1), Haputo Beach, P.K.L. Ng, 04.08.2001.—ZRC 2000.0573, female (15.2× 11.2 mm), Pago Bay, outside of University of Guam Marine Laboratory, P.K.L. Ng & C.H. Wang, 15– 18.04.2000.—UFUF-90, 2 females (19.1×13.6, 19.0× 13.7 mm), Tagachang, near shore, karst rubble, under 15 m overhang, G. Paulay, 29.09.1996. All localities Guam.
Diagnosis. Carapace transversely subovate, c. 1.3 times broader than long; dorsal carapace, lateral branchial regions markedly swollen; exorbital tooth weakly oblique, forming rounded obtuse angle posteriorly marking widest point of carapace, especially in larger specimens; front c. 0.65 times carapace width, margin straight or slightly concave in dorsal view, appearing smooth but microscopically granular, with pair of low lateral swellings behind margin; supraorbital margin entire, semicircular; dorsal surface of cheliped carpus conspicuously granular; walking legs relatively short; P4 merus c.2.3 times longer than wide; P5 merus similar, 2.4 times longer; P4 propodus c.2.6 times longer than wide; P5 merus c. 2.3 times; male abdomen (Fig. 13A) moderately broad, somite 6 with lateral margins broadly divergent with margins relatively evenly convex, somite 3 width 3.2 times basal width of telson; G1 (Fig. 13B–E) relatively slender, weakly tapering to broadly convex subdistal shoulder; distally slender, strongly bent to 45° angle; terminal process long, with dorsal margin concave, apically pointed.
Description. Carapace transversely subovate, c. 1.3 (range 1.27–1.36) times broader than long; surface bare, lacking setal tufts; covered with fine, but distinct low granules arranged into short striae that become longer posteriorly; mesogastric regions well defined; lateral carapace surface generally without obvious, discrete oblique striae except for strong, concave epibranchial sulcus beginning just posterior to exorbital tooth. Dorsal carapace, lateral branchial regions markedly swollen. Exorbital tooth triangular, pointed, outer margin long, weakly oblique, forming rounded obtuse angle posteriorly marking widest point of carapace, especially in larger specimens; margin markedly constricted posteriorly, but no trace of second anterolateral tooth. Postfrontal lobes strongly demarcated, separated by broad grooves, median lobes similar in width to lateral lobes. Front c. 0.65 times carapace width, markedly deflexed, margin straight or slightly concave in dorsal view; laterally triangular, bluntly pointed; frontal margin emarginated, appearing smooth but microscopically granular (Fig. 8 B); frontal surface somewhat concave medially but with pair of low lateral swellings posterior to frontal margin. Supraorbital margin entire, semicircular but oblique medial section somewhat straight; secondary rim formed either side making smooth, broadly triangular, deflexed central plate posterior to ocular peduncle. Eye not extending beyond exorbital tooth. Frontal plate protruding as shelf; medial septum broad, largely covering anterior half of anterior half of epistome; antennae, antennules much reduced in size, lodged under overhanging front; basal antennular segment not much swollen; antennal, antennular basal segments adjacent, not separated by septum. Antennal flagellum very short, entering orbit.
Third maxilliped ischium, merus subequal in length; ischium with shallow longitudinal, curved, median sulcus. Inner margin of merus, ischium with long setae, proximal outer margin of ischium, base of exopod with dense setae; outer margin of merus with short scattered setae only. Exopod slender, hidden behind ishium, merus except near base, tip reaching half length of outer margin of merus, flagellum long, slender.
