Karstarma jacksoni ( Balss, 1934 )

Poupin, Joseph, Crestey, Nicole & Guelte, Jean-Paul Le, 2018, Cave-dwelling crabs of the genus Karstarma from lava tubes of the volcano ‘ Piton de la Fournaise’, in Réunion Island, with description of a new species and redescription of Karstarma jacksoni, Zootaxa 4497 (3), pp. 381-397: 390-394

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Karstarma jacksoni ( Balss, 1934 )


Karstarma jacksoni ( Balss, 1934) 

( Figs. 7A –EView FIGURE 7, 8A –MView FIGURE 8, 9C, DView FIGURE 9, 10D –F, HView FIGURE 10, 11A –DView FIGURE 11)

Sesarma (Sesarma) jacksoni Balss, 1934: 230  , fig. 3 (Type locality Grimes Cave, Christmas Is., 10°26’32”S, 105°39’37”E).

Sesarma jacksoni  — Gibson-Hill 1947: 43 (Christmas Is.).— Tweedie 1947: 33 (Christmas Is.).

Sesarmoides jacksoni  —Ng 1998: 186 (Christmas Is.).— Davie 2002: 226 (catalogue).— Ng 2002: 433 (discussion and key).

Karstarma  jacksoni— Davie & Ng 2007: 229 (New genus and key).— Ng et al. 2008: 221 (Checklist).— Orchard 2012: 215 (Field guide) 2018: webpage (colour photograph).— Takakura 2018: webpage (colour photographs).

Material examined. Christmas Island, Australia, Eastern Indian Ocean. 1 male 10.9 × 12.8 mm, 1 female ov. 13.7 × 16.8 mm, 1 female juv. 7.5 × 9.2 mm, 1 juv. 6.0 × 7.0 mm ( WAM C13970View Materials; ex. WA 531-85), Daniel Roux Cave, 30 feet below entrance, coll. and det. R.W. George, 21/02/78; 1 male 15.4 × 18.7 mm, ‘Golf Course’, 19/11/78, det. R.W. George, 1 female 14.3 × 18.05 mm, Silver City, ‘House 640 on porch during rain’ coll. D. Morton, 12/11/78, det. R.W. George ( WAM C13968View Materials, ex. WA 529-85a-b); 1 juv. 3.2 × 3.6 mm, 19th Hole Cave, 10°25’29”S, 105°42’04.24”E, coll. J. Anderson, 30/03/2004, det. P. Davie ( WAM C54757View Materials); 1 juv. 2.1 × 2.3 mm, no data, Christmas Island, det. P. Davie ( WAM C54747View Materials); 1 juv. 2.5 × 2.8 mm, no data, Christmas Island, det. P. Davie ( WAM C54752View Materials); 1 juv. 4.5 × 5.3 mm, no data, Christmas Island, det. P. Davie ( WAM C54748View Materials).

Diagnosis. Carapace ( Fig. 7A, BView FIGURE 7) approximately trapezoidal, maximum width across base of second, third ambulatory legs (P3, 4), width 1.2–1.3 times length. Lateral margins of carapace sub-parallel, diverging very weakly posteriorly; dorsal surface gently convex, regions poorly marked, epigastric and urogastric grooves shallow, posterolateral regions with a few faint oblique striae. Lateral margins of carapace sub-parallel, diverging weakly posteriorly; anterolateral margin with 3 teeth including external orbital angle, first tooth (external orbital angle) acute, curved anteriorly at an angle of 50° ( Fig. 9C, DView FIGURE 9); second tooth with blunt tip separated from former tooth by V-shaped cleft; third tooth smallest poorly marked, separated from second tooth by clearly cut notch, tip of second tooth placed closer to first tooth than third.

Front ( Fig. 7EView FIGURE 7) similar to K. vulcan  sp. nov., 0.4–0.5 times fronto-orbital width (measured between tip of anterolateral teeth), 2.3–2.6 wider than high. Frontal margin, supraorbital margin, eyes, cornea, infraorbital margin, epistome, branchiostegite, and third maxilliped ( Fig. 7E, FView FIGURE 7) similar to K. vulcan  sp. nov.

Cheliped of male ( Fig. 8A, BView FIGURE 8) symmetrical, merus triangular in cross-section, outer ventral margin and dorsal margin denticulated, inner ventral margin with a few sharp teeth on proximal half and strong flange ( Fig. 8EView FIGURE 8) on distal half, inner face with a longitudinal line of pubescence, outer face with short transverse striae; carpus rounded unarmed, dorsal surface smooth with a few sparse granules, inner dorsal margin with line of low granules; chela higher than in females, 2.0–2.2 as long as high, inner and outer faces inflated, coarsely granulated, larger granules on inner face disposed on a salient transverse ridge forming a ‘stridulating’ ridge ( Fig. 8BView FIGURE 8; see remarks); fingers of chela with small sub-triangular teeth of unequal sizes all along cutting margins, with slight gape when fingers closed; tip of fingers corneous, hoof-like; movable finger 1.4–1.5 as long as palm, dorsal margin with granules. Cheliped of female with same flange on distal half of inner ventral margin of merus than in male, but reduced in size ( Fig. 8FView FIGURE 8); chela more elongated than in male, 2.3–3.1 as long as high, overall similar to that of K. vulcan  sp. nov., movable finger 1.3–1.6 as long as palm.

