Cymonomus aequilonius Dell, 1971

Ahyong, Shane T., 2019, The Cymonomid Crabs of New Zealand and Australia (Crustacea: Brachyura: Cyclodorripoida), Records of the Australian Museum 71 (2), pp. 33-69 : 36-38

Cymonomus aequilonius Dell, 1971

Figs 2 , 3

Cymonomus aequilonius Dell, 1971: 59–61 , figs 11–15.— Tavares, 1993b: 258.— Ahyong & Brown, 2003: 1372.— Ng et al., 2008: 32 .— Webber et al., 2010: 225.—Yaldwyn & Webber, 2011: 227.

Holotype: NMNZ Cr 1866, female (cl 7.1 mm, pcl 5.7 mm, cw 6.4 mm), NE of Mayor Island, Bay of Plenty, New Zealand, 37°10'S 176°23.5'E, 400 fm [732 m], trawl, BS 210, 28 February 1957.

Description of holotype. Carapace quadrate, almost square, lateral margins slightly divergent posteriorly; regions weakly indicated, cervical groove weakly indicated, broadly V-shaped; lower pterygostomian region swollen; anterolateral surfaces with few scattered setae.Anterolateral spine short, blunt, conical, directed anteriorly; smaller spine on lateral margin behind anterolateral spine. Dorsal and lateral surfaces entirely covered with minute rounded granules, granules becoming slightly larger and more elongate anterolaterally, bluntly conical, not globose. Fronto-orbital margin (excluding rostrum and outer-orbital processes) advanced beyond anterolateral margins; 0.6 anterior carapace width; outer orbital processes stout, elongate, directed anteriorly, situated below plane of rostrum, laterally spinulate, apex acute, half rostral length. Rostrum distinctly longer than eyestalks; 0.27 pcl; slender, tapering to acute apex, granulate dorsally and laterally.

Eyestalks strongly divergent (about 40° from median axis), stout, flattened, minutely granulate dorsally, acutely granulate and weakly spinulate along mesial margin, fused to carapace below rostral base but demarcation distinct, reaching anteriorly to midlength of antennular peduncle article 1; cornea apparently vestigial, not pigmented.

Epistome with tubercle mesial to base of antennules, otherwise smooth; strong spine with smaller secondary spine mesial to base of antenna.

Antennular peduncle 0.83 pcl (female); articles 1 and 2 minutely granulate; article 3 smooth. Antennal articles irregularly granulate or minutely spinular.

Maxilliped 3 ischiobasis subquadrate, sparsely granulate and minutely spinular; longitudinal sublateral groove; ischium and basis demarcated by faint groove. Merus slightly shorter than ischium; length about twice width, tapering distally to rounded apex; surface and margins spinulate. Dactylus conical, with scattered granules; propodus and carpus sparsely spinulate. Exopod surface sparsely granulate, distal margin spinulate; apex reaching beyond carpo-meral articulation but not reaching beyond end of endopod merus.

Chelipeds (pereopod 1) equal in size and ornamentation, setose. Merus finely granulate. Carpus granulate, dorsal margin with 4 spines. Propodus palm surfaces granulate, dorsal and ventral margins with few conical spines and tubercles. Dactylus longer than dorsal palm length; proximal dorsal half with few small spines; with faint longitudinal carina on outer surface, occlusal surfaces of dactylus and pollex weakly crenulate, without gape when fingers closed.

Pereopods 2 and 3 sparsely setose; all articles except for dactylus finely granulate; propodus and carpus with minutely spinular extensor margins; merus flexor margins minutely spinular on P2, unarmed on P3; dactylus broadly curved, smooth, with longitudinal rib. Pereopod 3 longest, merus 1.33 pcl (female); dactylus slightly shorter than combined length of propodus and carpus.

