Megaesthesius sagedae 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 : 89-90

Megaesthesius sagedae Rathbun, 1909

( Figs. 19 A, B, S, W; 27B; 34A; 48C; 55C; 62B; 82A–C; 88F; 92B)

Megaesthesius sagedae Rathbun, 1909: 112 [type locality: Singapore]; 1910: 344, figs. 30, 31, pl. 2, fig. 5.— Serène 1964b:

176, figs. 1–6 [South China Sea]; 1968: 92 [in list].— Zarenkov 1972: 241, fig. 6-1 [Vietnam].—Ng et al. 2008: 76 [in list].? Megaesthesius sagedae — Serène & Vadon 1981: 118, 119, 123 [Philippines].—Ng & Davie 2002: 378 [Western Thailand?] Not Megaesthesius sagedae —Davie 2013: 475, figs. 3J–N, 4. [= Alainthesius signatus ]

Type material. Holotype male (2.5 × 2.8 mm) ( ZMUC) [no pleon or gonopods], Singapore, Mortensen Expedition 1899–1900, 3.6–5.5 m.

Other material examined. Vietnam. 1 male, 1 female (MNHN-IU-2013-9473 [= MNHN-B10215), Gulf of Tonkin, trawl, Orlik, N. Zarenkov coll., 22.04.1963.

Malaysia. 1 male 2.3 × 2.8 mm ( ZRC 2015.303 ), stn F28 oilfield, offshore Bintulu, northern Sarawak, deep water, T.S. Leong coll., 2000s.

Singapore. 1 male (2.4 × 2.6 mm) , 1 female (cl 2.9 mm, carapace damaged) (ZRC 1995.883), Pulau Semakau, station 2, grab 2, Reef Ecology Study Team, 08.06.1995.—1 male (2.5 × 3.0 mm) (ZRC 1995.885), Reef Ecology Study Team, Pulau Semakau, station 2, grab 4, 08.06.1995.— 1 male (2.2 × 2.5 mm) ( ZRC 2015.264 ), station 5515- ORI-010, northeast of Sentosa I., 01°15.574' N –01°15.831' N to 103°51.675'E–103°51.524'E, rectangular dredge, 18‒19 m, mud, Comprehensive Marine Biodiversity Survey, 21.03.2013.—1 male (2.6 × 2.9 mm), 1 female (3.0 × 3.6 mm) (ZRC 2013.1804), Pulau Semakau, station PS25T2, Reef Ecology Study Team coll., 1990s; 1 female (3.3 × 4.0 mm), 1 female (3.3 × 3.7 mm) (QM), Pulau Semakau, Reef Ecology Study Team coll., 1990s.

Diagnosis. Small size. Carapace ( Fig. 19 A, B; Serène 1964b: fig. 1) subquadrate, 1.1‒1.2 wider than long; front bilobed, with well-defined median cleft; anterolateral margins straight, with numerous small, sharp teeth, spines; conspicuous granules through most of carapace, larger along posterolateral and anterior portions ( Fig. 19 A, B). Orbits long. Epistome ( Figs. 19 S; 27B) compressed, posterior margin with broad median lobe with fissure, lateral margins short, straight. Eye peduncle conspicuously long, immobile, cornea long ( Fig. 27 B; Serène 1964b: fig. 2), pigmentation reduced. Antennule ( Figs. 19 A, B; 27B) greatly enlarged (particularly in males), cannot be folded. Third maxillipeds ( Fig. 34 A; Zarenkov 1972: 6- 1III) nearly fill buccal cavern when closed; merus subtriangular, outer margin convex, no defined anteroexternal angle; ischium subquadrate, about same length than merus; outer margins of ischium, merus with small spines and teeth. Chelipeds ( Figs. 19 A; 48C; Serène 1964b: fig.

