Bresedium Serène & Soh, 1970

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 : 966

Bresedium Serène & Soh, 1970

Type species. Sesarma edwardsii brevipes De Man, 1889 , by original designation. Gender neuter.

Diagnosis. Carapace transversely rectangular; frontal margin bilobed, gently deflexed, wider than posterior carapace margin; lateral margins of carapace with epibranchial tooth, posterolateral part subparallel; regions of carapace demarcated; postfrontal and epigastric crests separated by relatively deep or distinct grooves, margin rounded, regions clearly separated; basal articles of antenna and antennules clearly separated by septum formed by extension of front; dorsal margin of palm without longitudinal pectinated ridge; in adult males, inner surface with submedian transverse swelling, outer surface of palm and pollex convex, with low striae or flattened granules; dorsal margin of chelipedal dactylus almost smooth; inner distal margin of merus of cheliped not lamelliform; inner surfaces of first to third ambulatory coxae with scattered short setae, not arranged into dense tufts; male thoracic sternites 2–4 relatively narrow in adults, suture between sternites 3 and 4 shallow; male sternopleonal cavity reaching three-quarters length of sternite 4 to just 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; male thoracic sternite 5 without depression on anterior part; G1 slighty stout to relatively slender, chitinous part elongate, tip dilated to some degree. Vulva on anterior part of sternite 6, anterior part presses against sternite 5; anterior sternal vulvar cover low, posterior sternal vulvar cover low; opening cylindrical, projecting with tip rounded, directed obliquely anteriorly.

Included species. Sesarma edwardsii brevipes De Man, 1889 (= Sesarma (Sesarma) edwardsi philippinense Rathbun, 1914 ); Sesarma Edwardsi var. laevimana Zehntner, 1894 (= Sesarma sediliensis Tweedie, 1940 ); Bresedium eurypleon Li, Shih & Ng, 2020 .

Remarks. Three species are now recognised in Bresedium Serène & Soh, 1970 : B. brevipes ( De Man, 1889) , B. sediliense ( Tweedie, 1940) , and B. eurypleon Li, Shih & Ng, 2019 . Bresedium sediliense is the only one known from the Sunda Shelf, and its generic position is not certain, although it does have a characteristic male pleon which has the telson distinctly sunken into the distal margin of somite 6 and the tip of the distal chitinous part of the G1 flared. Li et al. (2020) revised the Sesarmops and Bresedium species from Taiwan, Philippines, and Sulawesi and commented on the problems with the two genera. They showed that B. philippinense is a junior subjective synonym of B. brevipes ; recognised a new species, B. eurypleon , with a diagnostic G1 structure with an elongate distal chitinous part; described a new species of Sesarmops ( S. mora ); and redescribed Sesarmops mindanaoensis s. str.

Serène & Soh (1970) noted that both Sesarmops and Bresedium were related in having the male telson sunken into the distal margin of somite 6. They separated Sesarmops as having a more trapezoidal carapace versus a rectangular carapace for Bresedium . Ongoing studies now show that neither the carapace shape, nor the male telson are reliable characters at the generic level. Both genera are very close, and some species now placed in these genera do not possess the distinctive telson-somite 6 feature. There are also several kinds of G1 structures present, and it is clear that a full revision of both genera will be needed. This is beyond the scope of the present paper and will have to be undertaken at a future time.

For the present work, only one species is treated as it had been placed in Pseudosesarma previously— Sesarma edwardsi var. laevimana Zehntner, 1894 —and we find that it is actually a senior subjective synonym of Bresedium sediliense ( Tweedie, 1940) . Specimens from Lorentz River in New Guinea which have been identified as “ Sesarma laevimana ” in NMBA are here identified as a new species of Migmarma , M. lorentzi (see later). The placement of B. sediliense in Bresedium as presently diagnosed is mainly because of the male telson being prominently sunken into the distal margin of somite 6 and the G1 having the distal chitinous part elongate. In the phylogenetic tree ( Fig. 59 ), B. sediliense is not part of but adjacent to the main clade of Bresedium and Sesarmops , although the bootstrap support is not considered strong enough to be definitive.

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.