Cymonomus suluensis, Takeda & Ohtsuchi & Komatsu, 2021

Takeda, Masatsune, Ohtsuchi, Naoya & Komatsu, Hironori, 2021, Crabs (Crustacea, Decapoda) from the Sea off East and Southeast Asia collected by the RV Hakuhō Maru (KH- 72 - 1 Cruise) 1. Sulu Sea and Sibutu Passage, Bulletin of the National Museum of Nature and Science. Series A, Zoology 47 (2), pp. 65-97 : 67-71

Cymonomus suluensis sp. nov.

( Figs. 2–3 )

Material examined. RV Hakuhō Maru KH-72-1 cruise, sta. 08 (Sulu Sea ; 08°44.6′N, 119°05.4′E – 08°44.8′N, 119°06.2′E; 2,030 – 2,030 m deep); 3 m beam trawl; 25 May, 1972; ˂ holotype, NSMT-Cr 28965 .

Measurements. CB 5.1 mm; CL 4.4 mm excluding rostrum; length of rostrum, 2.5 mm; length of eyestalk, 1.5 mm including basal segment; length of ambulatory leg, 14.5 mm.

Diagnosis. Small species. Carapace subquadrate in outline, slightly wider than long, with weakly convex branchial regions; carapace dorsal surface and lateral margins covered and fringed with many spinules and granules. Rostrum long, about one-third as long as carapace, directed forward, weakly tapering, armed with three spinules at each side along distal half. Inner half of supraorbital margin with some tubercles; external orbital tooth spiniform, about half as long as rostrum, directed obliquely outward. Eyestalk fixed, slender, directed obliquely outward along whole length, obliquely downward for its basal half, horizontal for its distal half; in dorsal view, distal end of eyestalk just at level of basal one third of rostrum; eyestalk armed with 2 distant spinules on upper surface; cornea distinct at tip of eyestalk, without pigments. Chelipeds armed with spinules along margins of merus, carpus and palm; distal part of carpus and proximal part of palm deeply excavated to form a deep cavity. Ambulatory leg slender.

Description of holotype (Female). Carapace ( Figs. 2A , 3A ) subquadrate in outline, only slightly wider than long; carapace lateral margins weakly convex outward along hepatic and branchial parts, shallowly concave between both parts; carapace dorsal surface ( Fig. 2C–D ) not convex as a whole, roughened with small, sharp dispersed granules mainly on branchial regions; gastric, branchial and cardiac regions weakly demarcated with shallow depressions or indistinct furrows; anterior extension of mesogastric region narrow, reaching nearly to basal part of rostrum; a suberect spinule at anterior part of protogastric region; protogastric and hepatic regions indistinctly confluent, separated from branchial region by an oblique furrow from shallow depression between hepatic and branchial outer margins to a transverse shallow, short furrow separating cardiac and intestinal regions; gastro-branchial oblique furrow turned obliquely outward to lateral end of carapace posterior margin along intestinal region.

Rostrum ( Figs. 2A , 3A–C ) long, about one-third as long as carapace, directed forward, weakly tapering, with 3 spinules on each margin along distal half; proximal spinules of both sides alternate in position, diminishing in length distally. Supraorbital margin ( Figs. 2A , 3A–C ) nearly concave along inner half, with some marginal spinules weakly directed outward; outer half of supraorbital margin unarmed, weakly curving downward toward external orbital tooth. External orbital tooth ( Fig. 3A–C ) spiniform, slender, with tip bifid in left side, broken off in right side, extending beyond tip of eyestalk. Subhepatic tooth ( Fig. 3A–B ) shorter than external orbital and hepatic spiniform teeth, but sharp, directed obliquely outward. Hepatic margin ( Figs. 2A , 3A ) weakly convex, with a strong spine at anterior one third, a shorter spine on posterior slope and some subsidiary spinules. Carapace branchial margin ( Figs. 2A, C , 3A ) twice as long as hepatic margin, armed with a series of 10–15 spinules of approximately similar size. Carapace posterior margin ( Figs. 2A, C–D , 3A ) as long as front-orbital margin, rather strongly concave at median part, with a narrow but deep marginal furrow along whole length of posterior margin.

Eyestalk ( Fig. 3A–C ) slender, directed obliquely outward for whole length along external orbital tooth, obliquely downward for its basal half, horizontally for its distal half, distal end of eyestalk attaining to tip of inner infraorbital tooth; in dorsal view, distal end of eyestalk slightly exceeding one-third of, or slightly less than half of rostrum; cornea distinct at distal end of eyestalk, without pigment; upper surface of eyestalk armed with 2 outward-directed spinules with some distance.

Third maxilliped ( Fig. 2B ) pediform; ischium elongated, rectangular, curved outward distally; merus half as long as ischium, weakly curved inward as a whole, with rounded disto-lateral angle; merus with spine at median part of basal one-third of outer surface; outer margin armed with 4 strong spines, basal 2 directed laterally, distal 2 obliquely forward, inner margin with a series of spines, 2 sub-erect spines at subdistal part much stronger than others; outer margin of exopod armed with 3 erect equidistant spines at side of ischium, each side of merus with an obliquely-forward directed spinule.

Pleon with 6 pleomeres and telson ( Fig. 2B, D ); pleomere 1 small, quadrate, lateral margins weakly concave for whole length; pleomere 2 prominent, widening posteriorly, with 2 spines along median line and a cluster of some spines at each lateral part; pleomere 3 similar to pleomere 2 in shape and armature, with much smaller spines and spinules; pleomere 4 as wide as pleomere 3, with vestigial tubercles in median line; pleomeres 5–6 and telson weakly tapering distally, with thin plate-like appearance, unarmed, each pleomere distinctly articulated, separated laterally from other pleomeres by V-shaped notch; telson obtuse at tip, forming rounded triangle as a whole.

