Munidopsis ryukyuensis Cubelio, Tsuchida & Watanabe, 2007

Dong, Dong, Seid, Charlotte A., Li, Xinzheng & Rouse, Greg W., 2025, Taxonomic revision of two dominant Munidopsis species (Decapoda, Anomura, Munidopsidae) from the cold seeps in the northern South China Sea: new records and complementary descriptions, ZooKeys 1261, pp. 165-188 : 165-188

Munidopsis ryukyuensis Cubelio, Tsuchida & Watanabe, 2007

Figs 2 , 3 , 4 A – D, I – L, O – R, U

Munidopsis ryukyuensis Cubelio, Tsuchida & Watanabe, 2007 c: 5 , figs 3 b, 5 (type locality Hatoma Knoll).

Material examined.

Southern East China Sea – Okinawa Trough, Lion Chimney hydrothermal vent field • 2 males ( 17.1–19.7 mm); 24°51'N, 122°42'E; 1374 m depth; 25 Aug. 2016; collected by television grab, R/V Kexue; GenBank no.: PX 111638 ; MBM 287952 . Northern South China Sea – Haima cold seep field ; 16°44'N, 110°28'E • 2 males ( 16.1–23.8 mm), 1 ovigerous female ( 14.2 mm); 1387 m depth; 8 Oct. 2024; ROV Faxian of R/V Kexue; GenBank nos: PX 111635 – PX 111637 ; MBM 287953 • 2 females ( 16.8–21.4 mm), 1 male (16.0 mm); 1380–1390 m depth; 18–19 May 2018; collected by manned submersible Shenhai Yongshi of R/V Tansuo 1; MBM 287954 .

Description.

Carapace: distinctly longer than broad (excluding rostrum). Frontal margins oblique, antennal spine absent or reduced as blunt process. Lateral margins subparallel or slightly divergent posteriorly; anterolateral spine small or absent; anterior branchial margin with strong spine at end of anterior cervical groove; posterior branchial margin with short elevated ridge at end of posterior cervical groove. Posterior margin straight, with uninterrupted submarginal ridge. Dorsal surface hairless, covered with numerous short, transverse rugae; rugae longer and more strongly elevated on posterior branchial region; cervical groove deep. Gastric region strongly elevated, with pair of low epigastric ridges. Cardiac region with deep, transverse median groove, posterior cardiac region weakly delineated, subtriangular. Rostrum narrow, triangular and horizontal, ~ 0.3 remaining carapace length, 2.3 × longer than broad; carinate dorsally. Pterygostomial flaps with oblique rugae on surface, anterior end rounded.

Sternum: approximately as long as broad, widening posteriorly. Sternite 3 1.9 × broader than long, separated into 2 parts by longitudinal median groove; anterior margin weakly serrated, with median notch. Sternite 4 broader than long; anterior margin ~ 0.5–0.7 width of sternite 3; anterolateral margins oblique; anterior part trapezoidal, with longitudinal median groove. Sternites 5–7 separated by transverse ridges, with shallow groove along midline.

Pleon: tergites smooth and unarmed. Tergites 2–4 each with 2 transverse ridges; anterior ridge in tergites 2–4 uninterrupted, posterior ridge in tergite 3 and 4 relatively low and interrupted. Tergite 5 smooth, without distinct groove and ridge. Tergite 6 with straight posterior median margin, with shallow notches on each side. Telson composed of 10 distinct plates.

Eye: eyestalk immovable. Peduncle short, broader than cornea. Mesial eyespine short, anterolaterally directed, reaching to proximal ~ 0.3–0.4 of rostrum. Lateral eyespine absent. Blunt distomesial process present on ventral surface of peduncle. Cornea globular.

Antennule: article 1 longer than broad; distal margin with strong ventrolateral spine and small dorsolateral spine; lateral surface slightly inflated, rugose, with oblique shallow groove anteriorly; mesial margin straight.

Antenna: peduncle nearly reaching level of proximal 0.3 of rostrum. Article 1 with strong distomesial spine exceeding distal margin of article 2 and small distolateral spine. Article 2 with small distolateral spine. Article 3 subrectangular, unarmed. Article 4 short.

Mxp 3: ischium shorter than merus, unarmed; crista dentata well-developed. Merus with extensor margin convex, armed with small but distinct disto-extensor spine; flexor margin with 3 small teeth; lateral surface with few rugae. Carpus unarmed. Propodus short, flexor margin gently convex distally.

P 1: subequal, moderately short, ~ 1.7 × longer than pcl, with short rugae bearing sparse fine setae (those much denser and thicker on larger individuals) on surfaces and margins. Ischium ~ 0.6 merus length, dorsodistal margin with short spine; ventrodistal margin produced, with 1–3 small subterminal spines sometimes reduced as tubercles. Merus ~ 0.6 pcl, distally with strong dorsomesial and ventromesial spines and small ventrolateral spine; dorsal surface with small subterminal spine followed by 1 or 2 spines along midline; ventromesial margin unarmed or with 1–3 additional spines on median part. Carpus half of merus length, slightly longer than broad, with strong dorsomesial spine; dorsal surface mesially with longitudinal, rugose ridge (ridge armed with spine on larger individuals). Chela compressed dorsoventrally, palm ~ 1 / 2 of merus length, 1.2 × longer than broad; lateral and mesial margins rugose; fixed finger rugose on lateral margin. Dactylus 1.2 × palm length; occlusal margins weakly crenulate and distally spooned, with blunt process on proximal 1 / 3.

