Sesarmidae Dana, 1851

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, Zootaxa 3609 (1), pp. 1-25 : 2-7

Family Sesarmidae Dana, 1851

Chiromantes obtusifrons (Dana, 1851)

( Figs. 1 , 4 A, 5A, B, 6A, B, 7A, B, 8A, 9A, B, 10A, 11)

Sesarma obtusifrons Dana, 1851: 250 ; 1852: 355, pl. 22, fig. 9.—H. Milne Edwards 1853: 183.—De Man 1887: 644.—Lenz 1901: 472.—Titcomb et al. 1979: 367.—Godwin & Bolick 2006: 39, 49.

Sesarma (Holometopus) obtusifrons —Rathbun 1906: 840; 1907: 35.—Tesch 1917: 179.—Edmondson 1946: 306, fig. 183e; 1959: 185, figs. 13c, 17c.

Chiromantes obtusifrons —Ng & Liu 1999: 230.—Castro 2011: 120. “ Chiromantes ” obtusifrons —Ng et al. 2008: 220, 224 (part).

Material Examined. ZRC 2002.0220, 2 males (16.8×12.9, 19.7× 14.7 mm), Malaikahana, O‘ahu, Hawaiian Is., C.M. Cooke et al., 0 8.07.1916 [ex. BPBM-266].—UFUF-14837, male (17.7× 13.2 mm), female (15.7× 11.6 mm), 21.2833°N, 157.667°E, Coco Head, O‘ahu, Hawaiian Is., G. Paulay et al., 10.2006.

Diagnosis. Carapace transversely rectangular, c. 1.3 times broader than long; dorsal carapace, lateral branchial regions weakly convex, not prominently swollen; exorbital tooth with outer margin broadly convex so greatest carapace width clearly posterior to exorbital tooth; front c. 0.65 times carapace width, margin broadly convex in frontal view with medial part relatively straight in dorsal view, beaded with row of small but distinct granules, with pair of low lateral swellings behind margin; supraorbital margin entire, semicircular; dorsal surface of cheliped carpus covered in small but conspicuous granules; walking legs relatively short; P4 merus c. 2.2 times longer than wide; P5 merus c. 2.3 times longer; P5 propodus c. 2.2 times longer; male abdomen moderately broad; somite 6 with distolateral margins strongly divergent, relatively straight over distal two-thirds; somite 3 width 2.9 times basal width of telson; G1 ( Fig. 11 D–G) relatively slender, weakly tapering to obtusely angled subdistal shoulder; distally slender, strongly bent to 45° angle; terminal process long, with slender narrow apex, dorsal margin slightly concave.

Redescription. Carapace transversely rectangular, c. 1.3 times (range 1.30–1.34) broader than long; surface bare, lacking setal tufts; anterior half conspicuously granular, granules become minute posterior, arranged into inconspicuous striae; mesogastric regions well defined; lateral carapace surface generally without obvious discrete oblique striae, except for strong, concave epibranchial sulcus beginning just behind exorbital tooth. Dorsal carapace, lateral branchial regions weakly convex. Exorbital tooth triangular, pointed, outer margin broadly convex so that greatest carapace width clearly posterior to exorbital tooth, margin not constricted behind (except on one side of larger male, but probably result of damage) so that entire lateral border appears convex, no trace of second anterolateral tooth. Postfrontal lobes well demarcated but relatively low, rounded, separated by broad grooves, median lobes similar in width to lateral lobes. Front c. 0.65 times carapace width, markedly deflexed, margin broadly convex in frontal view, but medial part relatively straight in dorsal view; laterally triangular, bluntly pointed; frontal margin emarginated, beaded with row of small but distinct granules; surface concave behind rim, but with pair of low lateral swellings behind frontal margin. Supraorbital margin entire, semicircular but oblique medial section somewhat straight; secondary rim formed either side making smooth, broadly triangular, deflexed central plate behind 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 distinctively short, entering orbit.

Third maxilliped ischium subequal to merus in length; ischium with shallow longitudinal, curved, median sulcus. Inner margins 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 ischium, 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, emarginated, minutely granular, without indication of subdistal spine or lobe; inner anterior border with conspicuous row of 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 covered in small but conspicuous granules. Dorsal surface of palm 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. Inner face of palm of males smooth except for curved projecting crest of large tubercles posterior to gape ( Fig. 6 A, B). Cutting margin of fixed finger typically with 3 or 4 blunt teeth, proximal 3 similar in size, distal-most 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. Without setae except for scattering of short dark bristles on dactyli, ventral face of propodi. Dorsal margin of meri with blunt subdistal shoulder, otherwise unarmed; outer surfaces of meri with minutely granular transverse striae. P4 merus c. 2.2 times longer than wide; P5 merus c. 2.3 times longer. Carpi with 2 well-developed accessory carinae on outer surface. Dorsal surfaces of carpi, propodi conspicuously granular. P4 propodus c. 2.1 times longer than wide; P5 propodus c. 2.2 times. Dactyli 0.8 times length of propodi, slightly recurved, terminating in acute chitinous tip.

