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

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Megaesthesius sagedae Rathbun, 1909


Megaesthesius sagedae Rathbun, 1909 

( Figs. 19View FIGURE 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) ( ZMUCAbout 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 ( ZRCAbout ZRC 2015.303View Materials), 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) ( ZRCAbout ZRC 2015.264View Materials), 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. 19View FIGURE 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. 19View FIGURE 19 A, B). Orbits long. Epistome ( Figs. 19View FIGURE 19 S; 27B) compressed, posterior margin with broad median lobe with fissure, lateral margins short, straight. Eye peduncle conspicuously long, immobile, cornea long ( Fig. 27View FIGURE 27 B; Serène 1964b: fig. 2), pigmentation reduced. Antennule ( Figs. 19View FIGURE 19 A, B; 27B) greatly enlarged (particularly in males), cannot be folded. Third maxillipeds ( Fig. 34View FIGURE 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. 19View FIGURE 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. 55View FIGURE 55 C; 88F) broadly triangular, proportionally wide, short; fused sternites 3, 4 relatively broad. Male pleon ( Figs. 55View FIGURE 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. 19View FIGURE 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. 19View FIGURE 19 W). G1 ( Fig. 82View FIGURE 82 A, B; Zarenkov 1972: 6- 1I) slender, distal segment straight with spinules, apex slanted. G2 ( Fig. 82View FIGURE 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. 92View FIGURE 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.


Zoological Museum, University of Copenhagen


Zoological Reference Collection, National University of Singapore














Megaesthesius sagedae Rathbun, 1909

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


Megaesthesius sagedae

Rathbun 1909: 112