Ambulatory legs 1–4 (P2–P5) ( Fig. 8G –MView FIGURE 8) of similar proportions in males and females, longest leg (leg 3, P4) 2.3–2.6 (male) and 2.4–2.8 (female) as long as CW, and 2.8–3.1 (male), 2.9–3.4 (female), as long as CL. Prominent tufts of setae present between coxae of legs 1, 2 (P3, 4) and legs 2, 3 (P3, 4) in male and female. Meri unarmed, dorsal and inner ventral margins with faint carina, outer faces convex with short transverse striae, inner faces smooth, length to width ratios of meri of legs 2, 3 (P3, 4) between 3.3–3.9 and 3.4–4.1, respectively. Carpi as in K. vulcan  sp. nov. Propodi and dactyli with fewer stiff setae on upper margins and outer faces than in K. vulcan  sp. nov., probably an artefact due to cleaning the specimens (see remarks), outer faces of propodi convex with granules, mat of setae present on distal half of inner faces of legs 1–3 (P2–4), in male only ( Fig. 8K –MView FIGURE 8). Dactyli as in K. vulcan  sp. nov.

Male pleon ( Fig. 7CView FIGURE 7) triangular, telson rounded at tip, as high as wide; 6th somite the highest, trapezoidal, anterior margin sinuous, lateral margins feebly convex; somites 3–5 elongated, somite 5 4.0 as long as high, somites 1–3 the longest, somite 3 not reaching base of P5 coxae. Female pleon similar in shape than in K. vulcan  sp. nov., about 1.5–1.6 as wide as long. G1, G2 ( Fig. 10D –FView FIGURE 10) and vulva ( Fig. 10HView FIGURE 10) as in K. vulcan  sp. nov.

Remarks. Karstarma jacksoni  is morphologically similar to K. vulcan  sp. nov. but can separated by a series of characters that are presented in Table 1. In the set of five juveniles of K. jacksoni  examined (CW <7.0 mm), the anterolateral armature of the carapace can still be used to separate this species from K. vulcan  sp. nov. but the aspect of the meral flange on cheliped is not present and cannot be used, this structure being noticed only for individuals with CW ± 9.2 mm. The setation of the ambulatory propodi and dactyli is more sparse in K. jacksoni  than in K. vulcan  sp. nov., stiff setae being almost absent on the inner and outer surfaces and on the upper margins. This seems to be an artefact due to brushing the specimens for observation because these setae were seen on photographs of live specimens examined for this work ( Fig. 11View FIGURE 11).

In the largest male examined the inner face of the chela has a salient ‘stridulating’ ridge with prominent granules, similar to that observed for some ocypodid crabs of the genus Ocypode  (e.g. O. pauliani Crosnier, 1965  ). This ridge is also observed in K. vulcan  sp. nov. but in that species it rubs against the infraorbital margin, while in K. jacksoni  it rubs against the flange of the merus, much more developed in that species. These structures, not present in females, are perhaps used by the males to produce sounds, for example during mating or fighting behaviour. In sesarmid crabs stridulation is performed by winners after fights, to reduce the chances of losers reinitiating another fight ( Chen et al. 2014, 2017).

Size. Specimens examined and measured for biometry range in size from 7.5 × 9.2 mm to 15.4 × 18.7 mm. A set of small specimens in poor condition (legs broken or missing) was also examined but not retained for biometry, ranging in size from 2.1 × 2.3 mm to 6.0 ×7.0 mm.

Live coloration ( Fig. 11A –DView FIGURE 11) Carapace and legs purple-red with paler areas at bases of ambulatory legs; cornea black-red; cheliped orange with fingers of chela paler or white.

Habitat and ecology. This crab is found in karstic caves but has also been reported wandering outside this habitat ( Gibson-Hill 1947; Tweedi 1947; Ng 1988; Orchard 2012). In the material examined a female (14.3 × 18.05 mm, WAM C13968View Materials) is labelled as being collected in urban habitat at “House 640 on porch during rain” and photographs by Takakura (2018) (reproduced on Fig. 11C, DView FIGURE 11) show the crab feeding outside between rubble and vegetal debris in the same biotope as the land crab Geograpsus grayi  . Orchard (2012: 215) indicated that the crab is observed from sea level to 50 m above sea level and that it may be observed outside caves when spawning, from November to April, with the eggs being released into the sea at night just after high tide.

Distribution. Known from Christmas Island only.


Western Australian Museum














Karstarma jacksoni ( Balss, 1934 )

Poupin, Joseph, Crestey, Nicole & Guelte, Jean-Paul Le 2018

Sesarma (Sesarma) jacksoni

Balss, 1934 : 230

Sesarma jacksoni

Gibson-Hill 1947 : 43
Tweedie 1947 : 33

Sesarmoides jacksoni

Davie 2002 : 226
Ng 2002 : 433


Davie & Ng 2007 : 229
Ng et al. 2008 : 221
Orchard 2012 : 215