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: 382-388
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Karstarma vulcan sp. nov.
Material examined. Holotype: male 16.2 × 18.9 mm (MNHN-IU-2013-7243), lava tube ‘ Brûlé des Citrons Galets’ , Le Tremblet, Saint Philippe, Réunion Island, coll. N. Crestey and J.- P. Le Guelte, 16 March 2018 . Paratypes (same data as holotype): 1 female 13.7 × 16.9 mm (MNHN-IU-2013-7240) , 1 female 13.1× 16.5 mm (MHNH-IU-2013-7241), coll. N. Crestey and J.-P. Le Guelte, 16 November 2017.
Additional observations. In situ photographs (no specimens collected) from lava tubes: ‘ Grotte au Trésor’ , photograph Christopher Lauret, 3 March 2016 ; ‘Coulée de 2004’, photograph F. Leveneur, 30 August 2014 and Julien Dez, 14 February 2018; ‘ Grotte des Hirondelles’ , photograph François Martel-Asselin, 2 May 2010 .
Type locality. Lava tube ‘ Brûlé des Citrons Galets’ , Le Tremblet, Saint Philippe, Réunion Island.
Diagnosis. Carapace ( Fig. 1A–CView FIGURE 1) approximately trapezoidal, maximum width across base of second and third ambulatory legs, width 1.2 times length; dorsal surface gently convex, slightly uneven, minutely pitted, not setose with only scattered short setae more abundant on posterolateral regions, regions poorly marked except for epigastric and urogastric grooves, posterolateral regions with a few faint oblique striae. Lateral margins of carapace sub-parallel, diverging weakly posteriorly. Anterolateral margin with 2 main teeth including external orbital angle ( Fig. 9A, BView FIGURE 9); first tooth (external orbital angle) blunt, directed inwards at angle of ~ 40°; second tooth with blunt tip separated from former tooth by shallow cleft; third tooth poorly marked, separated from second tooth by minute notch, or absent, tip of second tooth placed closer to tip of first tooth than third.
Front ( Fig. 1DView FIGURE 1) deflected at almost 90°, 0.4 times fronto-orbital width (measured between tip of anterolateral teeth), 2.2–2.4 wider than high, frontal margin minutely granulated, slightly sinuous with a large median V-shaped notch, postfrontal cristae composed of two large protuberances placed transversely between base of ocular peduncles on each side of epigastric groove. Supraorbital margin widely U-shaped, eyes well developed, cornea pigmented as wide as ocular peduncle; infraorbital margin composed of strong ridge with transverse granulation ventrally and line of short curved setae dorsally. Epistome obtusely triangular on upper side, lower side with 1 median, 2 lateral triangular projections. Branchiostegite ( Fig. 1D, EView FIGURE 1) covered with reticulate uniform network of short setae. Third maxilliped ischium ( Fig. 1EView FIGURE 1) slightly longer than merus, median groove with row of setae, merus with oblique crest adjacent to inner margin, exopod with flagellum as long as width of merus.
Chelipeds of male ( Figs. 1A–DView FIGURE 1, 2A, B, EView FIGURE 2) symmetrical; merus triangular in cross-section, all margins denticulated, inner face ( Fig. 2EView FIGURE 2) with longitudinal row of pubescence, inner ventral margin with about 10 denticles on proximal ¾ and a denticulated flange on distal ¼, outer face with transverse striae furnished with short setae; carpus rounded, unarmed, dorsal face with short setiferous striae; chela much higher than in female, 2 times as long as high, inner and outer faces of palm inflated, coarsely granulated, larger granules of inner face disposed on a salient transverse ridge that rubs against the infraorbital margin (‘stridulating’ mechanism; see remarks under K. jacksoni ), upper and lower margins rounded, unarmed, cutting edges of fingers not gaping when fingers closed, with 8–12 large sub-triangular teeth of unequal sizes all along margin; tip of fingers corneous, hoof-like; movable finger 1.2 as long as palm, dorsal margin with granules. Chelipeds of female symmetrical; inner ventral margin of merus minutely denticulated, without distal flange ( Fig. 2FView FIGURE 2); chela ( Fig. 2 C, DView FIGURE 2) much elongated than in male, 2.8– 3.0 as long as high, inner and outer face of palm moderately inflated sparsely pitted, upper and lower margins rounded, unarmed, cutting edges of fingers not gaping when fingers closed, with minute sub-equal triangular teeth on proximal ¾, unarmed on distal ¼, tip of fingers corneous, hoof-like, incurved, movable finger 1.4 as long as palm.