1; Zarenkov 1972: 6- 1I) nearly identical in both sexes; cutting margins of both chelae with broad, shallow teeth in both sexes; small chela laterally flattened, fingers short, prominently flattened laterally, cutting margins blade-like; ventral margin of palm dentiform; ventral surface of cheliped merus with smooth or with obtuse tubercles. Inner margin of cheliped carpus with small acute tooth. Ventral surface of cheliped merus smooth in both sexes. Proportionally long ambulatory legs ( Serène 1964b: fig. 1), P5 merus not reaching front when folded. Minute teeth on anterior or posterior or both margins of most articles ( Serène 1964b: fig. 1). Fused thoracic sternites 1, 2 ( Figs. 55 C; 88F) broadly triangular, proportionally wide, short; fused sternites 3, 4 relatively broad. Male pleon ( Figs. 55 C; 62B) with lateral margins of somite 6, fused somites 3–5 slightly convex; postero-lateral regions slightly swollen; telson proportionally 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 ( Figs. 19 W, 62B) 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 ( Fig. 19 W). G1 ( Fig. 82 A, B; Zarenkov 1972: 6- 1I) slender, distal segment straight with spinules, apex slanted. G2 ( Fig. 82 C; Zarenkov 1972: 6- 1II) nearly as long as G1, straight, slender, distal segment long, as long as basal segment, directed inward. Somites of female pleon with gently convex margins; telson proportionally short. Sterno-pleonal cavity of female ( Fig. 92 B) deep, vulvae far apart from each other on outer margins of cavity close to suture 5/6.

Remarks. The figure given in Rathbun (1910: figs. 30, 31, pl. 2, fig. 5) of this species is relatively simple and the photograph of her only specimen, a male, is too small to discern any details. Serène (1964b: figs. 1–6) examined the specimen and the carapace, chelipeds, and mouthparts in his figures agree with those of Megaesthesius sagedae examined here from the type locality in Singapore. Unfortunately, the present condition of the type specimen in ZMUC is very poor, and the pleon, G1, and G2 are all missing.

We are confident that Zarenkov’s (1972: 241, fig. 6-1) record of this species from Vietnam is M. sagedae as it agrees very well with the specimens from Singapore. Serène & Vadon (1981: 123) reported on a female, 4.5 × 6.0 mm, collected by MUSORSTOM 2 from station CP15 from the entrance of Luzon and Lubang in the western Philippines (13°55’N, 120°29'E, 326–330 m, 21 November 1980) but we have not been able to find the specimen in MNHN. Like many of the identified specimens from this expedition reported by these authors, the specimens could not be found (e.g., see Ng & Chen 2005; Richer de Forges & Ng 2012). Its identity must be regarded as incerta sedis until it can be found. Megaesthesius migmus n. sp. has been recorded from deep waters in Philippines, and it is possible their record is this new species instead. Megaesthesius migmus n. sp. was found in the Bohol and Luzon Seas to the east. The record of “ Megaesthesius sagedae ” by Ng & Davie (2002: 378, Western Thailand) will also need to re-examined to ascertain its actual identity.

The present specimens from Singapore were all collected from soft substrates in relatively shallow water, all less than 20 m.

The major differences between the four taxa are discussed under the Remarks for M. migmus n. sp. and M. yokoyai (see below and the key to the species).

Distribution. Western Pacific Ocean ( Vietnam and Singapore; questionably from Andaman Sea). Depth: 3.6– 19 m.

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 27. Frontal view of species of Megaesthesius and Microtopsis. A, Microtopsis takedai, Japan: paratype female 3.6 × 2.7 mm (ZRC 2012.0146); B, Megaesthesius sagedae, Singapore: male 2.5 × 3.0 mm (ZRC 1995.883); C, Megaesthesius migmus n. sp., Philippines: holotype male 4.4 × 4.9 mm (NMCR); D, Alainthesius signatus n. sp., Madagascar: paratype female 2.7 × 3.0 mm (MNHN-IU- 2013 - 9471); E, Alainthesius bertrandi n. sp., New Caledonia: holotype male 3.3 × 4.1 mm (MNHN-IU- 2013 - 9073); F, Alainthesius bertrandi n. sp., Papua New Guinea: female 5.6 × 6.9 mm (MNHN-IU- 2013 - 9012).

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).

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 92. Vulvae of species of Microtopsis and Megaesthesius. A, Microtopsis takedai, Japan: paratype female 3.6 × 2.7 mm (ZRC 2012.0146); B, Megaesthesius sagedae, Singapore: female carapace length 2.9 mm, sides damaged (ZRC 1995.883); C, Megaesthesius migmus n. sp., Philippines: paratype female 3.9 × 4.5 mm (ZRC 2015.245); D, Alainthesius bertrandi n. sp., Papua New Guinea: female 5.6 × 6.9 mm (MNHN-IU- 2013 - 9012).