Orbicantha, Ng & Forges & Lee, 2023

Ng, Peter K. L., Forges, Bertrand Richer De & Lee, Bee Yan, 2023, New and rare deep-sea majoid crabs (Crustacea: Brachyura) from Papua New Guinea, Zootaxa 5318 (1), pp. 1-44 : 24-25

publication ID	https://doi.org/ 10.11646/zootaxa.5318.1.1
publication LSID	lsid:zoobank.org:pub:DB9D6B30-5A8C-486A-BF67-0965E4C026DA
DOI	https://doi.org/10.5281/zenodo.8158143
persistent identifier	https://treatment.plazi.org/id/9B094F56-FA5F-FFE2-FF20-C18FFA9BF9AC
treatment provided by	Plazi
scientific name	Orbicantha
status	gen. nov.

Treatment

Genus Orbicantha View in CoL n. gen.

Diagnosis of female. Carapace subcircular; gastrohepatic and frontal regions not elongated, lateral constriction with branchial regions weak ( Fig. 19A, B View FIGURE 19 ); dorsal surface not swollen, gently convex; mesogastric region with 1 median spine with rounded tip; protogastric region with 2 small spines arranged obliquely; metagastric region with 1 median spine with rounded tip cardiac region with 2 short spines positioned side by side; intestinal region with 1 large median tubercle and 1 short spine lateral to it on each side; hepatic region not swollen with 2 or 3 spines with round tips; branchial region with oblique median row of 3 spines; lateral branchial margin with 6–8 large and many small spines (larger ones round-tipped); dorsal carapace surface with numerous distinct rounded granules ( Figs. 19A–E View FIGURE 19 , 22A–C View FIGURE 22 ); pterygostomial region protruding anteriorly, forming angular shelf-like structure in lateral view ( Fig. 19D, E View FIGURE 19 ); suborbital region narrow; pseudorostrum short, as long as supraorbital eave, base wide with 2 distal spines; interantennular spine distinct, tip bifurcated, clearly visible in dorsal view; postorbital spine gently curved with rounded tip; intercalated spine distinct with rounded tip; margin of supraorbital eave with 4 short spines, antorbital spine relatively short, no obvious preorbital spine ( Figs. 19B, C, F View FIGURE 19 , 20B View FIGURE 20 ); basal antennal article elongate, outer margin lined with short spines, third article short, subquadrate, ca. one-third length of basal article ( Fig. 19F View FIGURE 19 ); eyes with short, stout peduncle ( Fig. 19C, F View FIGURE 19 ); epistome is distinctly quadrate. Third maxilliped covered with sharp and rounded granules, those on inner margins sharper, ischium subrectangular, merus subtriangular, proximal margin with strong sharp tubercles, anteroexternal angle expanded, auriculiform ( Fig. 20A View FIGURE 20 ). Chelipeds slender, relatively short ( Fig. 19A View FIGURE 19 , 22A View FIGURE 22 ). Ambulatory legs relatively long, slender, P2 and P3 propodus and dactylus with long stiff setae that resemble spines and setae on ventral margins, appearing comb-like ( Figs. 19A View FIGURE 19 , 20E, F View FIGURE 20 , 22A View FIGURE 22 ), P4 and P5 prominently reduced in size, ventral margins of propodus and dactylus without long spines or long stiff setae ( Figs. 19A View FIGURE 19 , 20G, H View FIGURE 20 , 22A View FIGURE 22 ). Thoracic sternite 4 with prominent broad V-shaped ridge medially that demarcates sternopleonal cavity ( Fig. 14A View FIGURE 14 ). Pleon subcircular in shape, domed; covering most of thoracic sternum; somites 1–5 free but barely mobile; somite 6 and telson functionally fused ( Fig. 21A, C, D View FIGURE 21 ); vulvae on anterior part of sternite 6, subovate, with narrow rim ( Fig. 21B View FIGURE 21 ).

Etymology. The name is derived from the Latin “orbis” for round, alluding to the shape of the carapace; in arbitrary combination with the genus name Pleistacantha . Gender feminine.

Remarks. Orbicantha n. gen. is clearly related to the following oregoniid pleistacanthine genera: Pleistacantha Miers, 1879 , Parapleisticantha Yokoya 1933 , and Pleisticanthoides Yokoya, 1933 . It is markedly different from all of these genera in that its carapace shape is semicircular, with the gastrohepatic and frontal regions not elongated ( Figs. 19A, B View FIGURE 19 , 22A, B View FIGURE 22 ); the pseudorostral spines are very short with the distal spines rounded at the tip ( Figs. 19B, F View FIGURE 19 , 22B View FIGURE 22 ); the gastric, cardiac and branchial regions possessing distinct spines which have a rounded tip ( Figs. 19A–D View FIGURE 19 , 22A–C View FIGURE 22 ); the ocular peduncle is very short ( Fig. 19F View FIGURE 19 ); the third antennal article is very short, being almost quadrate and much shorter than the basal antennal article ( Fig. 19F View FIGURE 19 ); the epistome is distinctly quadrate ( Fig. 19F View FIGURE 19 ); the buccal cavity is proportionately much wider ( Fig. 19E View FIGURE 19 ); and the P4 and P5 are prominently reduced in size and more slender in form compared to P2 and P3, with the propodus and dactylus lacking long spines or setae ( Figs. 19A View FIGURE 19 , 20D, E View FIGURE 20 , 22A View FIGURE 22 ). In species of Pleistacantha , Parapleisticantha and Pleisticanthoides , the carapace shape is distinctly pyriform, with the gastrohepatic regions separated from the branchial regions by a lateral constriction; the pseudorostral spines usually relatively longer with the distal spines sharp or distinctly tapering; the gastric, cardiac and branchial regions possessing normal sharp spines; the ocular peduncle is elongate; the third antennal article is at least half the length of the basal antennal article, often being subequal in length, being almost quadrate; the epistome is transversely narrower; the buccal cavity is proportionately narrower; and the P5, while reduced in size compared to the other pereopods, is not otherwise different in form, with P3 and P4 similar in form and structure (cf. Guinot & Richer de Forges 1986, 1992; Ahyong & Lee 2006; Ahyong & Ng 2007; Ng & Richer de Forges 2012; Richer de Forges et al. 2013; Ahyong et al. 2015, 2019; Ng et al. 2017). In addition, the ornamentation on most of the dorsal carapace surface of Orbicantha n. gen. consists of small, rounded granules, not spines ( Figs. 19B–D View FIGURE 19 , 22B View FIGURE 22 ); those on Pleistacantha and Parapleisticantha are invariably covered with small spines. While Pleisticanthoides simplex ( Rathbun, 1932) has a spine-covered carapace, the other two species ( P. cameroni Ng & Richer de Forges, 2012 , and P. piccardorum Ng & Richer de Forges, 2012 ) have much smoother carapaces with no spines on most of the surface (cf. Ng & Richer de Forges 2012).

The presence of four short spines on the supraocular eave ( Fig. 19B, C View FIGURE 19 ) and two distal spines on the ventral edge of the basal antennal article ( Fig. 19F View FIGURE 19 ) are characters shared with Parapleistacantha ; although they completely differ in the above discussed features (cf. Figs. 15C View FIGURE 15 , 16C, D View FIGURE 16 ; Richer de Forges et al. 2013). The differences with the Pleistacantha Miers, 1879 s. lato are obvious, with the latter possessing long pseudorostral spines usually diverging in V bearing long accessory spines versus very short triangular spines; pyriform carapace covered with long sharp spines in Pleistacantha versus rounded carapace with short granule-like spines in Orbicantha n. gen.