Ambulatory legs 1–4 (P2–P5) ( Fig. 2G–MView FIGURE 2) long, third leg longest 3.1–3.4 and 2.6–2.8 as long as CL and CW, respectively, tufts of setae present between coxae of ambulatory legs 1, 2 and 2, 3 ( Fig. 1CView FIGURE 1); outer faces of meri with short striae (reduced in females),with median rounded carina in distal half separating dorsal and ventral sulci (more pronounced in leg 3), upper and lower margins carinated, unarmed, length to width ratios of meri of legs 2, 3, 3.8–4.1, 3.9–4.6, respectively; carpi 0.4–0.6 meri length, unarmed, outer faces with 2 longitudinal carinae, lower one finely granulated; propodi 0.6–0.7 meri length, unarmed, furnished with longitudinal rows of long stiff setae on upper and lower margins and on inner/outer faces (1 or 2 rows on each face), outer faces flat and smooth, mat of setae ( Fig. 1 K, LView FIGURE 1) present on distal lower margin of legs 1, 2 in males only; dactyli sub-cylindrical, unarmed, as long or slightly longer (1.1) than propodi, gently curved in distal third, with similar longitudinal rows of setae than on propodi, terminating in corneous claws.
Male pleon ( Fig. 1CView FIGURE 1), sub-triangular, telson rounded at tip, as high as wide; 6th somite the highest, trapezoidal, anterior margin sinuous lateral margins convex; somites 3–5 elongated, somite 5 3.5 as long as high, somites 1–3 the longest, somite 3 not reaching base of P5 coxae. G1 ( Fig. 10A, CView FIGURE 10), stout, distal part directed outward at an angle of about 90°, terminating in chitinous tip. G2 ( Fig. 10BView FIGURE 10) approximately half as long as G1, barely curved, with Ushaped corneous extremity.
Female pleon rounded, about 1.5–1.6 as wide as long, lateral margins markedly convex, maximum width between somites 4, 5, telson sub-triangular, rounded at tip, somite 6 the highest, somites 4, 5 largest of similar height. Vulva ( Fig. 10GView FIGURE 10) on sternite 6, with broad sternal rim covered with folded central operculum.
Size. From 13.0 × 16.5 mm to 16.2 × 18.9 mm.
Live coloration ( Fig. 3A–FView FIGURE 3). Carapace and ambulatory legs brown or purple. Male chelipeds light brown to brown orange or yellow on chelae. Female chelipeds paler brown to white on chelae. Male pleon white with brown at junctions of somites, female pleon white. Overall live coloration similar to K. boholano ( Ng, 2002) in colour in Ng (2002: fig. 17) and Fujita & Naruse (2016: fig. 1A–D), K. jacksoni in colour herein ( Fig. 11View FIGURE 11) and Orchard (2012: 215–217), and K. waigeo Wowor & Ng, 2009 in colour in Wowor & Ng (2009: fig. 7C, D).
Habitat. All crabs were captured in the lava tube of ‘Brûlé des Citrons Galets’ ( Fig. 4View FIGURE 4). This tunnel is discussed in Audra (2009) and Michon (2018). Its total length is about 700 m and it is perpendicular to the coast. Its lower part is 120 m from the shoreline at an altitude of about 50–60 m above sea level and its upper part is about 750 m far from the shoreline at an altitude of about 160–170 m above sea level. It has several openings allowing the crabs to enter or leave the tunnel easily. The crabs were all collected near the sea in the lower part of the tunnel (point ‘A’ on Fig. 4View FIGURE 4). During heavy rains, this section receives sediments and vegetal debris from the gutter of the road (N 2 in red on Fig. 4View FIGURE 4) where the main entrance is situated. The crab apparently prefers the moist and muddy parts of the tunnels where it has been observed scraping the substrate for food.
Photographs of probably the same species have been obtained from three other tunnels, ‘Grotte au Trésor’,
‘Coulée de 2004’, and ‘Grotte des Hirondelles’, respectively points B, C, D ( Fig. 4View FIGURE 4). The crab seems therefore
distributed only along the south-eastern coast of Réunion Island where are situated the lava tubes coming out near the coastline suggesting that it is perhaps closely associated to this kind of cave.
