Diyutamon cereum, Huang, Chao, Shih, Hsi-Te & Ng, Peter K. L., 2017

Huang, Chao, Shih, Hsi-Te & Ng, Peter K. L., 2017, A new genus and new species of Potamidea (Crustacea: Decapoda: Brachyura: Potamoidae), the first stygomorphic cave crab known from China and East Asia, Zootaxa 4232 (1), pp. 71-84 : 73-76

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Diyutamon cereum

n. gen.

Diyutamon cereum n. gen., n. sp.

( Figs. 1 View FIGURE 1 A, 2–4)

Material examined. Holotype: male (32.3 × 23.9 mm) ( SYSBM 001551 ), Anlong , Guizhou, China, subterranean stream inside a cave at the bottom of sinkhole, coll. C. Huang, May 2016 . Paratypes: 1 female (allotype) (27.3 × 20.4 mm) ( SYSBM 001552 ), same data as holotype ; 5 males (30.6 × 22.6 mm, 30.4 × 21.7 mm, 28.5 × 21.5 mm, 23.5 × 17.2 mm, 17.6 × 13.2 mm), 4 females (25.8 × 18.3 mm, 26.5 × 19.2 mm, 21.3 × 15.8 mm, 18.6 × 14.2 mm) ( SYSBM), same data as holotype .

Description of male. Carapace transversely subquadrate; dorsal surface gently convex transversely, longitudinally, generally smooth, weakly pitted, appearing almost flat in frontal view; regions barely indicated ( Figs. 2 View FIGURE 2 A, B, 3A, 7A). Frontal margin not distinctly protruding anteriorly, very broad, deflexed, margin divided into 2 broad, gently convex lobes on dorsal view; margin lined with closely packed, low rounded granules; confluent with supraorbital margin ( Figs. 2 View FIGURE 2 A, 3A, 7A). Epigastric cristae very low, almost undiscernible, area marked by distinct transverse rugosities ( Figs. 2 View FIGURE 2 A, B, 3A–C, 7A). Postorbital cristae undiscernible ( Figs. 2 View FIGURE 2 A, B, 3A–C, 7A). Branchial regions gently swollen ( Figs. 2 View FIGURE 2 A, B, 3A–C, 7A). Cervical groove very shallow, barely visible; H-shaped gastro-cardiac groove shallow but visible ( Figs. 2 View FIGURE 2 A, 3A, 7A). External orbital angle distinct, very low, broadly triangular, lined with spinules, separated from supraorbital margin by small spine, just discernible from rest of anterolateral margin, separated by broad shallow cleft ( Figs. 2 View FIGURE 2 A, B, 3A–D, 7A). Anterolateral margin convex, cristate, not clearly dentate or lobate, with approximately 20 low spines, sharp granules, some in clumps ( Figs. 2 View FIGURE 2 A, B, 3A, C, 7A). Posterolateral margin uneven, gently sinuous, surface lined with oblique striae; strongly converging towards gently concave posterior carapace margin ( Figs. 2 View FIGURE 2 A, 3A, 7A). Orbits large, clearly demarcated; supraorbital, infraorbital margins cristate, lined with numerous low granules ( Figs. 2 View FIGURE 2 B, 3B, D). Eye mobile, greatly reduced, reaching mesial third of orbit width; peduncle short, stout; cornea small, without pigmentation ( Figs. 2 View FIGURE 2 B, 3B, D). Suborbital region smooth; pterygostomial region covered with distinct granules; subhepatic, subbranchial regions smooth or with low striae ( Figs. 2 View FIGURE 2 B, 3B, D). Antennules relatively short, folding transversely into rectangular fossae ( Figs. 2 View FIGURE 2 B, 3B, D). Antennae very short; first (urinary) article round, prominent; basal article subquadrate, mobile, lodged in orbital hiatus; remaining articles very short, just touching basal part of ocular peduncle ( Figs. 2 View FIGURE 2 B, 3B, D). Epistome longitudinally narrow; posterior margin with median lobe broadly triangular, lateral margins unevenly sinuous, separated by distinct clefts ( Figs. 2 View FIGURE 2 B, 3B, D).

Third maxilliped with merus subtrapezoidal, with submarginal depression, about 1.2 times as broad as long; ischium rectangular, about 1.5 times as long as broad, with shallow, broad median sulcus; exopod slender, reaching to proximal third of merus, flagellum distinct, reaching lateral three-fifths or almost to width of merus ( Figs. 2 View FIGURE 2 B, 3B, D, 5D).

Chelipeds unequal ( Figs. 2 View FIGURE 2 A, 7A). Basis-ischium fused, separated by clear suture; ventral margin with row of low granules ( Figs. 2 View FIGURE 2 B, 4C, D). Merus cross-section trigonal; dorsal margin lined with low, uneven rounded granules; ventro-inner margin lined with relatively sharper granules, subdistal part with short spines; surfaces otherwise smooth, pitted ( Fig. 2 View FIGURE 2 B). Carpus subovate; surfaces pitted; outer surface with low sharp granules on distal part; inner surface with 1 or 2 prominent sharp spines at inner-distal angle, with several sharp granules basal to these ( Figs. 2 View FIGURE 2 A, 4B). Palm of major chela about 1.5 times as long as high; fingers subequal in length to palm; dactylus, pollex subequal in length, dactylus gently curved inwards; cutting margin of dactylus lined with denticles except for basal molariform tooth; cutting margin of pollex lined with denticles along distal half, proximal part with prominent molariform tooth with gently concave occlusal surface ( Fig. 4 View FIGURE 4 C); fingers forming small gap when closed ( Figs. 2 View FIGURE 2 A, 4B, C, 7C). Palm of smaller chela slender; fingers longer than palm; fingers slender, cutting edges evenly lined with denticles ( Figs. 2 View FIGURE 2 A, 4D, 7D).

