Megaesthesiinae Števčić 2005

Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), Zootaxa 4209 (1), pp. 1-182 : 87

Subfamily Megaesthesiinae Števčić 2005

Subfamily Megaesthesiinae Števčić 2005: 108 ; 2013: 186.—Ng et al. 2008: 27, 76.— De Grave et al. 2009: 33 [in list].— Davie et al. 2015c: 1082.

Type genus. Megaesthesius Rathbun, 1909 Other genera included:

Alainthesius n. gen.

Diagnosis. Minute size, carapace subquadrate, high. Eye peduncle conspicuously long, immobile, cornea long, eye reduced, eye pigmentation reduced. Antennule greatly enlarged (particularly in males), cannot be folded. Buccal cavern conspicuously arched, sunken. Chelae nearly equal in both sexes. Outer (ventral) surface of penis calcified, resembling plate between “supplementary plate”, sternite 8.

Remarks. The structure of the penis and the calcified plate covering it is of interest. Like all chasmocarcinids, megaesthesiines possess a distinctive “supplementary plate” between thoracic sternites 7 and 8. This “plate” is an invagination of sternite 8, and in chasmocarcinids the elongated penis is positioned just below this plate and the rest of sternite 8. In megaesthesiines there is a wider space between the “supplementary plate” and the rest of sternite 8 where the penis sits, and most of the penis (except for the distal part) is surrounded by a calcified tube, with the upper part forming an elongated plate ( Fig. 19 W‒Y). This penis plate sits exactly in the space between the “supplementary plate” and the rest of sternite 8. In Megaesthesius sagedae , Alainthesius signatus n. sp. and A. bertrandi n. sp., this penis plate does not move at all when the coxa of the fifth pereiopod (from which the penis emerges) is engaged and is effectively immobile, although the margins do not appear to be fused with the thoracic sternite ( Fig. 19 W‒Y). In Megaesthesius migmus n. sp., however, the penis plate is mobile ( Fig. 19 X). This is a feature that will need further study to determine if it is affected by growth or if it varies. The lack of additional male specimens of Megaesthesius migmus n. sp., Alainthesius signatus n. sp., and A. bertrandi n. sp. (only one of each is known) poses limitations to study at present.

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.