Cymonomus clarki, Ahyong, 2008

Ahyong, Shane T., 2008, Deepwater crabs from seamounts and chemosynthetic habitats off eastern New Zealand (Crustacea: Decapoda: Brachyura), Zootaxa 1708 (1), pp. 1-72 : 13-14

Cymonomus clarki sp. nov.

( Figs 1D , 6 )

Type material. HOLOTYPE: NIWA 29666 , ovigerous female (cl 6.9 mm, cw 6.6 mm), Graveyard Seamount, 42°45.91’S, 179°59.26’W, 993–1090 m, coral rubble and rocks, TAN0604/16, 29 May 2006 . PARATYPES: NIWA 34965 , 1 ovigerous female (cl 7.3 mm, cw 7.1 mm), Ritchie Hill, 39°29.44–28.51’S, 178°25.05– 25.48’E, 1000– 980 m, KAH9907/37, 3 Jun 1999 ; NIWA 29663 , 1 ovigerous female (cl 6.7 mm, cw 6.6 mm), Ghoul Seamount, 42°47.85’S, 179°59.26’E, 925–1054 m, coral rubble and rocks, TAN0604/118, 7 Jun 2006 .

Diagnosis. Carapace almost square, lateral margins almost parallel; with anterolateral spine; anterior and anterolateral surfaces with long, fine setae, other surfaces with sparse, short fine setae; dorsal and lateral surfaces entirely covered with minute granules. Fronto-orbital margin (excluding rostrum and lateral projections) not advanced beyond anterolateral margins; lateral frontal projections subequal to length of rostrum. Rostrum about half-length of eyestalks. Eyestalks distinctly divergent, reaching anteriorly to end of first antennular peduncle segment. Third maxilliped sparsely granular; merus as long as ischium, about 2.5 times longer than wide; tapering distally to rounded apex. Chelipeds granular; carpus with 2 or 3 dorsal spines; palm without dorsal spines. P2 and P3 setose; all segments except for dactylus finely granular. P3 merus as long as carapace length (excluding rostrum). P4 and P5 longer than merus of P3; dactyli with corneous spines on ventral margins. P5 merus when folded against carapace, reaching to the anterior quarter of carapace. Female telson subtriangular, margins concave near midlength, apex rounded.

Description. Carapace quadrate, almost square, lateral margins almost parallel; regions weakly indicated; with blunt anteriorly directed anterolateral spine and one or two smaller laterally directed spines on lateral margin behind anterolateral spine; lower pterygostomian region swollen; anterior and anterolateral surfaces with long, fine setae, other surfaces with sparse, short fine setae. Dorsal and lateral surfaces entirely covered with minute granules, with granules becoming larger and more elongate anterolaterally. Fronto-orbital margin (excluding rostrum and lateral projections) not advanced beyond anterolateral margins; about half anterior carapace width; lateral frontal projections slender, elongate, situated below plane of rostrum, laterally spinulate, with acute apices, subequal to length of rostrum. Rostrum 0.12–0.15 cl, half or slightly less than halflength of eyestalks; slender, tapering to acute apex, minutely granular laterally and dorsally. Eyestalks distinctly divergent, flattened, fused to carapace below rostral base; reaching anteriorly to end of first antennular peduncle segment; granular and spinulate; cornea apparently vestigial, not pigmented. Epistome smooth except for blunt tubercle at base of rostrum and blunt tubercle mesial to base of antennules.

Antennular peduncle shorter than carapace length; first segment laterally granular; second segment minutely granular; third segment smooth. Basal antennal segment fused to epistome; second segment granular, laterally and distally spinulate; third segment granular; fourth segment smooth.

Third maxilliped ischiobasis subquadrate, sparsely granular, with longitudinal sublateral groove; ischium and basis demarcated by shallow groove. Merus elongate, as long as ischium, about 2.5 times longer than wide; tapering distally to rounded apex; surface sparsely granular or spinulate, margins spinulate. Propodus and carpus sparsely spinulate. Dactylus conical, unarmed. Exopod sparsely granular, not exceeding merus of endopod.

Chelipeds (P1) equal in size and ornamentation, setose. Merus finely granular. Carpus finely granular, dorsal margin with 2 or 3 spines. Palm surfaces with fine granules and few scattered acute granules, slightly more prominent on upper margin, but none produced to spines. Dactylus and pollex about 1.5 times upper palm length; without spines; with faint longitudinal carina on outer surface, occlusal surfaces of dactylus and pollex crenulate, with slight gape when closed; proximal half of dactylus with scattered granules.

P2 and P3 long, slender, setose; all segments except for dactylus finely granular. P3 longest, as long as carapace length (excluding rostrum). Dactyli broadly curved, smooth, with longitudinal rib. P3 dactylus slightly shorter than combined length of propodus and carpus.