P 2–4: surfaces and extensor margins with scale-like rugae and sparse setae on (those much thicker on larger individuals). P 2 ~ 1.8 pcl, reaching base of P 1 dactylus. Meri subequal in width and decreasing in length posteriorly, P 2 merus 0.7 pcl and 4.4 × longer than broad, P 3 and P 4 meri ~ 0.9 and 0.8 P 2 merus length, respectively; extensor margin with short distal spine and tubercle-like proximal spines; flexor margin rugose, with blunt distal spine. Carpi subequal in length on P 2–4, ~ 0.4 P 2 merus length; extensor margin ridged, with small distal spine; lateral surface with low, submarginal ridge along extensor margin ending in short distal spine (P 2) or unarmed (P 3 and P 4). Propodi subequal in length and width on P 2–4, 0.9 P 2 merus length, and 6.8 × longer than broad; extensor margin unarmed; flexor margin with pair of small distal corneous spines only. Dactyli ~ 1 / 2 of propodus length; extensor margin proximally straight; distal claw strongly curving; flexor margin straight, with 17–19 movable corneous spines on nearly entire length, each spine of distal 1 / 2 margin present on low triangular tooth, ultimate spine closer to penultimate spine than to distal claw.

P 1 –4 without epipods.

Coloration.

Entirely whitish.

Habitat.

Chemosynthetic environments: cold seep and hydrothermal vent.

Distribution.

Northeastern South China Sea and Okinawa Trough, southern East China Sea; depth 1374–1487 m.

Genetics.

The genetic distance between M. ryukyuensis and M. lauensis is 13.1 % – 13.8 %. The genetic distances between specimens of M. ryukyuensis from the Okinawa Trough and the Haima field are 0.5 % – 0.6 % (Table 2 ).

Remarks.

Munidopsis ryukyuensis is morphologically very similar to M. lauensis in almost every aspect. Cubelio et al. (2007 c) noted that M. ryukyuensis differs from M. lauensis in having a broadly triangular and straight rostrum, the mesial eyespine more than half the length of the rostrum and directed almost laterally, and a strongly curved distal claw of the P 2–4 dactyli. We compared specimens of M. ryukyuensis with the material of M. lauensis from the Manus Basin and found that the these characters are variable in both species: the length / width ratio of the rostrum in both species ranges from 1.8–2.3, the mesial eyespine only reaches the proximal 0.3–0.4 of rostrum (Fig. 4 A – H ), and the distal claw of the P 2–4 dactyli is strongly curved in most specimens of both species (Fig. 4 U – W ). Likewise, in the carapace, the antennal spines (situated above the antennal peduncle) and the anterolateral spines vary from acute spines to blunt projection (Fig. 4 A – H ). In both species, the chelipeds and meri of P 2–4 are usually more spinose in males than in females; the males have a row of spines on the ventromesial margin of the P 1 merus and a stout distal spine or tubercle on the extensor margin of the P 2 merus, whereas these spines are reduced in the females. Larger individuals occasionally bear long setae on the surfaces of P 1 –4. These variations make the recognition of the two species challenging. Nevertheless, we found some minor but consistent differences based on the specimens examined. In M. ryukyuensis , the anterior margin of the sternite 4 is relatively broad, 0.5–0.7 width of the sternite 3 (Fig. 4 I – L ), and the posteromedian margin of the pleonal tergite 6 is relatively straight with shallow lateral notches (Fig. 4 O – R ). In contrast, M. lauensis usually has a narrower sternite 4 with the anterior margin 0.4–0.5 width of the sternite 3 (Fig. 4 M, N ), and the posteromedian margin of the pleonal tergite 6 is rather weakly concave (Fig. 4 S, T ). Baba and de Saint Laurent (1992) also described a narrow sternite 4 in the type material of M. lauensis , consistent with our specimens, but illustrated a straight posteromedian margin of pleonal tergite 6, suggesting that this character should be re-examined with more specimens. Notably, M. lauensis also occurs at the Site F cold seep field ( Lin et al. 2013; Dong and Li 2015). Our observations reveal that the Site F specimens possess a broad sternite 4, resembling that of M. ryukyuensis . However, the genetic distance based on the COI gene indicates that M. lauensis in Site F is genetically consistent with populations from the Manus Basin and Lau Basin, whereas the nuclear gene genes suggest a more complex relationship (unpublished). We therefore temporarily assign the Site F specimens to M. lauensis .

Figures

Figure 2. Munidopsis ryukyuensis Cubelio, Tsuchida & Watanabe, 2007, MBM 287953, Haima cold seep, male, pcl 16.1 mm. A. Carapace and pleonal tergites 1–3, dorsal view; B. Carapace and left pterygostomian flap, lateral view; C. Sternal plastron, ventral view; D. Telson; E. Right antennular and antennal peduncles, ventral view; F. Left Mxp 3, ventral view. Scale bars: 1.0 mm.

Figure 3. Munidopsis ryukyuensis Cubelio, Tsuchida & Watanabe, 2007, MBM 287953, Haima cold seep, male, pcl 16.1 mm. A. Right P 1, setae illustrated only on mesial margin of merus, dorsal view; B. Right P 1, ventral view; C. Left P 1 merus, ventral view; D. Left P 2, setae illustrated, lateral view; E. Left P 3, lateral view; F. Left P 4, lateral view; G. Dactylus of left P 2, lateral view. Scale bars: 1.0 mm.

Figure 4. Munidopsis ryukyuensis Cubelio, Tsuchida & Watanabe: (A, B, I, J, O, P, U) Lion Chimney hydrothermal vent, MBM 287952; (C, D, K, L, Q, R) Haima cold seep, MBM 287953. Munidopsis lauensis Baba & de Saint Laurent, 1992: (E – H, M, N, S, T, V, W) Manus Basin, M 041. A – H. Anterior part of carapace, epigastric spines omitted, dorsal view; I – N. Sternite 3 and 4, ventral view; O – T. Posterior margin of pleonal tergite 6; U – W. Dactylus of P 2, lateral view.