Megaesthesius migmus Ng & Castro, 2016, n. sp.
Creators: name: Ng, Peter K. L.
name: Castro, Peter
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Chasmocarcinidae
Megaesthesius
Megaesthesius migmus

description: Megaesthesius migmus n. sp.
(Figs. 19 T, X; 20A, B; 27C; 34D; 48D; 55D; 62C; 82D–F; 88G; 92C; 103)
Type material. Holotype male (4.4 × 4.9 mm) (NMCR) [photographed], Philippines, Bohol Sea, off Pamilacan I., PANGLAO 2005, stn CP2344, 09°28.4’N, 123°50.1’E, 128–142 m, 23.05.2005.
Paratypes: 1 female (4.6 × 5.2 mm) (ZRC 2015.243) [photographed], Phillippines, Bohol Sea, off Balicasag I., PANGLAO 2005, stn CP2240, 09°29’N, 123°44'E, 271–318 m, 27.05.2005, 23.05.2005; 1 paratype female (4.7 × 5.7 mm) (ZRC 2015.244), Bohol Sea, Maribojoc Bay, PANGLAO 2005, stn CP2407, 09°41’N, 123°48'E, 256–268 m, 01.06.2005.— 1 female (3.9 × 4.5 mm) (ZRC 2015.245) [photographed], northwestern Philippines, AURORA 2007, stn CP2711, 15°19’N, 121°32’E, 184–200 m, 28.05.2007.
Diagnosis. Carapace (Figs. 20 A, B; 103A–C) subquadrate, 1.1 wider than long; front bilobed, with deep median cleft; dorsal surface with low, spherical tubercles; post-orbital margins sloping to angular anteroexternal angle marked by small tooth in some specimens. Lateral margins straight, parallel to each other, each with 3 straight, dented regions separated by deep notches, acute or obtuse tooth on posterior third of margin. Posterior margin nearly straight. Long orbits, eye peduncle (Figs. 27 C; 103D) conspicuously long, immobile, cornea reduced, non-pigmented. Epistome (Fig. 19 T) compressed, posterior margin with broad median lobe with fissure, narrow, nearly straight lateral margins without fissures. Antennule (Fig. 27 C) greatly enlarged, particularly in males. Third maxillipeds (Fig. 34 D) nearly fill buccal cavern when closed; merus subtriangular, outer margin dentate; ischium subquadrate, dentate outer margin, longer than merus; outer margins of merus, ischium dentiform. Chelipeds (Figs. 20 A, B; 103A–C) subequal in length, nearly similar in both sexes; cutting margins of both chelae (Fig. 48 D) with broad, shallow teeth in both sexes; ventral margin of palm dentiform; ventral surface of cheliped merus smooth. Proportionally long ambulatory legs (Fig. 20 A, B), P5 merus not reaching front when folded. Dorsal, ventral margins of meri, dorsal margin of carpus of ambulatory legs with minute teeth. P5 merus 0.7‒0.8 cl. Small, sharp tooth on inner margin of cheliped carpus, ventral surface of cheliped merus smooth in both sexes. Fused thoracic sternites 1, 2 (Fig. 55 D) broadly triangular, proportionally wide, short. Male pleon with lateral margins of somite 6, fused somites 3–5 nearly straight; postero-lateral regions slightly swollen; telson proportionally short (Figs. 55 D). Outer (ventral) surface of penis calcified, resembling plate between “supplementary plate”, sternite 8 (Fig. 19 X). G1 (Fig. 82 D, E) slender, distal segment, straight, with spinules, apex slanted. G2 (Fig. 82 F) nearly as long as G1, straight; slender, distal segment long, curved, as long as basal segment, directed inward. Somites of female pleon (Fig. 88 G) with gently convex margins; telson proportionally short. Sterno-pleonal cavity of female (Fig. 92 C) deep, vulvae far apart from each other on outer margins of cavity close to suture 5/6.
Colour. The carapace is white to pinkish-white beige in life, with white chelipeds and ambulatory legs (Fig. 103).
Etymology. From the Greek migma for “mixture” in reference to the mixture of characters of the other species of Megaesthesius that characterise the new species.
Remarks. Megaesthesius migmus n. sp. is distinct from M. sagedae in having the carapace proportionately wider (Fig. 20 A, B versus Fig. 19 A, B), the lateral margin of the carapace having distinct lobiform teeth (Fig. 20 A, B) (only small spines in M. sagedae, Fig. 19 A, B), the aesthetascs of the antennules are distinctly shorter (Fig. 27 C versus Fig. 27 B); the fingers of the chela are relatively more slender and longer (Fig. 48 C versus Fig. 48 D); and the G1 is relatively longer and more slender (Fig. 82 D, E in contrast to Fig. 82 A, B). As discussed earlier, the calcified plate on the surface of the penis is mobile in the single male of M. migmus examined (Fig. 19 X) but is immobile in M. sagedae (Fig. 19 W); but with one male of the former known, it is not clear if this is also a species-specific character.
Compared to M. yokoyai, M. migmus n. sp. is easily distinguished by its proportionately longer ambulatory legs (Fig. 20 A, B versus Fig. 19 E); the outer margins of the merus and ischium of the third maxilliped are lined with sharp teeth (Fig. 34 D) (unarmed in M. yokoyai, Fig. 34 B); the fingers of the chela are longer and the ventral margin of the palm is lined with sharp teeth (Fig. 48 D) (fingers short with the ventral margin of the palm smooth in M. yokoyai, Fig. 48 E); and the G1 is proportionately more slender with the distal part tapering and the opening large (Fig. 82 D, E) (G1 relatively stouter, with distal part not tapering and the opening rounded in M. yokoyai, Fig. 82 G–I).
Distribution. Known only from Philippines. Depth: 128‒ 318 m.
description: Published as part of Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), pp. 1-182 in Zootaxa 4209 (1) on pages 90-91, DOI: 10.11646/zootaxa.4209.1.1, http://zenodo.org/record/272646