Cheramus cavifrons, Komai, Tomoyuki & Fujiwara, Yoshihiro, 2012

Komai, Tomoyuki & Fujiwara, Yoshihiro, 2012, New records of callianassid ghost shrimp (Crustacea: Decapoda: Axiidea) from reducing environments in Kyushu, southwestern Japan, Zootaxa 3271, pp. 55-67 : 56-63

Cheramus cavifrons n. sp.

( Figs 1–4 )

Callianassa s.l. sp. — Fujiwara et al. 2007: 223.

Material examined. Holotype: ovigerous female (cl 3.7 mm), R/V Natsushima , NT07-09 cruise, ROV Hyper- Dolphin dive No. 689, off Cape Nomamisaki, Kagoshima Prefecture, 31°20.994’N, 129°59.159’E, 226 m, 8 June 2007, scoop sampler, NSMT-Cr 22092 (formerly JAMSTEC No. 072010).

Paratypes: 3 females (cl 2.0– 2.2 mm), R/V Natsushima , NT07-09 cruise, ROV Hyper-Dolphin dive No. 682, similar locality, 31°20.725’N, 129°59.289’E, 226 m, 4 June 2007, scoop sampler, JAMSTEC No. 071987 – 071990; 1 female (cl 2.2 mm), R/V Natsushima , NT04-08 cruise, ROV Hyper-Dolphin dive No. 328, similar locality, 31°20.721’N, 129°59.283’E, 225 m, 27 July 2004, scoop sampler, JAMSTEC No. 0 55524.

Description. Holotype female. Rostrum ( Figs 1 A, 2C)) narrowly triangular, acuminate, directed somewhat downward, reaching midlength of eyestalks. Carapace ( Figs 1 A, 2A, B) about 0.3 length of abdomen; orbital margins distinctly concave; anterolateral projections triangular with subacute tip; excavation inferior to anterolateral projections deep, U-shaped; dorsal oval well defined, smooth, occupying about three-fourths of carapace length; linea thalassinica distinct.

Length ratio of first to sixth abdominal somites measured along midline 1: 2: 1.1: 1.1: 1.1: 1.5. First abdominal somite ( Figs 1 B, 2E) generally narrowing anteriorly in dorsal view, with slight constriction just anterior to midlength; pleuron weakly developed but with clearly defined ventral margin. Second somite ( Figs 1 B, 2E) distinctly longer than other somites; pleuron with posterolateral margin slightly expanded, bearing tuft of long setae directed laterally adjacent to posterolateral margin. Third to fifth pleura ( Figs 1 C–E, 2E) each with patch of soft plumose setae; posterolateral margins slightly expanded; third and fourth pleura further each with short row of long setae near posterolateral margin, fifth pleuron with row of long setae on posterior half of ventrolateral margin. Sixth somite ( Figs 1 F, 2E) 1.1 times longer than wide, subquadrate with nearly parallel lateral margins in dorsal view, lacking ventrolateral projection; lateral margin with shallow notch at about posterior 0.2; posterolateral margin with tuft of moderately long to long setae. Telson ( Fig. 1 F) trapezoidal, slightly narrowing posteriorly, approximately as long as wide, broadest at anterior 0.2; dorsal surface nearly flat, with short, low transverse ridge anteromedially, bearing row of long setae; lateral margin unarmed, 2 pairs of minute spiniform setae at rounded posterolateral angles; posterior margin slightly concave, with minute median tooth and row of thin setae (these setae greatly unequal in length, lateral setae elongate) ( Fig. 4 I).

Eyestalks ( Figs 1 A, 2B, C) flattened distally, tapering to bluntly pointed mesiodistal projection, narrowly separated, slightly overreaching distal margin of first segment of antennular peduncle; dorsal surface closely attaching rostrum, sloping anteriorly; lateral margin sinuous with obtuse angle slightly distal to midlength. Cornea subterminal and lateral, darkly pigmented, its width approximately half of greatest width of eyestalk.