Both chelipeds ( Figs. 2E–F , 3D ) equal in size and shape, not inflated. Merus weakly curved, with about equidistant 10 spinules of alternate sizes on posterior margin of merus. Carpus armed with some longer spinules and some smaller spinules on outer and upper surfaces, respectively; inner angle produced to be a tubercle directed forward. Palm and fingers together weakly curved inward; inner proximal part of palm ( Figs. 2E–F , 3D ) deeply excavated together with distal inner part of carpus to form a deep cavity. Palm and fingers ( Fig. 3E ) subequal in length, with untoothed cutting edges of fingers. A detached ambulatory leg of left side long, slender ( Fig. 3F ).

Remarks. Ng et al. (2008) enumerated 24 species in the genus Cymonomus A. Milne-Edwards, 1880 and five species in the genus Cymonomoides Tavares, 1993 . Both genera are distinguished only by the fusion of the telson and the sixth pleomere being indistinguishably fused as a pleotelson in Cymonomus , but separated by a visible suture in Cymonomoides . Ahyong and Ng (2009), however, doubted the validity of Cymonomoides on the basis that in Cymonomus diogenes newly described by them from the Philippines, the sixth pleomere and the telson are immovably fused with a clear suture in males, but completely fused without a visible suture in females. Recently, Ahyong (2019) decidedly returned Cymonomoides delli ( Griffin and Brown, 1976) , C. cubensis ( Chace, 1940) and C. valdiviae ( Lankester, 1903) to the original position in the genus Cymonomus . As rightly mentioned by Ahyong and Ng (2017), the genus Cymonomoides may be restricted to the type species, C. guinotae (Tavares, 1991) from Brazil, and C. fitoi Lemaitre and Bermudez, 2000 from the Caribbean coast of Colombia.

Since the enumeration of the species by Ng et al. (2008), altogether 18 species were additionally described as follows: 1) one species from New Zealand ( Ahyong, 2008); 2) four species from the Philippines ( Ahyong and Ng, 2009); 3) one species from the Philippines ( Ahyong and Ng, 2011); 4) one species from off Madagascar ( Ahyong, 2014); 5) two species from Taiwan and Japan ( Ahyong and Ng, 2017); 6) eight species from Australia and New Zealand (Ahyong, 2019); 7) one species from Indonesia ( Ahyong et al., 2020).

Ahyong and Ng (2017) reduced Cymonomus sagamiensis Sakai, 1983 from Japan to a synonym of C. umitakae Takeda, 1981 from Japan. As a result, the genus Cymonomus is composed of 31 Indo-West Pacific, 11 West Atlantic, and 2 Northeast Atlantic and Mediterranean species, and in this paper, C. suluensis is described as the 45 th Cymonomus species and 32 nd Indo-West Pacific species.

Ahyong (2019) revised the New Zealand and Australian cymonomid crabs and distinguished six species groups in the genus Cymonomus : 1) C. bathamae group, 2) C. curvirostris group, 3) C. delli group, 4) C. granulatus group, 5) C. karenae group, and 6) C. soela group.

The new species described in this paper, C. suluensis , is quite distinctive in having the long rostrum and the long eyestalks directed obliquely outward as a whole, and downward along the basal half and horizontally along the distal half, with the distinct transparent cornea, and readily distinguished from all the known species. As far as the long rostrum concerned, the new species corresponds to the C. granulatus group which is composed of six species with the combination of a long rostrum that overreaches the eyestalks and well-developed outer orbital tooth. The six species referred to this group by Ahyong (2019) are C. aequilonius Dell, 1971 from New Zealand, C. alius Ahyong, 2019 from New Zealand, C. granulatus (Norman, in Wyville Thomson, 1873) from the Northeast Atlantic and the Mediterranean Sea, C. indicus Ihle, 1916 from Indonesia, C. japonicus Balss, 1922 from Japan, and C. magnirostris Tavares, 1991 from Brazil. Of them, C. granulatus and C. magnirostris from the Atlantic Ocean are closer to the new species rather than four species from the Pacific Ocean.

The new species, C. suluensis , however, is quite characteristic in the carapace margin fringed with many spinules, and the rostrum armed with some spinules of good length at each side, and each eyestalk directed obliquely outward as a whole and downward for its basal half and horizontally for its distal half. These characters make this species distinct from all the species including the C. granulatus group and the sole representative of the seventh group in the genus Cymonomus , herein called the C. suluensis group.

Etymology. Named after the type locality, the Sulu Sea.

Distribution. Known only from the type locality. The bathymetric record, 2,030 m, is the deepest for any cymonomid species.

Figures

Fig. 2. Cymonomus suluensis sp. nov., female, holotype (NSMT-Cr 28965. CB 5.1mm; CL 6.9mm including rostrum). Carapace in different views (A–D), and both chelipeds in dorsal view (E–F).

Fig. 3. Cymonomus suluensis sp. nov., female, holotype (NSMT-Cr 28965. CB 5.1mm; CL 6.9mm including rostrum). Carapace in dorsal view (A), right front-orbital region in dorsal view (B), left front-orbital region in lateral view (C), left cheliped in upper and outer views (D–E), and left ambulatory leg in dorsal view (F). Scale bars=2mm.