Thoracic sternites ( Fig. 9 A, B) 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. 11 C) moderately broad; telson broadly rounded apically, 1.1 times longer than wide, slightly longer than somite 6 (1.15 times); somite 6 c. 2.0 times wider than long, distolateral margins strongly divergent, relatively straight over distal two-thirds; somites 3–5 trapezoidal, lateral margins of somites 4, 5 relatively straight, lateral margins of somite 3 convex; somite 3 width 2.9 times basal width of telson; somites 1, 2 transversely narrower, longitudinally narrow. G1 ( Fig. 11 D–G) relatively slender, almost straight, weakly tapering to obtusely angled subdistal shoulder; distally slender, strongly bent to 45° angle; terminal process long, with slender narrow apex, dorsal margin slightly concave. G2 short, as for the genus.

Females. Chelipeds slightly smaller, less robust than in males; 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 given in Fig. 1 . Background colour of carapace, legs fawn to bluish gray, covered in fine darker speckling with scattering of slightly larger irregular spots. Legs with darker, broad transverse bands that are more noticeable on the carpi and meri. Chelipeds dorsally similar in colour to carapace and legs but becoming greyish white in frontal and ventral view. Ocular peduncles and corneas pale, similar to carapace in colour.

Remarks. Chiromantes obtusifrons was originally described by Dana (1851) from Maui, Sandwich Archipelago [= Hawaiian Is.], and since then it has been reported from a number of Indo-West Pacific localities stretching as far west as Christmas Island in the Indian Ocean (Davie 2002). Like most of Dana’s specimens, the type material is now considered lost, and so, to be certain of its identity we obtained topotypic material from the Hawaiian Is. for redescription and refiguring. Although we have concluded that C. obtusifrons sensu lato consists of at least five sibling species, we feel confident that each can be reliably recognised based on differences in the morphological characters given here for the Hawaiian populations. We therefore do not consider it necessary to designate a neotype at this time, especially because it appears that most of the species have relatively narrow distributions, and this will make it unlikely that there will be future taxonomic confusion.

Chiromantes obtusifrons can be identified using the characters given in Table 1 . Of all the members in the species-complex it has the most conspicuously granular carapace (over anterior half), frontal margin and claws; and the carapace is more flattened and less convex longitudinally.

C. obtusifrons C. garfunkel C. silus C. leptomerus C. eurymerus

Carapace dorsal Weakly convex Prominently convex Prominently convex Prominently convex Prominently convex

surface ( Fig. 10 A) ( Fig. 10 B)

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C. obtusifrons C. garfunkel C. silus C. leptomerus C. eurymerus We have included previous reports of C. obtusifrons by other authors under the synonymies of our new species as appropriate, however the record of De Man (1895, 1898) from the southern coast of Sumatra must remain incertae sedis until his material can be reexamined. It occurs relatively close to Christmas I., but we believe it to be different from C. garfunkel sp. nov., and probably represent a yet undescribed species (see Remarks for C. garfunkel ).

Distribution. Only known from the Hawaiian Islands.

Ecology. Edmondson (1959) noted that C. obtusifrons was found intertidally and “even above the high water mark”, and Paulay & Starmer (2011: 11) reported finding it living in a supratidal boulder field on O‘ahu. Paulay & Starmer (2011: 11) nevertheless also suggested that this species may not be as terrestrial as Chiromantes silus sp. nov. (see below), which they had observed in Micronesia, but indicated that additional observations on its ecology in the Hawaiian Is. would be useful.

Figures

FIGURE 1. Colours in life. Chiromantes obtusifrons (Dana, 1851). A – C, UFUF- 14837, male (17.7 × 13.2 mm), Hawaiian Is.; D, UFUF- 14837, female (15.7 × 11.6 mm), Hawaiian Is. Photographs by G. Paulay.

FIGURE 4. Overall dorsal views. A, Chiromantes obtusifrons (Dana, 1851), UFUF- 14837, male (17.7 × 13.2 mm), Hawaiian Is.; B, C. silus sp. nov., ZRC 2012.0787, holotype male (16.3 × 12.8 mm), Guam; C, C. leptomerus sp. nov., NMNS 7028 - 001, holotype male (16.8 × 12.7 mm), Taiwan; D, C. eurymerus sp. nov., NMNS- 7028 - 002, holotype male (18.7 × 14.8 mm), Taiwan; E, C. garfunkel sp. nov., QM-W 29172, holotype male (17.0 × 13.0 mm), Christmas I.