Antennular peduncle ( Figs 1 A, 2B) subequal in length to antennal peduncle; first segment short, hardly visible in dorsal and lateral views; second segment shorter than first segment; third segment about 2.6 times longer than second segment; second and third segments with row of sparse setae ventrally; antennular flagella both slightly longer than peduncle; dorsal flagellum thicker and slightly shorter than ventral flagellum. Antennal peduncle ( Figs 1 A, 2B) with distal two segments subcylindrical; fourth segment about 1.3 times longer than fifth segment; scaphocerite rudimentary, subovate; flagellum about 1.6 times longer than carapace.

Prolongations of cephalothorax lateral to epistome bearing few long setae ( Fig. 2 B).

First maxilliped ( Fig. 2 F) with endopod greatly reduced to small rounded lobe located proximally; endopod slender, tapering to acute tip; exopod elongate subovate; epipod distinctly bilobed. Second maxilliped ( Fig. 2 G) with exopod slightly falling short of distal margin of merus. Third maxilliped ( Fig. 4 A, B) without exopod; ischium-merus broadly subrectangular, operculiform, 1.3 times longer than wide; ischium 1.1 times wider than long, crista dentata consisting of row of small slender spines; merus about 0.6 times as long as ischium, about 1.6 times wider than long, distolateral margin slightly produced in broadly rounded lobe, but unarmed; distomesial margin also unarmed; carpus subovate, subequal on length to merus; propodus 2.0 times longer than high and subequal in length to carpus; dactylus moderately slender, digitiform, about 0.7 times as long as propodus.

First pereopods (chelipeds) greatly unequal and dissimilar. Major cheliped ( Fig. 3 A, B) massive, about 2.3 times longer than carapace (chela and carpus combined 1.4 times longer than carapace). Ischium becoming slightly higher distally in general contour, dorsal margin nearly straight, unarmed; lateral surface gently convex; ventral margin with serration consisting of 8 small acute spines increasing in size distally. Merus about 0.9 times as long as ischium; dorsal margin convex, unarmed; lateral surface forming concavity ventrally to accommodate proximoventral margin of carpus; mesial surface generally flat; ventral margin with moderately large, hook-like, sharply pointed spine proximally and row of small tubercles distal to hook-like spine. Carpus subsemicircular, about 1.3 times higher than long; dorsal and ventral margins sharply carinate, almost smooth; lateral surface smooth, convex; mesial face nearly flat. Palm subrectangular, about 1.2 times longer than wide; lateral surface smooth, convex, with few short setae; mesial surface slightly convex generally, but with shallow depression at base of fixed finger, having row of tufts of short setae or individual setae adjacent to dorsal margin and with row of long setae along ventral margin; dorsal and ventral margins sharply carinate, smooth. Fixed finger about 0.6 times as long as palm, slightly curving distally, terminating in subacute tip; cutting edge with single, subacute tooth slightly distal to midlength; lateral surface convex, scarcely carinate. Dactylus about 0.7 times as long as palm, hooked distally, crossing at midlength of fixed finger, terminating in acute tip; lateral surface with rows tufts of long setae dorsally and ventrally; dorsal surface rounded; cutting edge sinuous, without conspicuous teeth; mesial surface convex, also with tufts of long setae.

Minor cheliped ( Fig. 3 C, D) moderately stout. Ischium slightly curving; dorsal margin unarmed; ventral margin concave, with serration consisting of 6 small acute spines. Merus 0.9 times as long as ischium; dorsal margin convex, unarmed; ventral margin nearly straight, unarmed. Carpus cup-shaped, widened distally, subequal in length to merus and about 1.3 times longer than wide; dorsal margin nearly straight, rounded; ventral margins arcuate, sharply carinate. Palm subrectangular, about 1.3 times longer than wide; dorsal margin nearly straight, bluntly carinate, few short to long setae; ventral margin sharply carinate, with row of tufts of setae; lateral and mesial surfaces weakly convex, smooth. Fixed finger about 0.7 times as long as palm, slightly curving, terminating in acute tip; cutting edge with small acute, forwardly directed tooth arising slightly distal to midlength. Dactylus damaged at tip, crossing at midlength of fixed finger; dorsal margin rounded, almost glabrous; cutting edge unarmed.

