A new dwarf sentinel crab from Mayotte Island, western Indian Ocean (Decapoda: Brachyura: Macrophthalmidae) Author Poupin, Joseph Author Bouchard, Jean-Marie text Zootaxa 2010 2501 61 67 journal article 10.5281/zenodo.294100 45529c16-4d98-40eb-8542-129667952117 1175-5326 294100 Macrophthalmus (Chaenostoma) lisae sp. nov. ( Figs 1 a–c, 2a–o) Type material. Mayotte . Holotype : 1 male 3.75 × 4.9 mm , stn 26, 12°45’15.60”S , 45°02’49.86”E , upper intertidal beach on Mliha, “Platier de Mutsumbatsou”, coll. J.-M. Bouchard, R. Cléva, J. Poupin, J. Dumas, V. Dinhut, KUW fieldwork 16.xi.2009 ( MNHN B32254). Paratypes : same data as holotype , 1 female ov. 3.85 × 5.1 mm ( MNHN B32362); 1 male 4.1 × 5.4 mm , 3 females ov. 3.1 × 4.1–3.95 × 5.3 mm , 3 females 2.6 × 3.3– 3.7 × 4.9 mm , 1 female juvenile broken ( MNHN B32071); 1 female ov. 3.75 × 5.01 mm , stn 13b, 12°55’22.08”S , 45°09’22.08”E , Malamani mangrove, intertidal, coll. J.-M Bouchard, 08.x.2008 ( MNHN B32072). Comparison material: Macrophthalmus ( Chaenostoma ) boscii Audouin, 1826 Mayotte . 1 female 7.5 × 9.1 mm , stn 1, 12°43’50.45”S , 45°11’39.66”E , intertidal beach of Trévani, coll. J. Delmas, J. Poupin, R. Cléva, 01.xi.2009 ( MNHN B32073); 1 male 3.6 × 4.6 mm , 1 juvenile , stn 31, low intertidal Brandélé, Musicale beach, 12°55’01.60”S , 45°11’12.43”E , coll. J. Poupin, R. Cléva, 19.xi.2009 ( MNHN B32074). Diagnosis. Small-size species with carapace breadth less than 6 mm , greatest breath occurs at tip of first anterolateral teeth (external orbital angle); carapace subrectangular, 1.3 wider than long; front broad, 0.2–0.3 distance between external orbital angles, not constricted between bases of ocular peduncles. Dorsal surface of carapace smooth. Anterolateral margin with 3 teeth, including external orbital angle; first two teeth separated by a V-shaped incision, last tooth feebly marked. Ocular peduncles short and stout, not projecting significantly beyond lateral carapace margin. Upper and lower orbital border with small granules; males without stridulating mechanism. Central region of posterior border of epistome straight. Ischium of third maxilliped 1.2–1.3 times merus length. Male chela with palm inflated, 1.1 times as high as long, furnished with mat of setae on inner face; fingers short 1.0–1.2 times as long as palm, cutting margins with sets of small triangular teeth, without differentiated larger teeth. Female chela with thinner palm, 0.9 as high as long, whithout mat of setae on inner face; fingers 1.0–1.2 as long as palm with smooth cutting margins; telson 3.4 times as wide as long, with lateral margins straight to feebly convex. Description. Carapace subrectangular, 1.3 wider than long ( Figs. 1 b, c; 2a). Anterolateral margin with three teeth ( Fig. 2 b), widest being between tips of first teeth. Outer orbital tooth, made by external orbital angle, broad, directed outwards; anterior and posterior margins minutely serrated. Second tooth smaller, separated from first one by V-shaped incision, with margins minutely serrated. Third tooth reduced, indistinct, separated from second tooth by minute incision. Dorsal surface of carapace smooth with sparse tufts of short setae, regions poorly defined; branchial regions with few scattered setae and few low tubercles. Lateral margins with row of long setae. Front deflexed with smooth margins, broad, width 0.2–0.3 the distance between external orbital angles, not constricted between bases of ocular peduncles; anterior margin forming somewhat large open V in frontal view ( Fig. 2 c); upper face smooth, with median furrow filled with short setae. Upper orbital border curved with minutely granular margin. Lower orbital border with regular tubercles along whole length; pterygostomian region with line of granules sub-parallel to lower orbital border; granules very small not forming stridulating ridge with large tubercles. FIGURE 1. Habitat and live coloration of Macrophthalmus ( Chaenostoma ) lisae sp. nov. : a, beach at Mliha, Mayotte, KUW fieldwork 2009, stn 26, low tide; specimens were collected on the upper part of the beach (foreground); b, paratype female ov. 3.85 × 5.1 mm (MNHN B32362); c, holotype male 3.75 × 4.9 mm (MNHN B32254). Ocular peduncles short, stout, reaching outer orbital tooth but not projecting significantly beyond carapace margin; diameter of cornea about 0.3 times length of peduncle. Posteromedian margin of epistome straight; central region of anterior buccal cavity without median ridge ( Fig. 2 c). Ischium of third maxilliped 1.2–1.3 times length of merus; carpus inserted at distolateral angle of merus ( Fig. 2 d). Dorsal surface of merus of male cheliped separated from lateral, mesial surfaces