Scallasis inermis, Komai & Poore & Fujita, 2020

Komai, Tomoyuki, Poore, Gary C. B. & Fujita, Yoshihisa, 2020, Redescription of the poorly known ghost shrimp species, Scallasis amboinae Bate, 1888, review of the genus, and description of a new species from the Ryukyu Islands, Japan (Decapoda: Axiidea: Callianassidae), Zootaxa 4766 (3), pp. 401-420 : 409-416

Scallasis inermis n. sp.

[New Japanese name: Mame-sunamoguri]

( Figs 4–9 )

http://zoobank.org/ urn:lsid:zoobank.org:act:68C54DF6-7729-4E82-8785-87AAD5CC1A14

Type material. Holotype: Japan, Okinawa Island, Ohura Bay, Umi-saboten diving point (stn D1), 3–20 m, 18 June 2009, SCUBA diving, yabby pump, male (cl 3.7 mm; photographed), coll. Y. Fujita and T. Naruse, CBM-ZC 14140 .

Paratypes: Japan, Okinawa Island, Ohura B ay, Tettou-oki diving point, 23 m, muddy sand, 9 November 2013, SCUBA diving, yabby pump, coll. Y. Fujita, 1 male (cl 3.7 mm; photographed), RUMF-ZC-04571; same data, 3 males (cl 3.6, 3.6, 3.7 mm), 1 ovigerous female (cl 3.6 mm), CBM-ZC 14141–14144.

Diagnosis. Pleomere 6 1.1 times as long as wide. Telson 1 times as long as wide; greatest width 1.5 times posterior width. Maxilliped 3 ischium-merus 2.6 times as long as wide; merus free distal margin oblique. Female major cheliped ischium lower margin with 1–5 small spines; merus lower margin unarmed; palm 0.7 times as long as carpus. Male major cheliped merus lower margin with bifid proximal spine; palm 1.4–1.6 as long as carpus; dactylus with 2 notches on cutting margin. Minor cheliped ischium lower margin with 2 or 3 small spines; merus lower margin unarmed. Male pleopod 1 of 1 minute article, or of 2 small articles. Male pleopod 2 biramous. Uropodal endopod anterior margin with small spine about two-thirds along; dorsal surface with 1–3 minute proximal spiniform setae; exopod anterior margin with small spiniform seta about three-quarters along.

Description. Rostrum narrowly triangular, spiniform, directed forward, with slight ventral carina, falling slightly short of distal ends of eyestalks. Carapace subequal in length to pleomeres 1 and 2 combined; orbital margins concave; anterolateral projections triangular with subacute tip; excavation inferior to anterolateral projections deep, U-shaped; deep cervical groove across 0.8 length of carapace, without postrostral transverse groove; linea thalassinica distinct, extending to posterolateral margin of carapace.

Thoracic sternite 7 subpentagonal with slightly projecting anteromedian margin; ventral surface with shallow median groove.

Pleomere 1 narrowing anteriorly in dorsal view; pleuron with poorly defined ventral margin. Pleomere 2 pleuron posterolateral margin slightly expanded, without tuft of setae. Pleomeres 3–5 pleura each with patch of plumose setae; posterolateral margins slightly expanded; pleuron 3 with tuft of long setae adjacent to posterolateral margin, pleura 4 and 5 with tuft or longitudinal row of setae on posterior parts, respectively. Pleomere 6 about 1.1 times as long as wide, subquadrate, slightly narrowing posteriorly, lacking ventrolateral projection; lateral margin with shallow notch at about posterior 0.2. Telson trapezoidal, narrowing posteriorly, approximately as long as wide, broadest at anterior 0.2; greatest width 1.5 times posterior width; dorsal surface medially with some spiniform setae in short transverse row anterior to midlength; lateral margin unarmed; 2 pairs of small spiniform setae at rounded posterolateral angles; posterior margin shallowly concave medially, unarmed or armed with minute median spine.

Eyestalks flattened distally, tapering to acutely or subacutely pointed distal projection, contiguous, slightly overreaching distal margin of article 1 of antennular peduncle; dorsal surface narrowly separated from rostrum, sloping anteriorly; lateral margin convex. Cornea subterminal, submedial, darkly pigmented, wider than half width of eyestalk.

Antennular peduncle slightly longer than or subequal to antennal peduncle; article 1 short, hardly visible in dorsal and lateral views; article 2 shorter than article 1; article 3 2.7 times as long as article 2; articles 2 and 3 with longitudinal ventral row of sparse long setae; antennular flagella both slightly shorter than peduncle; dorsal flagellum thicker and slightly shorter than ventral flagellum.

Antennal peduncle with distal two articles subcylindrical; article 4 1.9 times as long as article 5; scaphocerite minute, spiniform.

Maxilliped 1 with exopod moderately broad; epipod unequally bilobed. Maxilliped 2 moderately slender; exopod short, not reaching midlength of ischium-merus. Maxilliped 3 without exopod; ischium-merus narrowly subrectangular, non-operculiform, 2.8 times as long as wide; ischium tapering, 1.8 times as long as wide, crista dentata consisting of row of 11 strong erect spines; merus about 0.5 times as long as ischium measured along dorsal margin, about 1.2 times as wide as long, wider than merus, distolateral margin obliquely truncate, unarmed; carpus cupshaped, slightly shorter than merus; propodus narrowing distally, 2.0 times as long as wide, longer than carpus; dactylus slender, digitiform, about 0.7 times as long as propodus.

