Circulium, Naruse & Ng, 2020

Naruse, Tohru & Ng, Peter K. L., 2020, Revision of the sesarmid crab genera Labuanium Serène and Soh, 1970, Scandarma Schubart, Liu and Cuesta, 2003 and Namlacium Serène and Soh, 1970 (Crustacea: Decapoda: Brachyura: Sesarmidae), with descriptions of four new genera and two new species, Journal of Natural History (J. Nat. Hist.) 54 (7 - 8), pp. 445-532 : 455-458

Genus Circulium gen. nov.

Type species

Sesarma rotundatum Hess, 1865 , by present designation. Gender neuter. Included species

Circulium rotundatum ( Hess, 1865) ; C. navus ( Ng, 2012) ; C. papuomalesiacum ( Nobili, 1899) ; C. scandens ( Ng and Liu, 2003) ; and C. vitatum ( Ng and Davie, 2011) .

Diagnosis

Carapace rounded, as long as or longer than wide; branchial regions laterally inflated, usually with 1 distinct tooth and 1 rudimentary tooth or 2 teeth behind external orbital angle; dorsal surface slightly convex longitudinally, branchial regions inflated, clearly divided from gastric region by deep cervical groove.Front deflexed at anterior margin of postfrontal lobes,distally recurved, directed anteriorly; frontal margin gradually concave medially in dorsal view, overhanging onto antennular septum and fossae. Two pairs of postfrontal lobes present, lateral lobes exceeding mesial lobes anteriorly; anterior edge of lobes lined with granules, far from ( C. navus , C. papuomalesiacum , C. vitatum ) or close to ( C. rotundum , C. scandens ) but never reaching frontal margin in dorsal view. Antennular septum relatively narrow, short. Orbit, in dorsal view, wide U-shaped, median part of supraorbital margin horizontal in large individuals; inner orbital tooth short, directed dorsoanteriorly. External orbital angle with 1 longitudinal ridge of granules on ventral surface. Epistome posterior margin with 3 low lobes, lateral lobes terminally directed anteroventrally, median lobe terminally directed anteriorly ( C. navus , C. papuomalesiacus , C. rotundatum , C. scandens ) or lateral lobes terminally directed ventrally, median lobe terminally directed anteroventrally ( C. vitatum ). Antenna entering orbit through wide gap between inner orbital tooth and front. Mxp3 exopod with distinct flagellum. Chela palm of male (except for very large male) and female with 1 complete row of granules from hinge with carpus to base of inner angle of dactylus, row sometimes interrupted in very large males; outer surface granulated in various ways but without prominent protuberance; inner surface without transverse rows of granules; thick rim extending along the occlusal margin of immovable finger to dactylar articulation on both outer and inner surfaces, thick rims of both outer and inner surfaces not interrupted near dactylar articulation. Upper-inner margin of movable finger of male and female with 1 row of sparse, irregular low granules over most of length, tip of granules weakly corneous. Ambulatory legs (P2–5) moderately long, dorso-ventrally flattened, moderately wide; distal anterior corner of meri toothed, followed proximally by subdistal tooth; carpi and propodi narrower than respective meri; dactyli very short, shorter than half length of respective propodi. Male thoracic sternum transversely wide; sternite 8 clearly exposed. Male pleon bell shaped; lateral margins of somites 3–6 forming concavity, telson usually reaching proximal half of bases of cheliped coxae (reaching proximal three-fifths). G1 stout, relatively long, reaching distal end of sternite 6 to midlength of thoracic sternite 5; straight proximally, distally bent laterally, beak-like corneous process narrow. Vulvae opening near thoracic sternal suture 5/6 on distal third of sternite 6, ellipsoidal; raised posterolaterally, lateral to posterior margin rimmed, mesial end of posterior rim produced to wide, short triangular lobe, sternal vulval cover developed from lateral margin, covering almost entire vulva.

Etymology

The generic name ‘ Circulium ’ is derived from an arbitrary combination of ‘ circularis ’ (circular in Latin, alluding the shape of the carapace) and the suffix for the Labuanium , ‘ - ium ’. Gender neuter.

