Migmarma, Schubart & Ng, 2020

Schubart, Christoph D. & Ng, Peter K. L., 2020, Revision of the intertidal and semiterrestrial crab genera Chiromantes Gistel, 1848, and Pseudosesarma Serène & Soh, 1970 (Crustacea: Brachyura: Sesarmidae), using morphology and molecular phylogenetics, with the establishment of nine new genera and two new species, Raffles Bulletin of Zoology 68, pp. 891-994 : 967

Migmarma , new genus

Type species. Sesarma (Sesarma) modesta De Man, 1902 , by present designation. Gender neuter.

Diagnosis. Carapace squarish to trapezoidal, wider than long; frontal margin weakly bilobed, gently deflexed, wider than posterior carapace margin; lateral margins of carapace with one tooth, posterolateral part subparallel or gently diverging; regions of carapace demarcated; postfrontal and epigastric crests separated by relatively deep grooves, margin relatively distinct, regions clearly separated; basal articles of antenna and antennules separated by septum; dorsal margin of palm without longitudinal pectinated ridge, inner surface gently convex, without prominent granulated ridge, outer surface and pollex gently convex or almost flat, covered with small granules; dorsal margin of chelipedal dactylus smooth in adult males or lined with small granules of various sizes; inner distal margin of merus of cheliped not lamelliform; inner surfaces of first to third ambulatory coxae with only scattered short setae between them, not arranged into tufts; male thoracic sternites 2–4 relatively narrow in adults, suture between sternites 3 and 4 shallow but visible; male sternopleonal cavity reaching two-thirds length of sternite 4 to before anterior margin of sternite 2; without trace of press-button on sternite 5, posterior edge of sternite 4 not expanded, no trace of obvious pleonal locking mechanism; G1 stout to relatively slender, subdistal part slightly swollen or subequal to width of base; chitinous part short to relatively long. Vulva is on anterior part of sternite 6, anterior edge presses against sternite 5; anterior sternal vulvar cover very low, posterior sternal vulvar cover low, rim-like; opening cylindrical, projecting, tip rounded, directed obliquely anteriorly.

Etymology. The name is derived from the Latin “migma” for mixture, with the ending of the genus name Sesarma . This alludes to the mix of characters present in the type species. The gender is neuter.

Included species. Sesarma (Sesarma) modesta De Man, 1902 ; Migmarma lustrum , new species; Migmarma lorentzi , new species.

Remarks. Migmarma , new genus, is established for three species with a mix of characters that make their assignment to Pseudosesarma or Bresedium difficult; but which are nevertheless genetically related ( Fig. 59 ). The carapaces of M. modestum and M. lustrum , new species, are similar, being trapezoidal but clearly much wider than long ( Figs. 45B , 47B ); while that of M. lorentzi , new species, is distinctly quadrate ( Fig. 50B ). Their male pleons are triangular but the telson is normal in M. modestum and M. lustrum ( Figs. 45D , 46B , 47H ), but slightly sunken into somite 6 in M. lorentzi ( Fig. 50E ). Their G1s are relatively stout to more slender with the chitinous distal part relatively short or more elongate ( Figs. 46C, D , 49A, B , 51A, B ). None of them can be placed in Pseudosesarma as defined at present as the G1 is not sufficiently stout and the inner surface of sternite 5 of the sternopleonal cavity does not have a depression for the G1 ( Figs. 45E , 48A, B ). In B. brevipes and B. laevimanum , the male telson is prominently sunken into the distal margin of somite 6 (see Li, 2014: fig. 1B; unpublished data), distinct from the condition in M. lorentzi which is only slightly so ( Fig. 50E ) and not at all apparent in M. modestum and M. lustrum ( Figs. 45D , 46B , 47H ). In any case, the G1 of species in Bresedium s. str. are either stout with the chitinous distal process very long and straight, or the G1 is more slender with the distal process long and the tip dilated ( Fig. 42B–E ; Li, 2014: fig. 2A, B). Like Pseudosesarma , the inner surface of sternite 5 of the sternopleonal cavity in Bresedium species also do not have a depression for the G1.