Second pereopod ( Fig. 4 C) with ischium short, ventrodistal angle somewhat produced; merus with sinuous ventral margin, dorsal margin almost straight, but distal part sloping down distally; carpus cup-shaped, about 1.6 times longer than high; chela triangular; palm much higher than long, proximoventral margin rounded; cutting edges of fingers bordered by thin corneous ridge; dactylus 1.6 times longer than palm. Third pereopod ( Fig. 4 D) with carpus cup-shaped, about 1.6 times longer than high; propodus suboval, about 1.1 times higher than long (length measured along dorsal margin), lateral face slightly elevated on midline, ventral margin strongly convex, with slender spiniform seta subterminally; dactylus nearly straight, distinctly shorter than propodus. Fourth pereopod ( Figs 2 E, 4E) extended posterodorsally; coxa somewhat flattened ventrally, immovable ( Fig. 2 D); basis and ischium partially fused, extended posterolateraly; merus longer than ischium, distal margin deeply excavated; carpus slightly widened distally; propodus strongly compressed, slightly longer than carpus, with dense grooming setae on ventral margin; dactylus slightly curving, about half-length of propodus. Fifth pereopod ( Fig. 4 F, G) with chela distinctly longer than carpus, slightly curving; dactylus about 0.3 times as long as palm.

Branchial formula including two arthrobranchs on each third maxilliped to fourth pereopods; pleurobranchs absent.

First pleopod ( Fig. 2 E) uniramous, two-segmented; distal segment with low convexity bearing tuft of setae at midlength of mesial margin. Second pleopod biramous; endopod slightly longer than exopod, both slightly longer than protopod. Third to fifth pleopods ( Fig. 2 H) biramous, rami moderately broad; appendix internae ( Fig. 2 H, I) slender, digitiform, fully exceeding beyond mesial margin of endopod.

Uropod ( Figs 1 F, 4H) with both endopod and exopod subovate, both distinctly overreaching posterior margin of telson. Endopod about 1.6 times longer than wide; dorsal surface faintly elevated in midline and shallowly concave in mesial half; lateral margin nearly straight, with minute spinule somewhat posterior to midlength; mesial margin broadly convex; fringe of thin setae on distal to mesial margin. Exopod about 1.5 times longer than wide, exceeding beyond endopod; dorsal surface faintly elevated in midline, with short, sharp proximomedian carina; dorsal plate with distal row of spiniform setae distinctly separated from setal row of posterior margin; setal row on posterior margin consisting of stout and thin setae (stout setae located dorsally), thin setae extending to mesial margin.

Paratypes. Generally similar to holotype. Rostrum reaching proximal one-third to half of eyestalks. Ventral serration of meri of major and minor cheliped consisting of 4 or 5 spines or 4 spines, respectively. First and second pleopods very small and naked, clearly suggesting immaturity of specimens.

Distribution. At present known only from the type locality, off Cape Nomamisaki, Kagoshima Prefecture, Kyushu, Japan, 226 m.

Remarks. The genus level classification of the subfamily Callianassinae sensu Manning & Felder (1991) has been subject of debate. Manning & Felder (1991) proposed three new genera for species previously assigned to Callianassa and reinstated Trypaea Dana, 1852 on the basis of reexamination of species occurring in America; Cheramus Bate, 1888 and Scallasis Bate, 1888 were assigned to the separate subfamily Cheraminae Manning & Felder, 1991. Many subsequent taxonomic and phylogenetic works have followed the trend of splitting the genus Callianassa s.l. ( Manning & Felder 1992, Rodrigues & Manning 1992, Poore 1994, Heard & Manning 1998, 2000, Manning & Tamaki 1998, Tudge et al. 2000, Ngoc-Ho, 2003, Lin et al. 2007, Komai & Tachikawa 2008). On the other hand, Sakai (1999, 2005) maintained species once assigned to Biffarius Manning & Felder, 1991 , Cheramus Bate, 1888 , Gilvossius Manning & Felder, 1992 , Necallianassa Heard & Manning, 1998, Neotrypaea Manning & Felder, 1991 , Nihonotrypaea Manning & Tamaki, 1998 , Notiax Manning & Felder, 1991 , Poti Rodrigues & Manning, 1992 , Pseudobiffarius Heard & Manning, 2000, Scallasis Bate, 1888 , and Trypaea Dana, 1852 in Callianassa , but later he ( Sakai 2011) finally recognized six genera, i.e., Callianassa sensu stricto, Cheramoides Sakai, 2011 , Cheramus , Gilvossius , Notiax and Trypaea . Three genera proposed for species placed in Callianassa s.l., since Sakai (1999, 2005), i.e., Paratrypaea Komai & Tachikawa, 2008 , Pestarella Ngoc-Ho, 2003, and Rayllianassa Komai & Tachikawa, 2008 , were placed in the synonymy of Gilvossius ( Paratrypaea and Pestarella ) and Notiax (Rayllianassa) . Sakai’s (2011) classification is mainly based only on characters of the first and second pleopods in males, and does not fully reflect morphological diversity seen in species in this group of genera. We still prefer to follow the scheme of Manning & Felder (1991), Poore (1994) and Tudge et al. (2000) in recognizing these various genera as valid taxa because it reflects the phylogenetic structure and the morphological diversity in species previously referred to Callianassa s.l.