Ambulatory legs very long, slender, surfaces smooth or pitted; second leg longest ( Figs. 2 View FIGURE 2 A, 4A, 7A). Merus laterally flattened; dorsal margin subcristate, margin uneven but entire, subdistal angle low, without spine or tooth ( Figs. 2 View FIGURE 2 A, 4A, 7A). Carpus smooth, without distinct ridges ( Figs. 2 View FIGURE 2 A, 4A, 7A). Propodus elongated, ventral margin of that of fourth leg distinctly serrate ( Figs. 2 View FIGURE 2 A, 4A, 7A). Dactylus elongated, margins with short pectinate spines, those of second, third legs longest ( Figs. 2 View FIGURE 2 A, 4A, 7A). Fourth leg with propodus about 4.4 times as long as board, approximately same length as dactylus ( Figs. 2 View FIGURE 2 A, 4A, 7A).

Thoracic sternum almost smooth, pitted; sternites 1, 2 completely fused to form subtriangular structure, separated from sternite 3 by relatively shallow but distinct suture; sternites 3, 4 completely fused without trace of median suture; male sterno-abdominal cavity reaching to imaginary line joining posterior ends of coxae of cheliped; tubercle of male abdominal locking mechanism peg-like, on posterior third of sternite 5; sternites 4/5, 5/6 medially interrupted; median longitudinal groove between sternites 7, 8 deep, on sternite 7 extending along posterior three-quarters ( Figs. 2 View FIGURE 2 C, D, 3E, F, 6C). Part of sternite 8 clearly visible when abdomen closed ( Figs. 2 View FIGURE 2 C, 3E, F). Penis on condyle of coxa of fourth ambulatory leg.

Male abdomen narrowly triangular; all somites, telson freely articulating; somites 1–3 very broad, reaching to episternite 7, bases of coxae of fourth ambulatory legs; somites 3–6 progressively broader longitudinally; somite 6 about 2.1 times as board as long; telson about 1.6 times as board as long with a rounded tip, lateral margins of telson almost straight ( Figs. 2 View FIGURE 2 C, 3E, F).

G1 generally slender, terminal segment slightly curved anteriorly, tapered ( Figs. 5 View FIGURE 5 A, C, 6A, B); tip of terminal segment barely reaches tubercle lock of abdominal locking structure in situ ( Fig. 2 View FIGURE 2 D); subterminal segment about 2.7 times as long as terminal segment, lateral, mesial margins of distal part of subterminal segment almost parallel, ( Figs. 5 View FIGURE 5 A, 6A). Basal segment of G2 subrectangular; G2 basal segment about 2.6 times length of flagelliform distal segment ( Fig. 5 View FIGURE 5 B).

Variation. The male specimens on hand show no substantial variation in the key characters of the species discussed above.

Females. Carapace similar to male ( Fig. 7 View FIGURE 7 A). Abdomen broadly ovate, covering median parts of thoracic sternum ( Fig. 7 View FIGURE 7 B). Chelipeds unequal as male, but chelae not as strongly inflated ( Fig. 7 View FIGURE 7 C, D); pollex of major chela with relatively smaller basal molariform tooth on cutting edge ( Fig. 7 View FIGURE 7 C). Vulva relatively large, ovate, on anterior half of sternite 5, edge of vulvae on suture between sternites 5, 6, without operculum ( Fig. 7 View FIGURE 7 E).

Etymology. The species name, from the Latin cereum for wax-like referring to the white coloration and smooth surfaces of this species.

Color. Generally white to cream all over ( Fig. 1 View FIGURE 1 A).

Ecology. The new species inhabits subterranean streams. During the time of collection, the rainy season had started and the water in the stream was murky and fast flowing, making observation difficult. Crabs were nevertheless observed to intermittently appear near the water edge, waving their chelae and legs as if foraging for food. It is unclear whether this behavior is normal or induced by the presence of the survey team (e.g., vibrations from the team’s footsteps, water disturbance and lights). Crabs were abundant at the locality and were all found in water, indicating it is primarily an aquatic species. The subterranean stream at the type locality was less than 100 meters from the cave entrance where there was still a hint of light. The stream is nevertheless situated below an approximately 30 m flowstone drop off and can only be safely accessed with the aid of Single-Rope Technique (SRT) for caving. The cave entrance is at the bottom of a large sinkhole about 200 meters deep.

The presence of a molariform tooth on the major chela on both males and females is noteworthy. Such a character is present in molluscivorous gecarcinucid freshwater crab species from Sulawesi, Indonesia ( Chia & Ng 2006; Schubart & Ng 2008). It seems likely that the function of the molariform teeth in Diyutamon cereum n. gen., n. sp. is also for crushing the hard shells of freshwater snails. Though snails could not be observed in the subterranean stream due to the murky water, they were found clinging on the flowstone at the drop off. Further research using methods such as gut content analysis would be needed to further confirm this.