A new species of the coral-symbiont crab genus Cymo de Haan, 1833 (Decapoda, Brachyura, Xanthidae) from Nansha Islands, the South China Sea Author Yuan, Zi-Ming Laboratory of Marine Organism Taxonomy and Phylogeny, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China. yuanziming @ qdio. ac. cn; jiangwei @ qdio. ac. cn; shazl @ qdio. ac. cn & University of Chinese Academy of Sciences, Beijing, China yuanziming@qdio.ac.cn Author Jiang, Wei Laboratory of Marine Organism Taxonomy and Phylogeny, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China. yuanziming @ qdio. ac. cn; jiangwei @ qdio. ac. cn; shazl @ qdio. ac. cn & Shandong Province Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China & Laoshan Laboratory, Qingdao, China yuanziming@qdio.ac.cn Author Sha, Zhong-Li Laboratory of Marine Organism Taxonomy and Phylogeny, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China. yuanziming @ qdio. ac. cn; jiangwei @ qdio. ac. cn; shazl @ qdio. ac. cn & Shandong Province Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China & Laoshan Laboratory, Qingdao, China yuanziming@qdio.ac.cn text Zootaxa 2023 2023-11-01 5361 2 275 286 https://www.mapress.com/zt/article/download/zootaxa.5361.2.8/52183 journal article 277847 10.11646/zootaxa.5361.2.8 5bf93237-8248-486c-855f-c0f87553a671 1175-5326 10146585 9FB81B0C-412B-4978-B2CF-4151DA505F93 Cymo mazu sp. nov. Type material. Holotype : MBM287035 , male, CW 4.1 mm , CL 3.8 mm , Fiery Cross Reef , Nansha Islands , China , 112°59’E , 9°39’N , 3 m , in Acropora latistella ( Brook, 1892 ) , NS-YS-2022-1139, coll. Shaobo Ma, Yuli Sun & Ziming Yuan, 13 May 2022 . Etymology. Named after Mazu, a legendary Chinese woman, Mo Lin (A.D. 960–987) from the Song Dynasty, who made significant contributions to the rescue of shipwrecked people. In China , she is widely worshipped as the sea goddess who protects fishermen and sailors on their voyages. Diagnosis. Carapace slightly wider than long, dorsal surface smooth, with sharp granules on orbital region, frontal regions, gastric regions, anterolateral regions and posterolateral regions. Chelipeds with strong spines on outer surface of palm; large cheliped fingers totally white, small cheliped fingers white but with pale brown block. Endopodite of maxilliped 1 strongly concave. Description. Carapace ( Figs. 1 , 2A, B , 3A ) subcircular, slightly wider than long, armed with long setae, denser on front regions; dorsal flat and smooth, regions boundaries unclear; sharp granules on front, orbital region, gastric regions, anterolateral and posterolateral surfaces. Front ( Fig. 3B ) approximately 0.4× CW, divided into two lobes by wide v-shaped notch, margin oblique, with small spines, tip more-or-less truncate. Dorsal inner orbital angle with spine, separated from front by notch; dorsal orbital margin with submedian fissure; outer orbital angle and anterolateral margin confluent; infraorbital tooth sharp. Eyestalk short, with small granules. Anterolateral margin convex, lined with spines; posterolateral margin nearly straight, longer than anterolateral margin. Subhepatic and pterygostomial regions granulated. FIGURE 1 . Colour in life, Cymo mazu sp. nov. , holotype, male, CW 4.1 mm, CL 3.8 mm, MBM287035. Scale bar = 2 mm. Antennular fossae ( Fig. 2C ) subcircular, antennule situated obliquely. Basal segment of antenna subrectangular, anterolateral angle produced. Endopodite of maxilliped 1 ( Fig. 3C ) strongly concave. Maxilliped 3 ( Figs. 2C , 3D ) completely covering buccal frame; merus subpentagonal, dorsal margin concave, inner margin with prominent lobe; ischium subquadrate. Male thoracic sternum (Fig. D) smooth; sternites 1, 2 fused; suture between sternites 2, 3 and 6, 7 wide; suture of sternites 3 and 4 present on lateral edges, continuing via transverse depression; sternite 4 with distinct long median groove; press-button located on posterior of thoracic sternite 5. FIGURE 2. Cymo mazu sp. nov. , holotype, male, CW 4.1 mm, CL 3.8 mm, MBM287035: A, dorsal habitus; B, carapace; C, frontal view; D, thoracic sternites and pleon; E, chelipeds. Scale bar A–C = 2 mm, D–E = 1 mm. Chelipeds ( Figs. 2E , 3F ) markedly unequal, robust and covered with long setae; merus short, granulated on ventral and outer surface; carpus subsphaeroidal, with spines on outer and dorsal surface; palm covered with granules evenly arranged, forming larger, sharp spines on dorsal surface and dorsal part of outer surface, mixed with small granules, transitioning to smaller round granules on inner surface, ventral surface and ventral part of outer surface. Fingers deflexed, dorsal surface of dactylus lined with sharp spines. In preservation, finger white but with very pale brown block, more prominent on minor cheliped, almost invisible on major cheliped; with gape when closed, fingertips