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

Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), Zootaxa 4209 (1), pp. 1-182 : 90-91

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.

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 20. Overall dorsal view of species of Megaesthesius and Alainthesius n. gen. A, Megaesthesius migmus n. sp., Philippines: holotype male 4.4 × 4.9 mm (NMCR); B, Megaesthesius migmus n. sp., Philippines: paratype female 3.9 × 4.5 mm (ZRC 2015.245); C, Alainthesius signatus n. sp., Madagascar: holotype male 2.5 × 3.3 mm (MNHN-IU- 2013 - 9472); D, Alainthesius signatus n. sp., Madagascar: paratype female 2.7 × 3.0 mm (MNHN-IU- 2013 - 9471); E, F, Alainthesius bertrandi n. sp., New Caledonia: holotype male 3.3 × 4.1 mm (MNHN-IU- 2013 - 9073); G, Alainthesius bertrandi n. sp., Papua New Guinea: female 5.6 × 6.9 mm (MNHN-IU- 2013 - 9012); H, Alainthesius bertrandi n. sp., New Caledonia: female 3.4 × 4.0 mm (MNHN-IU- 2013 - 9007).

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 48. Chelae of species of Megaesthesius, Alainthesius n. gen. and Microtopsis. A, B, Microtopsis takedai, Japan: holotype male 2.4 × 3.1 mm (after Komai et al. 2012: fig. 3 B, D); C, Megaesthesius sagedae, Singapore: male 2.5 × 3.0 mm (ZRC 1995.883); D, Megaesthesius migmus n. sp., Philippines: holotype male 4.4 × 4.9 mm (NMCR); E, Megaesthesius yokoyai, Japan: male (after Takeda & Miyake 1969: fig. 4 d); F, Alainthesius signatus n. sp., Madagascar: holotype male 3.5 × 3.8 mm (after Crosnier 1975: fig. 1 d); G, Alainthesius bertrandi n. sp., New Caledonia: holotype male 3.3 × 4.1 mm (MNHN- IU- 2013 - 9073); H, Alainthesius bertrandi n. sp., Papua New Guinea: female 5.6 × 6.9 mm (MNHN-IU- 2013 - 9012).

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 88. Female thoracic sternum and pleon of species of Statommatia n. gen., Tenagopelta n. gen., Microtopsis and Megaesthesius. A, Statommatia apta, Philippines: female 8.5 × 10.6 mm (ZRC 2015.238); B, Statommatia malagasy n. sp., Madagascar: paratype female 9.4 × 8.1 mm (ZRC 2015.215); C, Tenagopelta pacifica n. sp., Philippines: female 9.2 × 10.4 mm (ZRC 2015.187); D, Tenagopelta brachyphallus n. sp., Australia: female 6.9 × 7.6 mm (NMV J 61680); E, Microtopsis takedai, Japan: paratype female 3.6 × 2.7 mm (ZRC 2012.0146); F, Megaesthesius sagedae, Singapore: female 3.0 × 3.6 mm (ZRC 2013.1804); G, Megaesthesius migmus, Philippines: paratype female 3.9 × 4.5 mm (ZRC 2015.245); H, Alainthesius bertrandi n. sp., Papua New Guinea: female 5.6 × 6.9 mm (MNHN-IU- 2013 - 9012). Vulvae exposed in E.

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).

FIGURE 103 Megaesthesius migmus n. sp. A, Philippines: holotype male 4.4 × 4.9 mm (NMCR); B – D, Philippines: paratype female 4.6 × 5.2 mm (ZRC 2015.243). [P. K. L. Ng]