P4 and P5 finely granular and setose; longer than merus of P3; dactyli markedly shorter than propodi, falcate, with corneous apex and 4–6 small, obliquely inclined, corneous spines on ventral margin. P5 merus, when folded against carapace, reaching anterior quarter of carapace.

Female abdomen with margins and surface finely granular; telson subtriangular, margins concave near midlength, apex rounded; length 0.5–0.6 times width. Egg diameter 1.2–1.5 mm (eyes not yet visible).

Etymology. Dedicated to Malcolm Clark, for his longstanding work on New Zealand seamounts.

Remarks. Three species of Cymonomus A. Milne-Edwards, 1880 are now known from New Zealand: C. aequilonius Dell, 1971 (type locality: northeast of Mayor Island, Bay of Plenty), C. bathamae Dell, 1971 (type locality: Papanui Canyon, off Otago coast), and C. clarki sp. nov. Cymonomus clarki is readily distinguished from C. bathamae in the form of the fronto-orbital region (not advanced anteriorly beyond anterolateral margins), and in the longer P2 and P3 (P3 merus as long as rather than distinctly shorter than postrostral carapace length). Rostral length immediately distinguishes C. clarki from C. aequilonius (about half the length, rather than distinctly longer than the eyestalks).

Of the known species of Cymonomus , C. clarki most closely resembles C. soela Ahyong & Brown, 2003 , from seamounts off Tasmania, Australia, sharing the combination of divergent eyestalks, a slender rostrum that reaches anteriorly to about the midlength of the eyestalks, well-developed lateral frontal projections of similar length to the rostrum, and granular carapace and pereopodal sufaces. Cymonomus clarki differs from C. soela in having distinctly setose rather than almost glabrous dorsal surfaces (few, scattered short setae on the rostrum and chelipeds of C. soela ); the third maxilliped ischium is proportionally longer (length 2.5 versus about 2 times width; Fig. 6C ) with a more slender distal half; the terminal abdominal segment is proportionally longer (length 0.6 versus 0.4 times width) with concave rather than angularly convex margins and a rounded rather than angular apex ( Fig. 6D ); having comparatively finer overall granulation, especially on the cheliped palm which lacks prominent, blunt, upright spines and elongate granules, and on the cheliped carpus, which bears only 2 or 3 rather than a cluster of dorsal spines; and in having a proportionally longer P4 and P5, which are distinctly longer than, rather than as long as the P3 merus ( Fig. 1D , 6A ). Additionally, when P5 is folded against the carapace, the distal end of the merus reaches the anterior quarter of the carapace in C. clarki , rather than the midlength as in C. soela .

The type specimens of C. clarki agree well in almost all respects, differing only marginally. The smaller paratype differs from the holotype in the length: width ratio of the telson (0.5 versus 0.6), in the slightly shorter rostrum (0.12 versus 0.15 cl), and in having slightly shorter anterolateral spines on the carapace. All specimens are ovigerous. The eggs are large (1.2–1.5 mm diameter) and few in number (25– 31 eggs), suggesting that C. clarki could be expected to have limited dispersal capabilities and a narrow range.

Takeda et al. (2005) reported as C. soela , a male and female from the Ryukyu Islands ( Japan), noting that it differed from the type description in having a much shorter rostrum and a more finely granular third maxilliped. According to Takeda et al. (2005: fig. 2), the Ryukyu specimens are also dorsally setose and the lengths of P4 and P5 are closer to those of C. clarki than C. soela . The Ryukyu specimens probably represent an undescribed species.

Distribution. Presently known only from seamounts on the northern Chatham Rise, New Zealand; 925– 1054 m.

Figures

FIGURE 1. A, Dagnaudus petterdi (Grant, 1905), female, pcl 44.1 mm (NIWA 34784). B, Homolodromia kai Guinot, 1995, female, cl 19.9 mm, cw 14.6 mm (NIWA 6761). C, Dicranodromia spinulata Guinot, 1995, male, cl 9.6 mm, cw 6.8 mm (NIWA 34933). D, Cymonomus clarki sp. nov., ovigerous female holotype, cl 6.9 mm, cw 6.6 mm (NIWA 29666). E, Lyreidus tridentatus De Haan, 1841, female, cl 46.6 mm (NIWA 6723). F, Cymonomus aequilonius Dell, 1971, female, cl 7.7 mm, cw 6.7 mm (NIWA 34966).

FIGURE 6. Cymonomus clarki sp. nov., ovigerous female holotype, cl 6.9 mm, cw 6.6 mm (NIWA 29666). A, dorsal habitus. B, frontal region. C, left third maxilliped. D, distal abdominal somites. Scale: A, D = 2.0 mm, B–C = 1.0 mm.