hollowed. Ambulatory legs ( Figs. 1 , 2A , 3E ) short, flat, covered with long setae; merus with spines on anterior and posterior margins; carpus short, spines present on anterior margin; propodus with spines on anterior margin; dactylus strong, hooked, with developed dactylo-propodal articulation. Male pleon smooth ( Figs. 2D , 3G ), pleonite 3 widest; pleonites 3–5 fused with visible suture; telson subtriangular, tip round, slightly wider than long. G1 ( Figs. 3H, I, J ) slender, slightly sinuous laterally; inner and outer surface with spines, distal lobe short, deflexed. G2 ( Fig. 3K ) small, length about 0.4× length of G1, distal part slightly upturned, subdistal part with spines. Live coloration. Body white to ivory, enveloped in long setae ranging from yellow to rufous, intermixed with bright red setae on carapace. Gastric, cardiac and intestinal regions adorned with fluorescent green vermicular pattern, interval being rufous. Cornea blue-green. Coloration of both cheliped fingers similar to that of palm, with no notable differentiation. Preserved in alcohol, almost entirely white, with only faint patches of brown discernible ( Fig. 1 ). Remarks. Cymo mazu sp. nov. distinctly differs from its congeners by several unique features: the carapace, despite being smooth, features anterolateral spines, chelipeds armed with well-arranged, potent, sharp spines, a pale carapace embellished with a fluorescent pattern, and the particularly concave distal end of the maxilliped 1 endopodite. Notably, the G1 distal lobe of C. mazu does not have the elongation observed in other species of Cymo ; rather, it is present as a short, deflexed opening lacking subdistal setae. This distinction could also result from incomplete development of the holotype . While uniquely characterized in many respects, the new species also exhibits combined characteristics shared with congeners. Cymo mazu bears the closest resemblance to C. deplanatus due to their similar setal distribution, smooth carapace and a spiny front. However, C. mazu can be distinguished by: the carapace being slightly wider than long (vs. carapace longer than wide in C. deplanatus ) ( Figs. 5A, B ); frontal lobes lined with small spines (vs. larger and longer spines adorning frontal lobes in C. deplanatus ) ( Figs. 5F, G ); chelipeds armed with large conical spines and hemispherical granules, evenly spaced and neatly arranged on the surface (vs. the irregular, smaller spines and granules with varying sizes and spacing on the chelipeds surface in C. deplanatus ) ( Figs. 5K, L )); a deeply concave endopodite of maxilliped 1 (vs. absence of concavity on the maxilliped 1 endopodite of C. deplanatus ) ( Figs. 5P, Q ). Cymo mazu can also be differentiated from C. barunae , a species described from the South China Sea, in features like those of C. deplanatus . Compared with C. mazu , C. deplanatus possesses a proportionally longer carapace, longer and more pronounced but fewer frontal spines, and the absence of the concavity on the maxilliped 1 endopodite. The morphology of the maxilliped 1 endopodite is an effective diagnostic characteristic for species of Cymo (see Galil & Vannini 1990 ). In this regard, C. mazu , with its significantly concave maxilliped 1 endopodite, closely resembles C. lanatopodus . However, C. mazu can be differentiated by the following characteristics: the carapace has distinct setae (vs. nearly glabrous in C. lanatopodus ); frontal lobes armed with spines (vs. bifurcated, flat and spineless frontal lobes in C. lanatopodus ); cheliped surface with notable long spines and granules (vs. rough cheliped surface with small granules in C. lanatopodus ). Additionally, compared to C. mazu , the carapace of C. lanatopodus presents a more elongated and inwardly tapered posterolateral margin. Cymo mazu , while bearing resemblance to C. quadrilobatus in the spiny chelipeds and concave endopodite of maxilliped 1, can be distinguished by: carapace spines and granules conspicuously smaller than on the chelipeds, never grouped in clusters (vs. spines and granules on carapace comparable to those on chelipeds, both large and grouped in clusters in C. quadrilobatus ); a simpler, bilobed front (vs. quadrilobed front in C. quadrilobatus ); a near-straight carapace posterolateral margin (vs. rounded posterolateral margin contributing to a more circular carapace appearance in C. quadrilobatus ); and a deeply concave endopodite of maxilliped 1 (vs. a slightly concave endopodite of maxilliped 1 in C. quadrilobatus ). Similarly, the present new species can also be distinguished from the lesser-known species, C. tuberculatus Ortmann, 1893 . The latter possesses a carapace and pereopods similar to C. quadrilobatus , characterized by nodules formed from aggregated granules, and a carapace length that exceeds its width. FIGURE 