Migmarma Schubart & Ng 2020, new genus
Creators: name: Schubart, Christoph D.
name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2020
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Sesarmidae
Migmarma

description: 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.
description: Published as part of 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, pp. 891-994 in Raffles Bulletin of Zoology 68 on page 967, DOI: 10.26107/RBZ-2020-0097, http://zenodo.org/record/5351295