Chartocinus, Ng & Rahayu, 2022

Ng, Peter K. L. & Rahayu, Dwi Listyo, 2022, A new genus and new species of Hymenosomatidae (Crustacea: Brachyura) from Indonesian Papua, Zootaxa 5092 (4), pp. 480-486 : 480-481

Chartocinus n. gen.

Type species. Chartocinus rarus n. sp., by original designation.

Diagnosis. Carapace longitudinally subovate, distinctly dorsoventrally flattened; dorsal surface almost flat, surrounded by continuous distinct rim except at base of rostrum; regions not discernible, without any distinct grooves, H-shaped gastric groove shallow, joining cervical groove, no grooves on posterior part; carapace rim and margin of rostrum lined with dense short, hooked setae that partially obscure surface; rostrum unilobate, subtriangular, no lateral lobes present; anterolateral margin unarmed; posterolateral margin with 3 low, wide lobes above P3 and P4 coxae; orbits clearly visible in dorsal view, eyes mobile; epistome broad, not projecting anteriorly, posterior margin with gently sinuous lateral margins, with 2 low, subtruncate median lobes; third maxilliped short, subquadrate, operculiform, covering buccal cavity when closed, merus short, subtrapezoidal, anteroexternal angle distinct, produced; ambulatory legs slender, very long, meri and dactyli unarmed; male thoracic sternum with sternites 3–8 fused, without trace of sutures or depressions, forming wide plate; male sternopleonal cavity short, with distal tip reaching to imaginary line joining anterior edge of P4 coxae; male pleon-pleotelson short, triangular, somites 1 and 2 free, somites 3–5 fused but sutures still visible, pleotelson semicircular, no intercalated plates visible; G1 short, stout, distal half tapering to slender sharp tip, directed towards median part of sternum, inner margin appears bilobed, with deep median cleft.

Etymology. From the Latin for paper (charta) in arbitrary combination with the genus name Carcinus ; alluding to the very flat carapace of the type species. Gender masculine.

Remarks. Although the general features of Chartocinus n. gen. do not stand out, the one character it has that is unusual in Hymenosomatidae is that its male sternopleonal cavity is short and prominently posterior in position, suggesting that thoracic sternites 3 and/or 4 may have been displaced posteriorly as well. This character easily distinguishes Chartocinus n. gen. from other genera where the male sternopleonal cavity is more elongate and more anteriorly positioned to different degrees. The male thoracic sternum in the new genus is completely smooth and other than sternites 1 and 2, there are no sutures or grooves that demarcate the boundaries of sternites 3–8 on the surface, even on the lateral margins. This is a relatively extreme condition but has precedence in the Hymenosomatidae for both sexes (cf. Guinot 1979: fig. 30A, pl. 20; Ng & Chuang 1996: fig. 6J; Guinot 2011a: fig. 1C; Guinot et al. 2013: 189).

Chartocinus n. gen. otherwise most closely resembles members of the Pacific genus Amarinus Lucas, 1980 , in possessing an ovate carapace without defined regions and grooves on the dorsal surface, a simple unilobate rostrum, without distinct lateral armature, relatively short and subquadrate third maxilliped with the anteroexternal part of the merus produced, simple chelipeds which are not armed or inflated, a relatively short male pleon, and a short and stout G1 (cf. Ng & Chuang 1996; Ng & Richer de Forges 1996).

Chartocinus n. gen., however, differs from Amarinus in that the ridge which demarcates the rim of the carapace only reaches the orbit, not being visible behind the rostrum ( Figs. 1A , 2A ) (versus rim is complete, clearly visible behind rostrum), ambulatory legs (P2–P5) are proportionately much longer ( Fig. 1A, B ) (versus short), the flexor margin of all ambulatory dactyli is unarmed ( Fig. 2G–J ) (versus all ambulatory dactyli with a subterminal spine), the male pleotelson is semicircular without any intercalated plate at the articulation with somite 5 ( Fig. 2E ) (versus triangular to rounded with distinct intercalated plates at the articulation with the pleotelson; cf. Guinot & Richer de Forges 1997: 460, fig. 4B–D; Guinot 2011a: 23, 26–28, fig. 3A–C), and the G1 has a prominent cleft on the inner margin giving it a bilobed appearance ( Fig. 2K, L ) (versus with margins entire) (cf. Holthuis 1968; Lucas 1980; Lucas & Davie 1982; Ng & Chuang 1996; Ng & Richer de Forges 1996; Rahayu & Ng 2004; Naruse et al. 2008b). The incomplete rim of the carapace is noteworthy as in known Amarinus and many other hymenosomatids, the rim is distinct even behind the rostrum. In the case of Chartocinus n. gen., the rim gets lower and weaker as it passes the orbit and is not obviously visible behind the rostrum ( Fig. 1A , 2A ). In addition, all Amarinus species are known from freshwater and low salinity habitats, including swamps at an altitude of 1600 m in New Guinea ( Holthuis 1968); whereas Chartocinus n. gen. was collected from a subtidal marine site.

One key feature of Chartocinus n. gen. is the prominent pattern of hooked setae that border the carapace, rostrum, margins of the pereopods and pleonal somite 1 which obscure the outline of the animal. This is a character similar to Odiomaris pilosus (A. Milne -Edwards, 1873) from New Caledonia, a species previously placed in Amarinus . The setae, however, are a bit different, as those in Chartocinus n. gen. are shorter, hooked and more densely arranged. Chartocinus n. gen. can easily be separated from Odiomaris Ng & Richer de Forges, 1996 , by the same characters as for Amarinus , except that in Odiomaris , the anteroexternal angle of the merus of the third maxilliped is not produced and the G1 is more elongate with the distal part with two tightly apposed processes (cf. Ng & Richer de Forges 1996; Davie & Richer de Forges 1996; Guinot & de Mazancourt 2020).

The male pleotelson has two depressions on the ventral surface (and are homologous to the pleonal sockets matching the intercalated plates in Amarinus and Odiomaris , representing the complementary pleonal parts for the locking of the pleon. Chartocinus represents the completely fused state of these small plates with the pleotelson (see Guinot 1979, 2011a).

Figures

FIGURE 1. Chartocinus rarus n. gen., n. sp., holotype male (2.9 x 3.1 mm) (MZB Cru 5342), Papua, Indonesia. A, overall dorsal habitus; B, ventral view.

FIGURE 2. Chartocinus rarus n. gen., n. sp., holotype male (2.9 x 3.1 mm) (MZB Cru 5342), Papua, Indonesia. A, dorsal view of carapace (setae not drawn); B, frontal view showing left antennule, antenna, orbit, proepistome and epistome (setae not drawn); C, right third maxilliped (setae not drawn); D, thoracic sternum and sternopleonal cavity; E, pleon (setae not drawn); F, left chela; G–J, P2–P5, respectively; K, left G1 (dorsal view); L, left G1 (ventral view); M, left G2. Scales: A, D, F–J = 0.5 mm; B, C, E = 0.2 mm; K–M = 0.1 mm.