Bioacoustics of Karstarma jacksoni

Diagnosis . Carapace ( Fig. 7A, B ) 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, D ); 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. 7E ) 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, F ) similar to K. vulcan sp. nov. Cheliped of male ( Fig. 8A, B ) 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. 8E ) 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. 8B ; 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. 8F ); 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–M ) 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–M ). Dactyli as in K. vulcan sp. nov. Male pleon ( Fig. 7C ) 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–F ) and vulva ( Fig. 10H ) 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. 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.