Alainthesius, Ng, Peter K. L. & Castro, Peter, 2016

Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), Zootaxa 4209 (1), pp. 1-182 : 94-106

Genus Alainthesius n. gen.

Megaesthesius —Crosnier 1975: 110 (part).

Type species. Alainthesius signatus n. sp. (gender masculine) Other species included:

Alainthesius bertrandi n. sp.

Diagnosis. Small size, carapace quadrate, high, front bilobed, with median cleft; anterolateral margins straight, parallel to each other, armed with small teeth arranged in 3 or 4 lobes; dorsal surface smooth. Epistome ( Fig. 19 U, V) compressed, posterior margin with broad median lobe with fissure, straight lateral margins. Eye peduncle conspicuously long, immobile, cornea long, pigmentation reduced. Antennule greatly enlarged (particularly in males), cannot be folded. Third maxillipeds nearly fill buccal cavern when closed; merus subtriangular, outer margin convex; ischium subquadrate, about same length or longer than merus; outer margin with teeth ( A. signatus n. sp.; Fig. 34 E) or smooth ( A. bertrandi n. sp.; Fig. 34 F). Chelipeds subequal, nearly identical in both sexes; cutting margins of both chelae with broad, shallow teeth in both sexes; fingers of minor chela relatively broad, proportionally short, prominently flattened laterally, cutting margins blade-like. Inner margin of cheliped carpus with small tooth. Ventral margin of cheliped propodus with teeth. Ventral surface of chelipeds merus smooth or with low tubercles. Ambulatory legs with minute teeth or tubercles on anterior or posterior or both margins of most articles. Fused thoracic sternites 1, 2 broadly triangular, short; fused sternites 3, 4 relatively broad. Male pleon with lateral margins of somite 6, fused somites 3–5 slightly convex; postero-lateral regions slightly swollen; telson proportionally short ( A. signatus n. sp.; Fig. 19 L) or long ( A. bertrandi n. sp.; Fig. 55 F). Sterno-pleonal cavity of male deep, press-button for pleonal holding as tubercle at middle portion of thoracic somite 5 near edge of sternopleonal cavity. 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 stout ( A. signatus n. sp.) or slender ( A. bertrandi n. sp.), distal segment straight with spinules. G2 about 1/2 G1, stout, distal segment long, wide, almost as long as basal segment, directed inward. Somites of female pleon with slightly convex lateral margins; telson proportionally long. Sterno-pleonal cavity of female deep, vulvae on outer margins of cavity between sutures 5/6, 6/7.

Etymology. The name is derived from Alain Crosnier , who described the morphology of this species (as Megaesthesius sagedae ) at length, in arbitrary combination with the genus name Megaesthesius . Gender of genus masculine.

Remarks. The differences between Alainthesius n. gen. and Megaesthesius Rathbun, 1909 , have been discussed under the remarks of the latter genus.

Distribution. Indo-West Pacific region: western Indian and western Pacific oceans.

Figures

FIGURE 19. Species of Megaesthesius and Alainthesius n. gen. A, B, Megaesthesius sagedae, Singapore: male 2.5 × 3.0 mm (ZRC 1995.883); C ‒ F, Megaesthesius yokoyai (after Takeda & Miyake 1969 a: figs. 3 a, 4 f, 4 a, 4 e, respectively); G ‒ L, Alainthesius signatus n. sp., Madagascar: holotype male 2.5 × 3.3 mm (MNHN-IU- 2013 - 9472) (after Crosnier 1975: fig. 1 a, 1 k, 1 j, 1 b, 1 c, 1 i, respectively); M ‒ R, Megaesthesius westralia, Australia: holotype male 3.4 × 3.8 mm (after Davie 2013: figs. 1 A, B, 2 A, D, E, 3 E); S, W, Megaesthesius sagedae, Singapore: male 2.5 × 3.0 mm (ZRC 1995.885); T, X, Megaesthesius migmus n. sp., Philippines: holotype male 4.4 × 4.9 mm (NMCR); U, Y, Alainthesius bertrandi n. sp., New Caledonia: holotype male 3.3 × 4.1 mm (MNHN-IU- 2013 - 9073); V, Alainthesius bertrandi n. sp., Papua New Guinea: female 5.6 × 6.9 mm (MNHN-IU- 2013 - 9012). A, overall habitus; B, C, G, M, dorsal view of carapace; E, left P 4; F, I, frontal view of cephalothorax; H, O, frontal and anterolateral margin of carapace; N, ventral view of cephalothorax; J, right P 3; K, buccal cavity and posterior margin of epistome; Q, right P 4; R, right P 5; D, L, P, male pleon; S ‒ V, posterior margin of epistome; W ‒ Y, left thoracic sternites 7 and 8, “ supplementary plate ”, penis tube and penis. Abbreviations: cx 4 = coxa of P 4; cx 5 = coxa of P 5; p = penis; pt = calcified penis tube; sp = “ supplementary plate ”; st 7 = thoracic sternite 7; st 8 = thoracic sternite 8.

FIGURE 34. Third maxilliped of species of Megaesthesius and Alainthesius n. gen. A, Megaesthesius sagedae, Singapore: male 2.5 × 3.0 mm (ZRC 1995.885); B, Megaesthesius yokoyai, Japan: male 3.5 × 3.8 mm (after Takeda & Miyake 1969: fig. 4 c); C, Megaethesius westralia, Australia: holotype male 3.4 × 3.8 mm (after Davie 2013: fig. 2 C); D, Megaesthesius migmus n. sp., Philippines: paratype female 4.6 × 5.2 mm (ZRC 2015.243); E, Alainthesius signatus n. sp., Madagascar: holotype male 2.5 × 3.3 mm (after Crosnier 1975: fig. 1 e); F, Alainthesius bertrandi n. sp., New Caledonia: holotype male 3.3 × 4.1 mm (MNHN-IU- 2013 - 9073). Scales = 0.5 mm.

FIGURE 55. Anterior thoracic sternum and male pleon of species of Statommatia n. gen., Microtopsis and Megaesthesius. A, Statommatia granulosa n. sp., New Caledonia: holotype male 4.1 × 4.7 mm (MNHN-IU- 2013 - 9454); B, Microtopsis takedai, Japan: paratype male, carapace damaged (ZRC 2012.0146); C, Megaesthesius sagedae, Singapore: male 2.5 × 3.0 mm (ZRC 1995.885); D, Megaesthesius migmus n. sp., Philippines: holotype male 4.4 × 4.9 mm (NMCR); E, Alainthesius signatus n. sp., Madagascar: holotype male 2.5 × 3.3 mm (MNHN-IU- 2013 - 9472); F, Alainthesius bertrandi n. sp., New Caledonia: holotype male 3.3 × 4.1 mm (MNHN-IU- 2013 - 9073).