Megaesthesius Rathbun, 1909

Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), Zootaxa 4209 (1), pp. 1-182 : 88-89

Genus Megaesthesius Rathbun, 1909

Megaesthesius Rathbun, 1909: 112 ; 1910: 344.— Serène 1964b: 175; 1968: 92 (in list).— Zarenkov, 1972: 241.—Crosnier 1975: 110.— Sakai 1976: 542 [in key], 552.—Ng et al. 2008: 76 [in list].— De Grave et al. 2009: 33 [in list].—Castro et al. 2010: 41.— Davie et al. 2015a: 43.

Type species. Megaesthesius sagedae Rathbun, 1909 (gender masculine) Other species included:

Megaesthesius migmus n. sp.

Megaesthesius westralia Davie, 2013

Megaesthesius yokoyai Sakai, 1939

Diagnosis. Small size, carapace subquadrate, high, strongly arcuate; front bilobed, with deep or shallow median cleft; anterolateral margins straight, armed with small teeth often arranged in 4 or 5 lobes; dorsal surface with conspicuous round granules. Epistome compressed, posterior margin with broad median lobe with fissure. Eye peduncle conspicuously long, immobile, cornea long, reduced, pigmentation reduced. Antennule greatly enlarged (particularly in males), cannot be folded. Third maxillipeds nearly fill buccal cavern when closed; merus subtriangular, outer margin convex, no defined anteroexternal angle; ischium subquadrate, about same length or longer than merus. Chelipeds subequal, nearly identical in both sexes; cutting margins of both chelipeds 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 major cheliped carpus with small tooth. Ventral surface of cheliped merus smooth or with teeth. Ambulatory legs with minute teeth 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 or long. Sterno-pleonal cavity of male deep. Press-button for pleonal holding as tubercle at middle portion of thoracic somite 5 near edge of sterno-pleonal 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” (unknown in M. westralia , M. yokoyai ), sternite 8. G1 slender, distal segment straight with spinules. G2 straight, slender, distal segment directed inwards, nearly as long as G1. Somites of female pleon with gently convex margins; telson proportionally short. Sterno-pleonal cavity of female deep, vulvae on outer margins of cavity between sutures 5/6, 6/7.

Remarks. Megaesthesius Rathbun, 1909 , is unusual in many respects, even within the Chasmocarcinidae . Most outstanding, a character that has been discussed at length by Serène (1964b) and Crosnier (1975), is the disproportionately large and exposed antennular peduncle that is densely covered with sensory setae (aesthetascs). The eye peduncle is also large and long, but is fused to the orbit with the cornea being barely discernible. It is probably for these reasons that Števčić (2005: 108) established a new subfamily, Megaesthesiinae , for the genus, although he did not explain why. We have also observed that the penis is covered by a calcified plate that lies between the “supplementary plate” and thoracic sternite 8 ( Fig. 19 W–Y), a character unique to Chasmocarcinidae . On the basis of these characters, it seems reasonable that Megaesthesius should be placed in its own subfamily (see also Ng et al. 2008: 76).

Crosnier’s (1975) record of “ Megaesthesius sagedae ” from Madagascar requires discussion. Although the carapace of the Madagascar material superficially resembles M. sagedae , it is clearly not M. sagedae as is known at present (see below) and must be recognised as a new genus and new species. More significantly, its G2 is short, with the basal segment short and the distal segment spatuliform with only with a relatively short elongation, and as such, is only about 2/3 length of the G1 ( Fig. 83 C) (G2 basal and distal segments markedly long, exceeding G1 length in other Megaesthesius species, Fig. 82 C, F, M). It makes more sense to separate the new Madagascar species into a new genus, here named Alainthesius n. gen. (see below). A second species of Alainthesius , A. bertrandi n. sp., is also described from the western Pacific .

Distribution. Indo-West Pacific region: eastern 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 82. Gonopods of species of Megaesthesius. A ‒ C, M. sagedae, Singapore: male 2.5 × 3.0 mm (ZRC 1995.885); D ‒ F, M. migmus n. sp., Philippines: holotype male 4.4 × 4.9 mm (NMCR); G ‒ I, M. yokoyai, Japan: male 3.5 × 3.8 mm (after Takeda & Miyake 1969: fig. 4 h, i); J ‒ M, M. westralia, Australia: holotype male 3.4 × 3.8 mm (after Davie 2013: fig. 3 A ‒ D). A, D, G, H, J, K, ventral view of left G 1; B, E, I, K, L, dorsal view of left G 1; C, F, M, left G 2. Scales = 0.1 mm.

FIGURE 83. Gonopods of species of Alainthesius n. gen. A ‒ C, A. signatus n. sp., Madagascar: holotype male 2.5 × 3.3 mm (MNHN-IU- 2013 - 9472) (after Crosnier 1975: fig. 1 f ‒ h); D ‒ H, A. bertrandi n. sp., New Caledonia: holotype male 3.3 × 4.1 mm (MNHN-IU- 2013 - 9073). A, ventral view of right G 1; D, ventral view of left G 1; B, dorsal view of right G 1; E, G, dorsal view of distal part of left G 1; F, ventral view of distal part of left G 1; C, right G 2; H, left G 2. Scales = 0.1 mm.