Alainthesius signatus n. sp.

(Figs. 19 G–L; 20C, D; 27D; 34E; 48F; 55E; 83A–C)

Megaesthesius sagedae —Crosnier 1975: 110, fig. 1 [Madagascar]; Davie 2013: 475, figs. 3J–N, 4. Not Megaesthesius sagedae Rathbun, 1909 .

Type material. Holotype male (2.5 × 3.3 mm) (MNHN-IU-2013-9472, ex MNHN-B10214), Madagascar, Tulear, 23°23.0’S, 43°36.5’E, dredging, 72 m, C. Jouannic coll., 20.02 . 197372 m.

Paratype: female (2.7 × 3.0 mm) (MNHN-IU-2013-9471, ex MNHN-B10213), collected with holotype.

Diagnosis. Small size. Carapace (Figs. 19 G; 20C, D; 27D; Crosnier 1975: fig. 1a, as Megaesthesius sagedae) subquadrate, 1.1‒1.3 wider than long, dorsal surface smooth; front bilobed, with well-defined median cleft; anterolateral margins straight, with small, sharp teeth arranged in 3 or 4 lobes. Orbits long. Epistome (Fig. 19 K; Crosnier 1975: fig. 1b, as M. sagedae) compressed, posterior margin with broad median lobe with median fissure, straight lateral margins without fissures. Eye peduncle (Figs. 19 I; 27D; Crosnier 1975: fig. 1k, as M. sagedae) conspicuously long, immobile, cornea long, pigmentation reduced. Antennule (Figs. 19 G, H; 27D; Crosnier 1975: 1a) greatly enlarged (particularly in males), cannot be folded. Third maxillipeds (Fig. 34 E; Crosnier 1975: 1e) nearly fill buccal cavern when closed; merus subtriangular, outer margin convex; ischium subquadrate, about same length or longer than merus; outer margins of merus, ischium with teeth. Chelipeds (Fig. 48 F; Crosnier 1975: fig. 1d) nearly identical in both sexes; cutting margins of both chelae with broad, shallow teeth in both sexes; small chela laterally flattened, shear-like; ventral surface of cheliped merus sometimes with obtuse tubercles. Inner margin of cheliped carpus (Fig. 20 D) with one or more small, sharp tubercles or teeth. Ventral surface of chelipeds merus with low tubercles. Proportionally long ambulatory legs (Fig. 19 J; Crosnier 1975: fig. 1i), P5 merus not reaching front when folded; minute teeth on anterior or posterior or both margins of most articles. Fused thoracic sternites 1, 2 (Fig. 19 K; 55E) broadly triangular, proportionally wide, short; fused sternites 3, 4 relatively broad. Male pleon (Fig. 19 L; Crosnier 1975: 1b) with lateral margins of somite 6, fused somites 3–5 slightly convex; postero-lateral regions slightly swollen; telson proportionally short. Male thoracic sternite 8 proportionally long, rectangular; “supplementary plate” conspicuously narrow, only reaching median portion of exposed thorax, short, slightly longer at rounded outer margin. Outer (ventral) surface of penis calcified, resembling plate between “supplementary plate”, sternite 8. G1 (Fig. 83 A, B; Crosnier 1975: fig. 1f, g, as M. sagedae) stout, distal segment straight with spinules. G2 (Fig. 83 C; Crosnier 1975: fig. 1h, as M. sagedae) about 1/2 G1, stout, distal segment long, wide, almost as long as basal segment, directed inward. Adult female unknown.

Etymology. The species name is derived from signum, Latin for “signal”, alluding to the large antennules of the species.

Remarks. The differences with A. bertrandi n. sp. are discussed under that species. Distribution. Known only from Madagascar, 72 m.