Remarks

The taxonomy of the Labuanium rotundatum species group has been revised by Ng and Liu (2003), Ng and Davie (2011) and Ng (2012). The type specimen of Sesarma rotundatum Hess, 1865 , the type species of Circulium , is not extant ( Ng 2012, p. 269), and since the type locality of Se. rotundatum (Sydney) has been questioned ( McCulloch 1913, p. 323; Davie 2002, p. 221; Ng 2012, p. 268), a neotype was selected from Tonga by Ng (2012). This action restricted Circulium rotundum to the population distributed to the east of the Caroline Islands and Solomon Islands, ranging across Pohnpei, Fiji, Samoa, Tonga, Cook Islands, Marshall Islands, and with doubt from the Hawaiian Islands ( Castro 2011; Ng 2012, p. 265). Other members of the genus – C. scandens Ng and Liu, 2003 , C. vitatum Ng and Davie, 2011 and C. navus Ng, 2012 – were described from Taiwan, the eastern Indian Ocean and Guam, respectively (as Labuanium ), whereas Sesarma (Episesarma) rotundata var. papuo-malesiaca Nobili, 1899, was resurrected by Ng and Davie (2011) as a valid species of Labuanium based on material from Irian Jaya (= Western New Guinea, Indonesia) and additional material from the Caroline Islands and St. Cruz Group, Solomon Islands.

Circulium gen. nov. is characterised by its laterally and dorsally inflated branchial regions of the carapace and relatively slender ambulatory legs with more or less parallel margins of the meri and distally narrowed propodi. The genus can be distinguished from Labuanium s.s. by characters of the cephalothorax, male thoracic sternum, male pleon, male chela, ambulatory legs and G1 ( Table 1 ). Circulium has a proportionally wider carapace than Labuanium ; the width-to-length ratio of Circulium species is usually more than 1 (rarely 0.98), whereas that of L. politum is less than 0.91. These proportional differences also appear in the relative length of the male thoracic sternum; the distal end of the male telson just reaches the midlength of sternite 4 in Circulium , whereas it clearly reaches beyond the midlength in Labuanium . The lateral margins of the carapace of Circulium are, overall, distinctly inflated around the branchial regions, so that the general outline of the carapace is somewhat circular in appearance. In contrast, the lateral margins of the carapace of Labuanium s.s. are more sinuous but they appear more or less subparallel. The frontal margin of Circulium is of the typical sesarmid form, with a slightly recurved entire frontal margin overhanging onto the antennuler fossae and narrow antennular septum. In Labuanium , however, the front is strongly bent for more than 90°, and the frontal margin is widely depressed medially so the margin of the depressed part is attached to the wide antennular septum. There is a thick rim extending along the occlusal margin of the immovable finger to the dactylar articulation on both outer and inner surfaces of the chela. Circulium has a continuous rim throughout, but that of Labuanium is interrupted near the dactylar articulation. In addition, Circulium has proportionally more slender ambulatory legs, especially the meri and propodi, compared to Labuanium . The male pleon of Circulium is widest at somite 3 with the lateral margins of somites 3 to 5 strongly converging distally. The male pleon of Labuanium is proportionally much narrower, and while somite 3 is still widest, the lateral margins of somites 3–5 are only weakly narrowed distally. These differences warrant the recognition of a separate genus for five species heretofore assigned to Labuanium .

Key to species of Circulium gen. nov

1. Outer surface of male chela with sparsely arranged large granules .................................. 2

– Outer surface of male chela with closely arranged small granules .................................... 3

2. Colouration in life uniformly yellow, with white fingers of chelipeds; G1 relatively more slender, distally bent about 60–80° from vertical, distal corneous process longer ................. ........................................................................................................................... C. rotundatum ( Hess, 1865)

– Colouration in life uniformly greyish-white; G1 relatively stouter, distally bent about 80° from vertical, distal corneous process truncated ........................................ C. navus ( Ng, 2012)

3. Upper surface of chelae of large male with interrupted rows of granules; lateral carapace margin of larger adult (e.g.> CW 34 mm) relatively less convex; G1 distally bent about 45° from vertical ...................................................... C. vitatum ( Ng and Davie, 2011)

– Upper surface of chelae of large male with a complete traversing row of granules; lateral carapace margin of larger adult (e.g.> CW 34 mm) relatively more convex; G1 distally bent 60° or more from vertical ............................................................................................... 4

4. G1 distally bent almost 90° from vertical.............. .............. C. scandens ( Ng and Liu, 2003)

– G1 distally bent almost 60° from vertical .................... C. papuomalesiacum ( Nobili, 1899)