Trapezarma 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
Trapezarma

description: Trapezarma, new genus
Type species. Sesarma angolensis Brito Capello, 1864, by present designation. Gender neuter.
Diagnosis. Carapace trapezoidal, distinctly broader than long; frontal margin entire or slightly bilobed, gently deflexed, wider than carapace margin; lateral margins of carapace gently convex, entire in adults, posterolateral part prominently converging; regions of carapace demarcated; postfrontal and epigastric crests separated by relatively shallow grooves, margin relatively sharp, almost straight, appearing contiguous; basal articles of antenna and antennules separated by septum; dorsal margin of palm without longitudinal pectinated ridge, inner surface not swollen, without granulated ridge, outer surface of palm and pollex gently convex to almost flat, smooth or with small granules, palm and pollex high in adult males, outer margin of palm rounded, without elbow-like projection; dorsal margin of chelipedal dactylus lined with irregularly arranged small granules of various sizes; inner margin of the chelipedal merus rounded, inner distal margin not lamelliform or with distal part partially expanded into projection; inner surfaces of first to third ambulatory coxae with only scattered short setae between them, not arranged into dense tufts; male thoracic sternites 2–4 relatively narrow, suture between sternites 3 and 4 distinct; male sternopleonal cavity reaching three-quarters length of sternite 4 to just before anterior margin of sternite 2; male thoracic sternite 5 smooth, without depression on anterior part; male pleonal somite 6 proportionately more narrow, telson linguiform; G1 relatively stout, subdistal part dilated, chitinous part short. Vulva on median part of sternite 6; sternal vulvar covers lateral, anterior sternal vulvar cover low, lobiform, posterior sternal cover very low; opening short, projecting, directed obliquely posteriorly.
Etymology. The genus name alludes to the trapezoidal shape of the carapace of the type species, in combination with the genus name Sesarma. The gender is neuter.
Included species. Sesarma angolensis Brito Capello, 1864.
Remarks. The two West African species, Sesarma angolense Brito Capello, 1864, and Sesarma buettikoferi De Man, 1883, form a discrete genetic clade in the molecular analysis, indicating they are sister taxa. They both possess relatively trapezoidal carapaces, a broad frontal margin, flattened to relatively flattened adult male chela and a sternopleonal cavity that reaches the junction between sternites 3 and 4, however they differ markedly in many features. Sesarma angolense is a much larger species with a more trapezoidal carapace (Fig. 13G) compared to S. buettikoferi (Fig. 13H–J). Adult males of S. buettikoferi also have a prominently flattened outer face of the chela with the outer angle elbow-like (Fig. 17G–J), while that of S. angolense is rounded and there is no elbow-like structure laterally, even if the pollex is enlarged and laterally flattened (Fig. 16A–F). The inner margin of the merus of the cheliped of S. angolense is also rounded (Fig. 16B, E) while that of S. buettikoferi has the distal part expanded and somewhat wing-like (Fig. 17I, J). In S. angolense, the male anterior thoracic sternum is relatively narrower and the male pleon has somite 6 proportionately narrower and a linguiform telson (Figs. 18E, 21A). The male anterior thoracic sterum of S. buettikoferi is relatively broader and the male pleon has a proportionately broader somite 6 with the telson more rounded (Figs. 18F, 21H). The G1 of S. angolense is also obviously stouter with the distal chitinous tip short and truncate (Fig. 21B–G), while that of S. buettikoferi is slender with the distal chitinous part elongate and subspatuliform (Fig. 21I–L). Most remarkably, their vulvae are completely different—that of S. angolense is positioned on the median part of sternite 6 and spaced far apart, almost to the edge of the sternopleonal cavity (Fig. 43I) while that of S. buettikoferi is positioned more medially and at the anterior edge of sternite 6, touching sternite 5 (Fig. 43J). These are very substantial differences and argue for separating the two species into separate genera. While the two species are in one clade, the branches are long, suggesting they have been separated for a substantial period of time, which would explain the morphological differences observed. As such, Trapezarma, new genus, and Platychirarma, new genus, are established for Sesarma angolense Brito Capello, 1864, and Sesarma buettikoferi De Man, 1883, respectively. Schubart et al. (2009) retained two morphologically different arboreal species in one Indo-West Pacific Selatium Serène & Soh, 1970, but in this case, the differences were mainly with carapace shape and leg proportions. Unlike the case of Selatium, however, the differences between S. angolense and S. buettikoferi are far more substantial, and the different position of the vulvae on sternite 6 is a compelling generic level difference.
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 934, DOI: 10.26107/RBZ-2020-0097, http://zenodo.org/record/5351295