Pseudosesarma , Serene & Soh 1970
Creators: name: Schubart, Christoph D.
name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2020
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Sesarmidae
Pseudosesarma

description: Pseudosesarma Serène & Soh, 1970
Type species. Sesarma edwardsii De Man, 1887, by original designation. Gender neuter.
Diagnosis. Carapace squarish to transversely subrectangular; frontal margin bilobed, gently deflexed, subequal to posterior carapace margin; lateral margins of carapace entire in adults or with one tooth, posterolateral part subparallel; regions of carapace clearly demarcated; postfrontal and epigastric crests separated by relatively deep grooves, margin relatively rounded, regions clearly separated; basal articles of antenna and antennules separated by septum; dorsal margin of palm without longitudinal pectinated ridge, inner surface may be gently swollen, but without prominent granulated ridge, outer surface and pollex convex or almost flat, covered with small granules; dorsal margin of chelipedal dactylus smooth in adult males or lined with irregularly arranged 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 dense tufts; male thoracic sternites 2–4 relatively narrow in adults, suture between sternites 3 and 4 distinct or obsolete; male sternopleonal cavity reaching two-thirds length of sternite 4 to just before anterior margin of sternite 2; pleonal locking mechanism formed by small angular projection on posterior edge of sternite 4 of sternopleonal cavity, no trace of tubercle on sternite 5; male thoracic sternite 5 usually with slight depression on anterior part for tip of G1 when pleon closed; G1 very stout or if more slender, subdistal part distinctly swollen, much wider than base; chitinous part almost absent to relatively short or long. Vulva on submedian part of sternite 6 or more anterior, close to margin with sternite 5; sternal vulvar covers small, rounded, flat, tips sometimes partially overlapping, opening not projecting or small, sometimes extending beyond sternal vulvar covers.
Included species. Sesarma edwardsii De Man, 1887; Sesarma edwardsii var. crassimana De Man, 1887; Pseudosesarma anteactum Ng & Schubart, 2017; Pseudosesarma glabrum Ng, Rani & Nandan, 2017; Pseudosesarma brehieri Ng, 2018; Chiromantes boulengeri Calman, 1920.
Remarks. As discussed earlier, Serène & Soh (1970) had already realised their genus was heterogeneous, which is confirmed in the present study. Of the nine species recognised by Ng et al. (2008a), five (P. modestum, P. patshuni, P. moeschii, P. johorensis, P. laevimanum) need to be transferred to other genera (see later). Instead, one other species, Chiromantes boulengeri, is here transferred into Pseudosesarma.
Pseudosesarma species are characterised by their G1 being stout, with the distal part (before the chitinous tip) being very wide; this has necessitated the anterior part of sternite 5 of the sternopleonal cavity developing a complementary concavity to accommodate this structure (Fig. 32A, B). In addition, the species have two kinds of vulva; one in which it is positioned submedially on sternite 5 with a distinctive plate-like covering and the opening is not projecting, and the other has the vulva closer to the margin of sternite 6 with two low sternal vulvar covers and a short projection (Fig. 44A–F).
The variation in the form of the vulva in Pseudosesarma species merits discussion. The type species, P. edwardsii, is the only one where the vulva is submedian in position on sternite 6 and the opening does not project at all (Fig. 44A). In all the other species examined, the vulvae are positioned more anteriorly, and are closer or adjacent to the margin with sternite 5 (Fig. 44B–F). In these other species, the opening is also visible as a short projection that may be directed obliquely anteriorly (Fig. 44B, D, F) or obliquely posteriorly (Fig. 44E). In one large specimen of P. crassimanum, the right vulva appears to have a short opening protruding above the posterior sternal vulvar cover but on the left side, only a very low opening is visible (Fig. 44C). In any case, the opening in P. crasssimanum is low (Fig. 44B), so some degree of variation is to be expected. In all the Pseudosesarma species examined, the relative position of the vulvae is the same, i.e., they are all submedially positioned in the female sternopleonal cavity.
Pseudosesarma patshuni Soh, 1978, is quite different from the species now recognised in Pseudosesarma in the form of the chela, male pleon, proportionately longer ambulatory legs, and having a short, straight, and stout G1. As discussed earlier, it has been transferred to Orisarma, new genus, with two other Sesarmops species. Its vulva is similar to those of Orisarma species (Fig. 43D).