by finely tuberculated edges; mesial surface smooth, without horny ridge; ventral margin with fringe of long setae ( Fig. 2 e). Carpus with few indistinct tubercles on dorsal surface, without spine, with long setae on disto-mesial angle. Chela with palm inflated, 1.1 times as high as long, furnished with mat of setae on inner surface, not extending on inner surface of fingers; upper margin rounded with small tubercles, lower margin rounded and smooth ( Figs. 2 f, g). Fingers 1.0–1.2 times as long as palm, cutting margins with sets of small triangular teeth but without differentiated tooth. Immovable finger undeflexed, outer surface smooth, with median longitudinal ridge, extending on distal half of outer surface of palm; inner surface smooth, with median longitudinal ridge not extending on inner surface of palm. Movable finger slightly curved; outer surface smooth with longitudinal ridge parallel to upper margin in proximal half; inner surface smooth. Merus and carpus of female cheliped similar to that of male in terms of tubercules and setae. Chela more elongated than in male, without mat of setae on inner surface of palm. Palm 0.9 as high as long, outer surface slightly convex, not inflated; upper margin slightly curved, weakly tuberculated, lower margin almost straight, smooth ( Fig. 2 m). Fingers 1.0–1.2 as long as palm, cutting margins smooth, distal thirds as corneous tips; immovable finger with median, sharp longitudinal ridge, extending onto distal half of palm. FIGURE 2 . Macrophthalmus ( Chaenostoma ) lisae sp. nov. , a–l, holotype male 3.75 × 4.9 mm (MNHN B32254); a, carapace dorsal face, b, detail of anterolateral margin; c, front view; d, left third maxilliped; e, left cheliped dorso-mesial view; f, left chela, outer view; g, same, inner view; h, left second pereopod, lateral view; i, left third pereopod, lateral view; j, abdomen and telson, outer view; k, right first pleopod, ventral view; l, same, dorso-mesial view, m–o, paratype female ov. 3.85 × 5.1 mm (MNHN B32362); m, left chela, outer view; n, abdomen and telson, outer view; o, left genital vulva, with indication of suture between fifth and sixth thoracic sternites. Second to fifth pereopods stout, setose on dorsal, ventral margins, P3–P4 the longest ( Figs. 2 h, i). Upper and lower margins of P3 merus granular with scattered setae, 2.4 (female) to 2.6 (male) times as long as high, distodorsal angle with small spine; carpus, propodus and dactyl unarmed, with smooth upper and lower margins, dactyl slightly longer than propodus. P4 merus granular on upper, lower margins, 2.2 (female) to 2.4 (male) times as long as high, distodorsal angle with small spine; carpus unarmed with few granules on distoventral margin, distal part of upper margin; propodus faintly granulated on upper, lower margins; dactyl unarmed slightly longer than propodus. Male abdomen narrow, somite 3 without transverse ridge, somite 6 the highest; telson with regularly convex lateral margins, 1.6 times as wide as long ( Fig. 2 j). Male first pleopod moderately compressed, slightly curved; lateral margin with long spaced setae, mesial margin with scattered short setae; terminal lobe with rounded horny apex, rows of stiffs bristles ( Figs. 2 k, l). Female abdomen wide, third somite without transverse ridge, somite 5 the highest; telson 3.4 times as wide as long, distolateral margins straight to feebly convex, basal margin feebly concave, as wide as distal margin of sixth somite ( Fig. 2 n). Female vulvae situated medially posterior to suture between sternites of fifth and sixth thoracic sternites, operculum situated postero-mesially, aperture oriented antero-laterally ( Fig. 2 o). Size. Males 3.75 × 4.94 mm and 4.06 × 5.44 mm ; females 2.61 × 3.33–3.7 × 4.9 mm ; ovigerous females 3.14 × 4.14–3.95 × 5.26 mm . Live coloration. Dorsal surface of carapace and appendages white-cream scattered with brownish patches ( Figs. 1 b, c). Coloration acts as camouflage, and the crabs were hardly noticeable on the substrate, when collected. Distribution. So far known only from the island of Mayotte , western Indian Ocean. Potentially also in neighbouring islands with similar biotopes ( Madagascar , Comoros , Glorieuses, Seychelles ) where the small size of the species may have made individuals difficult to detect. Habitat. Macrophthalmus lisae sp. nov. digs burrows on sandy mudflats. Entrances of burrows are located on the upper part of the intertidal area ( Fig. 1 a); they are only few millimetres in diameter and can remained unnoticed during sampling. Etymology. This new species is named after Lisa, ten-year old daughter of the second author, for enthusiastically participating in the collections.