Figures

Fig. 59. Phylogenetic relationships among selected taxa of Sesarmidae (91 OTUs), including all representatives of Chiromantes and Pseudosesarma previous to current revision of these genera. In addition, several species of Bresedium and Sesarmops are considered, as the revision also involves some of their former members. Topology and confidence values were determined by means of a phylogenetic reconstruction based on 1,096 basepairs of mtDNA (16S-tRNALeu-ND1), conducted with MrBayes, GTR+I+G as substitution model, and Parasesarma plicatum as outgroup (midpoint rooting). Posterior probabilities were determined after 4 million generations with a burn-in of 25%. All species so far classified within Chiromantes (without obvious anterolateral teeth) are shaded in blue, whereas those species that used to be classified in Pseudosesarma (carrying at least one pair of anterolateral teeth) are shaded in yellow.

Fig. 45. Migmarma modestum, holotype male (25.0 × 20.9 mm) (SMF 1989), Indonesia. A, overall habitus; B, dorsal view of carapace; C, frontal view of cephalothorax; D, anterior thoracic sternum and pleon; E, sternopleonal cavity; F, outer view of right chela; G, H, left G1; I–K, distal part of left G1.

Fig. 47. Migmarma lustrum, new species, holotype male (15.7 × 13.9 mm) (ZRC 2010.0021), Irian Jaya, Indonesia. A, overall habitus; B, dorsal view of carapace; C, frontal view of cephalothorax; D, left third maxilliped; E, outer view of right chela; F, outer view of left chela; G, anterior thoracic sternum and pleon; H, pleon.

Fig. 50. Migmarma lorentzi, new species, holotype male (15.8 × 14.0 mm) (NMBA 625IIa), Lorentz River, Irian Jaya, Indonesia. A, overall habitus; B, dorsal view of carapace; C, frontal view of cephalothorax; D, outer view of right chela; E, anterior thoracic sternum and pleon; F–H, left G1; I–K, distal part of left G1.

Fig. 46. Migmarma modestum, holotype male (25.0 × 20.9 mm) (SMF 1989), Indonesia. A, left third maxilliped (denuded); B, pleon; C, left G1 (ventral view); D, left G1 (dorsal view); E, distal part of left G1 (ventral view); F, distal part of left G1 (mesial view); G, distal part of left G1 (dorsal view); H, distal part of left G1 (mesial view); I, left G2. Scales: A, C, D, I = 1.0 mm; B = 5.0 mm; E–H = 0.5 mm.

Fig. 49. Migmarma lustrum, new species, holotype male (15.7 × 13.9 mm) (ZRC 2010.0021), Irian Jaya, Indonesia. A, left G1 (ventral view); B, left G1 (dorsal view); C, distal part of left G1 (ventral view); D, distal part of left G1 (dorsal view); E, distal part of left G1 (mesial view); F, left G2. Scales = 0.5 mm.

Fig. 51. Migmarma lorentzi, new species, holotype male (15.8 × 14.0 mm) (NMBA 625IIa), Lorentz River, Irian Jaya, Indonesia. A, left G1 (ventral view); B, left G1 (dorsal view); C, distal part of left G1 (ventral view); D, distal part of left G1 (dorsal view); E, distal part of left G1 (mesial view); F, left G2. Scales = 0.5 mm.

Fig. 48. Migmarma lustrum, new species, holotype male (15.7 × 13.9 mm) (ZRC 2010.0021), Irian Jaya, Indonesia. A, thoracic sternites 1–4; B, sternopleonal cavity; C, frontal view of cephalothorax; D, outer view of right chela; E, anterior thoracic sternum and pleon; C, D, left G1; E–G, distal part of left G1.

Fig. 42. A–E, Bresedium laevimanum, lectotype male (20.0 × 17.6 mm) (MNHG), Borneo. A, male pleon; B, left G1 (ventral view, denuded); C, left G1 (dorsal view, denuded); D, left distal part of G1 (ventral view, denuded); E, left distal part of G1 (dorsal view, denuded). Scales: A = 2.0 mm; B, C = 1.0 mm; D, E = 0.5 mm.