The placement of Sesarma (Sesarma) modesta De Man, 1902 (type locality Ternate) in Pseudosesarma is incorrect. We have examined the type specimen and it is better placed in a new genus, Migmarma (see later). Its subtrapezoidal carapace shape resembles many species of Pseudosesarma but its G1 is unlike any of those species, being more slender and without the subdistal part of the structure swollen (Figs. 45G–J, 46C–H), and the surface of thoracic sternite 5 does not have a depression (see discussion on this species later).
Pseudosesarma bocourti (A. Milne-Edwards, 1869) is here referred to a new genus, Contusarma. The chelae have the outer surface characteristically flattened (Fig. 28); the G1 is relatively slender and straight (Figs. 38D–H, 39F–J, L–M) and the vulvae are small with the sternal vulvar covers poorly developed, and the opening small and slightly projecting (Fig. 44G, H).
Pseudosesarma granosimanum (Miers, 1880) previously transferred to Chiromantes by Ng et al. (2008a) is also here referred to a new genus, Miersarma. It is unusual in having the anterolateral margins entire, the suture between thoracic sternites 3 and 4 almost undiscernible; press-button for the normal pleonal locking mechanism absent from sternite 5 (Fig. 32D); no depression on the anterior part of male sternite 5 for the G1 tip (Fig. 32D); an elongate, slender, and straight G1 (Fig. 40B–E, F–J); and a small vulva pushing prominently into the margin with sternite 5, and with the sternal vulvar covers and opening small (Fig. 44I).
Interestingly, the genetic data strongly supports a monophyletic lineage for P. bocourti and places it as sister to P. granosimanum (Fig. 59). This is despite the two taxa being morphologically very different.
Two species of Pseudosesarma, originally highlighted by Serène & Soh (1970: 400) as atypical of the genus, P. moeschii (De Man, 1892) and P. johorense (Tweedie, 1940), are also referred to a new genus Manarma (see Remarks for that genus). In addition to differences in the chelae, male thoracic sternum, male pleon and G1, this genus is distinct from Pseudosesarma in that its vulvae are located on the anterior edge of sternite 6 (rather than median) and have a different structure (Fig. 44J).
A re-examination of the types of Pseudosesarma laevimanum (Zehntner, 1894) show that it should be placed in Bresedium Serène & Soh, 1970, instead, and as a senior synonym of B. sediliensis (Tweedie, 1940). It is redescribed and figured later in this paper.
The remaining species historically recognised in Pseudosesarma, viz. Sesarma edwardsii De Man, 1887 and Sesarma crassimana De Man, 1887, are closely related with the three new species described since 1970 (Ng & Schubart, 2017). This also holds for Sesarma boulengeri Calman, 1920, despite the fact that it has no epibranchial tooth. The male pleonal somites 5 and 6 in all six species are proportionately broader when compared to those of Chiromantes, Orisarma, or Manarma, rendering the pleon a relatively broader appearance (widest in P. edwardsii and narrowest in P. boulengeri). The G1s are generally stocky, being shortest and stoutest in the type species, P. edwardsii. The distal chitinous part is generally short (e.g., P. crassimanum) or even absent (e.g., P. edwardsii). In species of Pseudosesarma s. str. (as in Contusarma species), there is also a shallow but distinct depression on sternite 5, just posterior to the ledge on sternite 4 that locks the pleon (Fig. 32A, B). In this depression sits the dilated distal part of the G1. In P. boulengeri, however, there is no trace of a depression, with the entire lateral surface of sternite 5 weakly convex.
It is interesting that Pseudosesarma and its related genera have only a very weak or even absent press-button pleonal locking system as described by Guinot & Bouchard (1998). Even when present, the press-button on sternite 5 is at most a small, low rounded granule. Instead, the male pleon appears to lock onto a small ledge on each side of sternite 4. The press-button on P. edwardsii is the most anteriorly positioned of the species studied, being just before sternal suture 4/5, and just below the ledge of sternite 4 (Fig. 32A). In P. crassimanum and P. boulengeri, the press-button is on the anterior third of sternite 4 (Fig. 32B, C). As noted above, P. granosimanum completely lacks any locking mechanism on sternite 4 (Fig. 32D).
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 pages 939-944, DOI: 10.26107/RBZ-2020-0097, http://zenodo.org/record/5351295