In Réunion Island more than 80 lava tubes or cavities, including about 15–20 near the coastline, are listed by Audra (1997) and Cailhol & Fulcrand (2011). A lot of them need official authorization to be visited, which is not the case for the lava tubes of ‘Brûlé des Citrons Galets’ and ‘Coulée de 2004’, which are open to the public, thus explaining why the crabs were first observed there. Karstarma vulcan sp. nov., however, is not often seen. It was not reported by Rochat et al. (2003) in their study on the arthropods of ‘Brûlé des Citrons Galets’, nor during several additional investigations conducted between 2003–2007 (J. Rochat, pers. comm. 16 November 2017). The only crab seen at that time, in the lower part of the tunnel, was Geograpsus grayi (H. Milne Edwards, 1853) , common along the littoral of Réunion Island, apparently having fallen from one of the numerous manholes present in that section. Lava tubes situated higher in the mountain at an altitude of>250–1000+ m above sea level have been also explored for cave-dwelling insects by Hoch et al. (2003) and Sendra et al. (2017), without mention of K. vulcan .
Distribution. Known only from Réunion Island. This is the first record of Karstarma in the WIO ( Fig. 6View FIGURE 6). This is a cryptic species that has remained unnoticed in Réunion for a long time despite numerous studies of wildlife in the Island in the past. It is probably most active at night and is potentially present in all lava tubes allowing an easy access to the shoreline where the crab is likely to lay its eggs. In WIO it is perhaps also present in the Islands of Madagascar, Mauritius and/or Rodrigues in similar cave biotopes. In Rodrigues Island (~ 820 km east to Réunion Island), for example, karstic caves are known in the wildlife park, ‘François Leguat Giant Tortoise and Cave Reserve’ where more investigations for Karstarma crabs would be interesting.
Etymology. This new species is named after ‘Vulcan’ (used as an appositive noun) the ancient Roman god of fire, including fire of volcanoes, in allusion to the crab’s being discovered in the lava tubes of the volcano ‘Piton de la Fournaise’.
Remarks. Karstarma vulcan sp. nov. is most closely related to K. jacksoni ( Balss, 1934) . The differences between the two species are presented in Table 1. This relationship with K. jacksoni , the only Karstarma species previously reported in the Indian Ocean, suggests that they have perhaps a common ancestor. Christmas Island is a much older geomorphological unit (~ 60–10 Myr; Bullough 2013) than Réunion Island (~ 2.1 Myr; Lénat et al. 2001). The South Equatorial current carries to the west of the tropical Indian Ocean, from Christmas Island to Réunion Island. This suggests that the ancestors of K. jacksoni may have colonized Réunion Island by marine dispersal of their larvae.
Karstarma vulcan sp. nov. is also related to K. waigeo Wowor & Ng, 2009 , with similar aspect of carapace and ambulatory legs (compare Fig. 1A, BView FIGURE 1 with Wowor & Ng 2009: fig. 2A, B). In K. waigeo , however, the first anterolateral tooth is more curved inwards, the notch between the first and second anterolateral teeth is deeper, the oblique striae on posterolateral carapace are better marked, the meri of ambulatory are proportionately less elongated with length to width ratios of second, third legs 3.5, 4.0, respectively (instead of 3.8–4.1 and 3.9–4.6 in K. vulcan sp. nov.), the cutting edges of the fingers of the male chela ( Fig. 2 A, BView FIGURE 2) have relatively much lower teeth ( Wowor & Ng 2009: fig. 3C), the lateral margins of the 6th male somite ( Fig. 1CView FIGURE 1) are more convex ( Wowor & Ng, 2009: fig. 3B), and the vulva ( Fig. 10GView FIGURE 10) has a different shape, lacking broad sternal rim ( Wowor & Ng 2009: fig. 6E).
In the form of the vulva ( Fig. 10GView FIGURE 10) and live colour ( Fig. 3View FIGURE 3), Karstarma vulcan sp. nov. also resembles K. boholano ( Ng, 2002) from Bohol ( Philippines) and the Ryukyu Islands ( Japan), being the single species of the genus found in two caves that are very distant geographically (~ 1700 km apart). Karstarma boholano is distinguished by: possessing a deep V-shaped tooth between the first and second anterolateral teeth of carapace (compare Fig. 1BView FIGURE 1, 9A, BView FIGURE 9 with Ng 2002: fig. 12A), having oblique striae on posterolateral carapace better marked, the meri of the ambulatory legs less elongated, length to width ratios of meri of second, third legs being 3.4, 3.7, respectively (versus 3.8–4.1 and 3.9–4.6 in K. vulcan sp. nov.), and with a much shorter chitinous distal part of G 1 in male (compare Fig. 10BView FIGURE 10 with Ng 2002: fig. 12E–G).
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