3. Cymo mazu sp. nov. , holotype, male, CW 4.1 mm, CL 3.8 mm, MBM287035: A, carapace; B, front; C, maxilliped 1; D, maxilliped 3; E, pereopod 5; F, large cheliped; G, pleon; H, left G1, ventral view; I, left G1, distal part, ventral view; J, same, dorsal view; K, right G2, ventral view. Scale bar A, E, F = 1 mm, C, D, G = 0.5 mm, H = 0.2 mm, I–K = 0.1 mm. Given the similar carapace contour, small dorsal spines or granules, and pale cheliped fingers, C. mazu bears a superficial similarity to C. andreossyi . Cymo mazu can be primarily distinguished by: carapace anterolateral margins lined with distinct spines (vs. anterolateral margin without spines in C. andreossyi ); chelipeds armed with large and acute spines (vs. chelipeds with low and round granules, densely arranged); front margins armed with simple spines (vs. two of the front margins spines larger and bifurcated); and a markedly concave endopodite of maxilliped 1 (vs. endopodite of maxilliped 1 slightly concave in C. andreossyi ). A notable variant is Cymo andreossyi var. maculata Klunzinger, 1913 , which is currently listed as a synonym for C. andreossyi (see Ng et al . 2008 ). According to its original description, this variant exhibits a longer and straighter posterior carapace margin, a feature that aligns with the new species under discussion. However, C. andreossyi var. maculata possesses a unique characteristic: a prominent angle at the junction of its anterior and posterior carapace margins. Further study might show it to represent a valid species. The coloration of the cheliped fingers often serves as a diagnostic characteristic within this genus. Cymo mazu can be readily differentiated from two morphologically similar species, C. melanodactylus and C. cerasma , by its overall lighter finger coloration. Both C. melanodactylus and C. cerasma present uniform black fingers, only showing white at the margin hollowed tips. Even in faded specimens of C. melanodactylus observed in the present study, the cheliped fingers retain their black coloration. In contrast, the new species, when alive, possesses white or ivory fingers with no apparent color separation from the palm. In faded specimens, the fingers are white, exhibiting only faint brown patches, a condition more akin to that of C. quadrilobatus . It is noteworthy that C. melanodactylus and C. cerasma are highly similar, with the primary distinguishing feature seemingly being the broader carapace of the latter. However, a nearly circular carapace outline has also been observed in C. melanodactylu s in this study, thus further inspection and confirmation of the identity and more detailed distinguishing features of the latter species are warranted. To further illuminate the relationships between C. mazu and other related species within the genus, we used COI sequences of C. andreossyi , C. deplanatus , C. mazu , C. melanodactylus , and C. quadrilobatus for molecular analyses. The interspecific genetic distances of COI among these five species were all greater than 12.3%, with intraspecific genetic distances being less than 0.7% ( Table 2 ). In the phylogenetic trees ( Fig.4 ), individuals of the same species clustered together, exhibiting high support values ( C. andreossyi : 100/91; C. deplanatus : 100/100; C. melanodactylus : 100/100; C. quadrilobatus : 100/99). The holotype of C. mazu remained distinct from all other species, exhibiting the closest relationship to C. deplanatus (87/60). Cymo quadrilobatus clustered with C. deplanatus and C. mazu (78/-). Additionally, C. melanodactylus and C. andreossyi formed a separate cluster (100/90). TABLE 2. Genetic divergence of the COI sequences among five s peciesof Cymo calculated using Kimura 2-parameter- corrected distances.

C. deplanatus	C. andreossyi	C. quadrilobatus	C. mazu	C. melanodactylus
C. deplanatus	0.001
C. andreossyi	0.170	0.007
C. quadrilobatus	0.147	0.132	0.007
C. mazu	0.162	0.187	0.174	n/c
C. melanodactylus	0.167	0.123	0.169	0.175	0.000

Species of Cymo present superficially similar morphologies, which can sometimes make them difficult to distinguish. However, based on current observations, we propose that the overall morphology of the carapace, appearance of the setae, features of the frontal margin, surface appendages and fingers coloration of the chelipeds, and characteristics of the first maxilliped endopodite serve as compelling identification features. Additionally, the body and corneal coloration of these species when alive may also be considerable diagnostic features. Based on present observations, we have compiled a comparative summary of these diagnostic features for the new species for C. deplanatus , C. quadrilobatus , C. andreossyi , and C. melanodactylus ( Fig. 5 , Tab. 3 ), and we provide updated the key for species within this genus, based on Ho & Ng (2005) .