Chelipeds subequal, robust. Merus trihedral; posterior border broadly convex emarginate, minutely granular, without indication of subdistal spine or lobe; inner anterior border armed with small pointed granules, broadly triangular; outer surface broadly convex, with fine transverse striae but appearing almost smooth. Carpus subquadrilateral, inner angle moderately produced, apically granular; outer margin, dorsal surface conspicuously granular. Palm dorsal surface slightly striated along superior margin, with scattering of moderate sized granules, otherwise smooth, punctate. Palm outer surface broadly rounded, smooth; no indication of subventral longitudinal ridge. Palm inner face of males smooth except for curved, projecting crest armed with large tubercles posterior to gape (Fig. 6 C), but which can vary in prominence among specimens. Cutting margin of fixed finger with 3 or 4 blunt teeth, proximal 3 similar in size, distalmost larger, rounded, conical, placed medially; dactylus with 2 teeth proximally, distalmost large, third much smaller blunt tooth slightly distal of medial tooth of ventral finger. Dorsal surface of dactylus smooth. Fingers with chitinous tips; adult males with narrow but distinct gape when fingers closed.
Walking legs relatively short, broad, flattened; second, third pairs sub-equal, longer than other walking legs. No setae except for scattering of short dark bristles on dactyli, ventral face of propodi. Dorsal margins of meri with blunt subdistal shoulder, otherwise unarmed; outer surfaces of meri with minutely granular transverse striae. P4 merus c. 2.3 times longer than wide; P5 merus similar, c. 2.4 times longer. Carpi with 2 accessory carinae on outer surface but not strongly marked. P4 propodus c. 2.6 times longer than wide; P5 propodus 2.3 times longer than wide. Dactyli 0.8 times length of propodi, slightly recurved, terminating in acute chitinous tip.
Thoracic sternites (Fig. 9 C) smooth, mostly bare of setae except for few scattered bristles; abdominal cavity reaching to thick transverse setal fringe at junction of sternites 3, 4. Male abdomen (Fig. 13A) moderately broad; telson broadly rounded apically, 1.1 times longer than wide, only slightly longer than somite 6; somite 6 c. 1.9 times wider than long, lateral margins broadly divergent with margins relatively evenly convex; somites 3–5 trapezoidal, lateral margins of somites 4, 5 relatively straight, lateral margins of somite 3 convex; somite 3 width 3.2 times basal width of telson; somites 1, 2 transversely, longitudinally narrow.
G1 (Fig. 13B–E) relatively slender, almost straight, weakly tapering to broadly convex subdistal shoulder; distally slender, strongly bent to 45° angle; terminal process long, with dorsal margin concave, apically pointed. G2 short, as for the genus.
Females. Chelipeds slightly smaller, less robust, lacking raised granular row on inner face of chela. Vulval morphology without clear species-specific characters.
Colour in life. Colour photographs taken from fresh specimens are shown in Fig. 3 A–C. Background colour of carapace typically maroon, with coarse paler orange to yellow-green blotching. Legs speckled with some indication of transverse banding. Legs and chelipeds dorsally reddish brown to dark orange, becoming paler orange in frontal and ventral view. Ocular peduncles speckled similar to carapace; corneas darker.
Remarks. The numerous features that distinguish this species from all others in the genus are given in Table 1. The greater marked constriction of the carapace behind the epibranchial teeth, the shape of the tip of the G1, the relatively straight frontal margin, the abdominal proportions, and the colour of the live crabs are all important characters to help identify this species.
Etymology. The name silus is Latin for “pug-nosed” in reference to the broad, flattened frontal plate that the new species shares with congeners.
Distribution. So far only known from Guam.
Ecology. Reported to live “under rocks and in karstic crevices in sparsely vegetated coastal platforms, up to ca. 50 m inland and ca. 10 m elevation” (Paulay & Starmer 2011: 11); and this is confirmed by personal observations of the second author.
description: Published as part of Davie, Peter J. F. & Ng, Peter K. L., 2013, A review of Chiromantes obtusifrons (Dana, 1851) (Decapoda: Brachyura: Sesarmidae), with descriptions of four new sibling-species from Christmas Island (Indian Ocean), Guam and Taiwan, pp. 1-25 in Zootaxa 3609 (1) on pages 14-17, DOI: 10.11646/zootaxa.3609.1.1, http://zenodo.org/record/283281