Pereopods 4 and 5 finely granulate, some minute spines, glabrous; longer than pereopod 3 merus (female); propodus distoextensor margin unarmed; dactylus markedly shorter than propodus, falcate, with corneous apex and 5 obliquely inclined, corneous spines on flexor margin. Pereopod 5 merus, when folded against carapace, reaching anterior one-fifth of carapace.

Thoracic sternite 3 pentagonal, width about 1.8 × length; lateral margins divergent posteriorly; surface sparsely granulate. Margins of sternites 4 and 5 weakly granulate.

Abdomen surface finely granulate or minutely spinulate; pleotelson without trace of demarcation between somite 6 and telson distally obtuse, bluntly rounded, length half width (female).

Remarks. Cymonomus aequilonius was described from a single spent female from the Bay of Plenty, New Zealand, and subsequently reported further south on the Chatham Rise ( Ahyong, 2008). Restudy of all previous records of C. aequilonius revealed that the southern specimens represent a separate species, herein named C. alius sp. nov. As such, C. aequilonius remains known only from the type locality.

As a member of the C. granulatus group, C. aequilonius shares the combination of well-developed outer orbital processes and the long rostrum overreaching the eyestalks ( Fig. 2A, D ) with C. alius sp. nov. ( Fig. 4A, D ) ( New Zealand), C. granulatus (Norman in Wyville Thomson, 1873) (northeast Atlantic), C. indicus Ihle, 1916 ( Indonesia) , C. japonicus Balss, 1922 ( Japan) , and C. magnirostris Tavares, 1991 ( Brazil) . Not surprisingly, C. aequilonius is morphologically closest to its New Zealand congener, C. alius , agreeing in almost all respects including the bifid epistomial spine mesial to the base of the antenna ( Fig. 2C , 4C ), but is readily distinguished by the distinctly longer walking legs (female pereopod 3 merus 1.33 versus 1.03–1.05 pcl, longer than cl versus shorter than cl) and longer antennular peduncle (females: 0.83 versus 0.58–0.74 pcl). Cymonomus aequilonius can be separated from C. indicus by the rounded versus polygonal or stellate carapace tubercles, and from C. japonicus by the longer pereopod 5, with the merus reaching to the anterior one-fourth, rather than the midlength of the carapace. Cymonomus aequilonius differs from C. magnirostris in the shorter maxilliped 3 exopod (which under- rather than overreaches the apex of the merus; Fig. 1E ), and from C. granulatus in the more slender rostrum ( Fig. 1A, D ) (basal width about half length in C. granulatus ).

Distribution. Known only from the Bay of Plenty, New Zealand ( Fig. 3 ); 732 m.

Figures

Figure 1. Measurements of carapace and antennules (A), and pereopod 3 (B).

Figure 2. Cymonomus aequilonius Dell, 1971, female holotype, cl 7.1 mm, pcl 5.7 mm, cw 6.4 mm, Bay of Plenty, New Zealand, NMNZ Cr1866. (A) dorsal habitus; (B) posterior abdomen; (C) right epistomial spine mesial to base of antenna; (D) fronto-orbital region; (E) right maxilliped 3; (F) thoracic sternite 3. Scale: A, B = 2.0 mm; C = 0.5 mm; D–F = 1.0 mm.

Figure 3. Cymonomus aequilonius Dell, 1971, distribution.

Figure 4. Cymonomus alius sp. nov., Tims Bank, New Zealand. (A–F) spent female holotype, cl 7.7 mm, pcl 6.2 mm, cw 6.7 mm, NIWA 34966. (G–I) male paratype, cl 6.0 mm, pcl 4.5 mm, cw 4.7 mm, NIWA 121121. (A) dorsal habitus; (B, I) posterior abdomen; (C) right epistomial spine mesial to base of antenna;(D) fronto-orbital region;(E) right maxilliped 3; (F) thoracic sternite 3; (G) right G1, abdominal view; (H) right G2, abdominal view. Scale:A–B = 2.0 mm; C = 0.5 mm; D–I = 1.0 mm.