Following Poore’s (1994) key, the present new species is assigned to Cheramus by the following characters: (1) cornea of eye subterminal and lateral; (2) no exopod on third maxilliped; (3) appendices internae on third to fifth pleopods slender, digitiform. The present new species is actually very similar to C. spinophthalmus (Sakai, 1970) known from the southern part of the Sea of Japan. Shared characters include: (1) rostrum narrowly triangular, acuminate; (2) sixth abdominal somite slightly longer than wide; telson trapezoidal, slightly narrowing posteriorly, bearing small median tooth and two pairs of minute spiniform setae at posterolateral angles; (3) ischium-merus of third maxilliped broad, operculiform; (4) major cheliped massive, carpus higher than long; (5) ischia of both chelipeds each with distinct serration; and (6) merus of major cheliped at least with prominent tooth proximally. Combination of these characters clearly separate these two species from other known members assigned to Cheramus by Tudge et al. (2000). Cheramus cavifrons n. sp. can be easily distinguished from C. spinophthalmus by the following characters (cf. Komai et al. 2002): (1) orbital margin distinctly concave in the new species, rather than sloping posteriorly in C. spinophthalmu s; (2) distomesial process of eyestalk blunt in the new species, whereas sharply pointed, spiniform in C. spinophthalmus ; (3) antennular peduncle subequal in length to antennal peduncle in the new species, but longer than the latter in C. spinophthalmus ; (4) ischium-merus of third maxilliped relatively higher in the new species than in C. spinophthalmus (about 1.3 times longer than high versus about 2.0 times as long); (5) merus of major cheliped bearing one prominent ventral spine in the new species, rather than two in C. spinophthalmus ; (6) merus of minor cheliped unarmed in the new species, but armed with tiny spine on ventral margin in C. spinophthalmus ; (7) propodus of third pereopod distinctly broader than carpus in the new species, nearly as wide as in C. spinophthalmus ; (8) uropodal rami less elongate in the new species than in C.

spinophthalmus ; (9) uropodal endopod bearing a minute spine on lateral margin distal to midlength in the new species, whereas unarmed in C. spinophthalmus .

Callianassa aqabaensis Dworschak, 2003 , known from the Gulf of Aqaba, northern Red Sea, is also very similar to the present new species, particularly in the shape of the rostrum, the shape and armature of the telson, and the general shape and armature of the chelipeds ( Dworschak 2003). Dworschak (2003) noted difficulty in the decision of the generic position of his new species, and thus he placed it in Callianassa s.l. Sakai (2011) assigned Callianassa aqabaensis to Cheramus mainly on the basis of the development of the male first and second pleopods, although Dworschak (2003) clarified the intraspecific variation in the structure of the male second pleopod. Nevertheless, Callianassa aqabaensis differs from other species assigned to Cheramus by Tudge et al. (2000) in the structure of the appendices internae of the third to fifth pleopods. As mentioned above, in species of Cheramus , including the present new species, the appendices internae are fully produced and digitiform in shape. In Callianassa aqabaensis , however, the appendices internae are stubby and partially embedded in the mesial margin of the endopod, though they are clearly projecting beyond the margins ( Dworschak 2003). In addition, the following characters distinguish the present new species from Callianassa aqabaensis : (1) ventral margin of propodus of third pereopod generally convex with one subterminal spine in the new species, while undulate and unarmed in Callianassa aqabaensis ; (2) uropodal endopod bearing one spinule on lateral margin and unarmed on dorsal surface in the new species, whereas unarmed on lateral margin but bearing two spiniform setae on dorsal surface posteriorly in Callianassa aqabaensis .

