Cristarma, 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 : 928

Cristarma , new genus

Type species. Sesarma (Sesarma) eulimene De Man , in Weber, 1897, by present designation. Gender neuter.

Diagnosis. Carapace transversely rectangular, distinctly broader than long; frontal margin gently bilobed, gently deflexed, subequal to posterior carapace margin; lateral margins of carapace entire in adults, posterolateral part subparallel; regions of carapace 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 with oblique pectinated ridge near base of finger; inner surface of palm with prominent submedian transverse swelling, highest point with transverse ridge of granules, outer surface of palm and pollex convex, lower surface of palm with 2 short, smooth, submedian oblique ridges; dorsal margin of chelipedal dactylus with differentiated or prominently shaped row of stridulatory granules; inner distal margin of merus of cheliped not lamelliform; inner surfaces of first to third ambulatory coxae with numerous long setae between them; male thoracic sternites 2–4 relatively narrow in adults, suture between sternites 3 and 4 distinct; male sternopleonal cavity reaching two-thirds length of sternite 4 to just before anterior margin of sternite 2; male thoracic sternite 5 smooth, without depression on anterior part; G1 relatively slender, chitinous part relatively short. Vulva on anterior part of sternite 6, anterior edge presses against sternite 5; anterior sternal vulvar cover very low, posterior sternal vulvar cover forming low rim; opening projecting obliquely anteriorly.

Etymology. The name is derived from the Latin term for “crest” (crista) with the genus name Sesarma . The gender is neuter.

Included species. Sesarma (Sesarma) eulimene De Man , in Weber, 1897; Sesarma ortmanni Crosnier, 1965 .

Remarks. The two species included in Cristarma , new genus, have been well described and discussed by De Man (1895) and Crosnier (1965). Although they have been treated as species of Chiromantes s. lato because their lateral carapace margins are entire, they have more features in common with Parasesarma De Man, 1895 , i.e., absence of an epibranchial tooth, presence of pectinated crest on the dorsal margin of the chela, and possession of distinct tubercles on the dorsal margin of the cheliped dactylus. Parasesarma species , however, have a relatively wider male thoracic sternum, a proportionately much broader pleon, the tubercles on their cheliped dactylus are almost always more distinctively structured, and the inner surface of the chela has a prominent transverse granulated ridge (low or not as prominent in species of Parasesarma ). In addition, Cristarma eulimene and C. ortmanni have two short parallel oblique ridges on the outer surface of their chelae, a character absent in all known Parasesarma species. The combination of characters on the chela is unique for these two species, and combined with the fact that the male palm is oblique to almost transverse in orientation, the glabrous lateral margins of the ambulatory dactyli, the presence of dense setae between the first to third ambulatory coxae and the vulva is very low, with two sternal vulvar covers bracketing the opening, we believe a new genus is warranted. The molecular tree also supports this, with the two species clustering in a distinct clade far from all the other genera ( Fig. 59 ).

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.