FIGURE 6. Inner views of chelae. A, Chiromantes obtusifrons (Dana, 1851), UFUF- 14837, male (17.7 × 13.2 mm), Hawaiian Is.; B, C. obtusifrons (Dana, 1851), ZRC 2002.0220, male (19.7 × 14.7 mm), Hawaiian Is.; C, C. silus sp. nov., ZRC 2012.0787, holotype male (16.3 × 12.8 mm), Guam; D, C. leptomerus sp. nov., NMNS 7028 - 001, holotype male (16.8 × 12.7 mm), Taiwan; E, C. eurymerus sp. nov., NMNS- 7028 - 002, holotype male (18.7 × 14.8 mm), Taiwan; F, C. garfunkel sp. nov., QM-W 29172, holotype male (17.0 × 13.0 mm), Christmas I.

FIGURE 9. Ventral views of thoracic sternum and anterior parts of male abdomens. A, Chiromantes obtusifrons (Dana, 1851), UFUF- 14837, male (17.7 × 13.2 mm), Hawaiian Is.; B, C. obtusifrons (Dana, 1851), ZRC 2002.0220, male (19.7 × 14.7 mm), Hawaiian Is.; C, C. silus sp. nov., ZRC 2012.0787, holotype male (16.3 × 12.8 mm), Guam; D, C. leptomerus sp. nov., NMNS 7028 - 001, holotype male (16.8 × 12.7 mm), Taiwan; E, C. eurymerus sp. nov., NMNS- 7028 - 002, holotype male (18.7 × 14.8 mm), Taiwan; F, C. garfunkel sp. nov., QM-W 29172, holotype male (17.0 × 13.0 mm), Christmas I.

FIGURE 10. Lateral carapace views. A, Chiromantes obtusifrons (Dana, 1851), UFUF- 14837, male (17.7 × 13.2 mm), Hawaiian Is.; B, C. garfunkel sp. nov., QM-W 29172, holotype male (17.0 × 13.0 mm), Christmas I.

FIGURE 11. Chiromantes obtusifrons (Dana, 1851), ZRC 2002.0220, male (19.7 × 14.7 mm), Hawaiian Is .. A, left third maxilliped; B, left side of anterior thoracic sternum; C, male abdomen (somites 3 – 6, telson); D, ventral view of left G 1; E, dorsal view of left G 1; F, ventral view of distal part of left G 1; G, dorsal view of distal part of left G 1. All structures denuded. Scales: A – E = 1.0 mm; F, G = 0.5 mm.

Tables

TABLE 1. Summary of major differences between species of the Chiromantes obtusifrons complex.

Carapace granulation	Anterior half conspicuously granular,granules become minute posteriorly	Covered with fine, but distinct low granules becoming arranged into short striae posteriorly	Fine, but distinct low granules arranged into short striae that become longer posteriorly	Fine, but distinct low granules becoming arranged into short striae posteriorly	Fine, but distinct low granules becoming arranged into short striae posteriorly
Exorbital tooth outer margin	Broadly convex	Long, slightly convex	Long, weakly oblique, forming rounded obtuse angle posteriorly	Long, moderately oblique, forming slight angle posteriorly	Broadly convex
Constriction posterior to epibranchial tooth	Absent	Present	Markedly constricted	Present	Absent
Postfrontal lobes	Relatively low, rounded	Strongly demarcated	Strongly demarcated	Strongly demarcated	Well demarcated but relatively low, rounded
Frontal width/ carapace width ratio	c. 0.65 times	c. 0.6 times	c. 0.65	c. 0.65	c. 0.65–0.7
Shape of frontal border	Broadly convex in frontal view, but medial part relatively straight in dorsal view	Broadly convex in dorsal view	Margin straight or slightly concave in dorsal view	Margin slightly to broadly concave in dorsal view	Margin broadly convex in frontal view, but medial part relatively straight in dorsal view
Granulation of frontal margin	Beaded with row of small but distinct granules	Minutely granular	Appearing smooth but microscopically granular	Microscopically granular	Microscopically granular but appearing relatively smooth
Post-frontal lateral swellings	Low	Low	Low	Prominent	Low
Supraorbital margin	Semicircular but oblique medial section somewhat straight	Medially straight	Semicircular but oblique medial section somewhat straight	Semicircular	Semicircular but oblique medial section somewhat straight
Cheliped merus	Inner anterior border with conspicous row of pointed granules	Minutely granular	Inner anterior border armed with small pointed granules	Minutely granular	Minutely granular
Cheliped carpus outer margin and dorsal surface P4 merus and propodus length/ breadth ratios, respectively	Covered with small but conspicuous granules 2.2; 2.1 times longer	Moderately granular 2.3; 2.8	Conspicuously granular 2.3; 2.6	Conspicuously granular 2.3; 2.8	Covered with small granules 2.0; 2.1–2.2