The new species also resembles Callianassa acutirostella Sakai, 1988 [assigned to Trypaea by Sakai (2011)], described on the basis of the damaged holotype from North West Shelf, Western Australia ( Sakai 1988) and subsequently recorded from the Arafura Sea ( Sakai 2005), in the shape of the rostrum, telson, eyestalks, and third maxilliped. However, the new species can be distinguished from the latter species by the following characters: (1) the orbital margin is evenly concave in Cheramus cavifrons , whereas it is mostly oblique in Callianassa acutirostella ; (2) the ventral margin of the first abdominal pleuron is deeply notched in Callianassa acutirostella , but such a deep notch is not seen in Cheramus cavifrons ; (3) the antennal scaphocerite is rounded in Cheramus cavifrons , rather than acuminate in Callianassa acutirostella ; (4) the chelipeds are greatly unequal even in females in the new species, rather than subequal in Callianassa acutirostella ; (5) the merus of the minor cheliped is unarmed on the ventral margin in Charamus cavifrons , but there is a conspicuous spine on that margin in Callianassa acutirostella .

Recent studies have demonstrated that chemosynthetic assemblages occur around the whale carcasses, with substantial similarities to hydrothermal vent or hydrocarbon seep communities (e.g., Smith et al. 1989; Bennett 1994). Since the first discovery in 1987 from the Santa Catalina Basin , California, at a depth of 1240 m ( Smith et al. 1989), many whale-fall communities have been reported, including modern and fossil assemblages ( Smith & Baco 2003). Fujiwara et al. (2007) reported on invertebrate community from sperm whale-fall ecosystems, based on mass sinking of whale carcasses at shelf depths in the northwest Pacific. Thirty-six species of the Decapoda were reported, including a number of unidentified species from various taxa. The present new species is the first representative of the Callianassidae to be associated with the whale-fall community.

Etymology. The specific name is a combination of the Latin, cavus (concave) and frons (front), in reference to the characteristic deeply concave orbital margins of the carapace of this new species. Used as a noun in apposition.

Figures

FIGURE 1. Cheramus cavifrons n. sp., holotype, ovigerous female (cl 3.7 mm), NSMT-Cr 22092. Body parts in dorsal view. A, carapace and cephalic appendages; B, first and second abdominal somites; C, third abdominal somite; D, fourth abdominal somite; E, fifth abdominal somite; F, sixth abdominal somite, telson and right uropod (setae partially omitted). Scale bar: 1 mm.

FIGURE 2. Cheramus cavifrons n. sp., holotype, ovigerous female (cl 3.7 mm), NSMT-Cr 22092. A, carapace and cephalic appendages, lateral view; B, anterior part of carapace and cephalic appendages, lateral view; C, rostrum and eyes, dorsal view; D, thoracic shield and coxae of fourth pereopods, ventral view; E, posterior part of cephalothorax and abdomen (left third pleopod removed), lateral view, showing direction of fourth pereopod; F, left first maxilliped, outer view (distal endite broken off); G, left second maxilliped; H, left third pleopod, posterior view (setae omitted); I, same, appendix interna. Scale bars: 1 mm for A – E; 0.5 mm for F – I.

FIGURE 3. Cheramus cavifrons n. sp., holotype, ovigerous female (cl 3.7 mm), NSMT-Cr 22092. A, left major cheliped, lateral view; B, same, mesial view; C, right minor cheliped, lateral view; D, same, mesial view. Scale bar: 1 mm.

FIGURE 4. Cheramus cavifrons n. sp., holotype, ovigerous female (cl 3.7 mm), NSMT-Cr 22092. A, left third maxilliped, lateral view; B, same mesial view; C, left second pereopod, lateral view; D, left third pereopod, lateral view; E, left fourth pereopod, outer view; F, left fifth pereopod lateral view; G, same, distal 2 segments, extensor view; H, left uropod, perpendicular dorsal view; I, posterior margin of telson. Scale bars: 0.5 mm.