Pereopods 1 (chelipeds) greatly unequal, dissimilar. Male major cheliped massive, carpus-chela combined length twice carapace length. Ischium becoming rather abruptly wider distally in general contour, upper margin strongly sinuous, unarmed; lateral surface gently convex; lower margin with row of 1–5 small spines. Merus as long as ischium; upper margin slightly convex, unarmed; lateral surface forming deep concavity ventral to distinct median ridge to accommodate proximoventral margin of carpus; mesial surface generally flat; lower margin with moderately strong, bifid, forwardly directed spine proximally and row of small tubercles distal to proximal spine. Carpus subquadrate, 1.0–1.2 times as wide as long; upper and lower margins sharply carinate, smooth, proximal lower margin broadly rounded; lateral surface smooth, convex; mesial surface nearly flat. Chela 2.2. times as long as carpus; palm subquadrate, 1.4–1.6 times as long as carpus, 1.2–1.4 times as long as wide; upper margin sharply carinate, almost glabrous; lateral surface smooth, convex; mesial surface slightly convex, with small shallow depression at base of fixed finger; lower margin sharply carinate, with row of short to moderately long setae extending onto fixed finger; fixed finger 0.5–0.8 times as long as palm, gently curving, terminating in acute tip; occlusal margin smooth, unarmed; lateral surface convex, with tufts of long setae along cutting edge; mesial surface depressed along occlusal margin, with tufts of setae in proximal half. Dactylus 0.5–0.9 times as long as palm, strongly hooked distally, crossing fixed finger, with acute tip; upper surface with blunt ridge laterally and with tufts of long setae; lateral surface with longitudinal row of tufts of long setae along cutting edge; occlusal margin sinuous, unarmed or armed with 2 large, truncate teeth; mesial surface slightly elevated in midline, also with tufts of setae along cutting edge.

Female major cheliped much slender than male major cheliped. Ischium armed with 1 spine on lower margin. Merus with lower margin slightly convex, unarmed. Carpus approximately twice as long as wide, widened distally. Chela 1.5 times as long as carpus. Palm 1.2 times as wide as long; no palmer process or excavation at base of fixed finger; fixed finger nearly straight. Dactylus subequal in length to palm; occlusal margin slightly sinuous, unarmed; setation of fingers less developed than in males.

Minor cheliped similar in male and female, slender; carpus-chela combined length 1.1 carapace length. Ischium slightly curving; upper margin smooth, lower margin with row of 2 or 3 small spines. Merus 0.9 times as long as ischium; upper and lower margins gently convex, unarmed. Carpus wider distally, 1.8 times as long as merus, 3.3 times as long as wide; upper margin nearly straight, rounded; lower margin slightly convex, carinate. Palm slightly widened distally, 1.4 times as long as wide; upper margin nearly straight, bluntly carinate, with row of setae; lower margin sharply carinate, with row of long setae extending onto fixed finger; lateral and mesial surfaces slightly convex, smooth. Fixed finger longer than palm, slightly curving, terminating in acute tip; occlusal margin without conspicuous teeth or spines; lateral surface with row of tufts of setae along cutting edge. Dactylus 1.6 times as long as palm, as long as fixed finger; dorsal margin rounded, with row of long setae; occlusal margin unarmed.

Pereopod 2 ischium short, lower distal angle produced; merus with sinuous lower margin, upper margin almost straight; carpus subtriangular, twice as long as wide; chela subtriangular; palm 1.8 times as wide as upper margin; occlusal margins of fingers bordered by thin corneous ridge; dactylus twice as long as palm. Pereopod 3 carpus subtriangular, 2.2 times as long as wide, lower margin concave, twice as long as wide; propodus suboval with produced lower proximal margin, upper margin 1.4 times width, lateral face obtusely carinate medially, lower margin convex, faintly undulate, with 1 slender spiniform seta subdistally; dactylus nearly straight, about 0.6 times as long as propodus upper margin. Pereopod 4 coxa flattened ventrally, immovable; basis and ischium partially fused, extended posterolaterally; merus longer than ischium; carpus wider distally; propodus compressed, as long as carpus, with dense grooming setae distally on lower margin, scattered stiff setae on outer surface; dactylus slightly curving, 0.3 times as long as propodus. Pereopod 5 moderately slender, with chela subequal in length to carpus, slightly curving; dactylus about 0.3 times as long as palm.

Male pleopod 1 papilla-like or very small, 2-articulated, distal article (ramus) much shorter than proximal article (= peduncle). Female pleopod 1 unequally biramous; peduncle gently arcuate; endopod very small, slender, weakly arcuate; exopod longer than peduncle, with slight shoulder slightly proximal to midlength on inner margin. Male pleopod 2 absent, or very small, terminally bilobed, or distinctly biramous (rami basally fused to peduncle). Female pleopod 2 subequally biramous; peduncle strongly curved outward, inner margin angular; endopod tapering distally, slightly shorter and broader than exopod; exopod slightly curved inward. Pleopods 3–5 biramous, rami narrow; appendices internae slender, rod-like, projecting beyond mesial margin of endopod, bearing coupling hooks on apical margin.

Uropod endopod and exopod subovate, overreaching posterior margin of telson. Endopod about 1.8 times as long as wide; upper surface shallowly concave in mesial half; anterior margin slightly sinuous, usually with small subdistal spine at about distal 0.2 (subdistal spine absent in female specimen); posterodistal margin convex, with fringe of setae. Exopod 1.3 times as long as wide, exceeding endopod by half length; anterior margin convex, bearing 1 small subdistal spiniform seta; posterior margin with about 10 blade-like distal setae, connected to distal margin by rounded corner; upper surface faintly slightly carinate in midline; dorsal plate extending about half distal exopod width, with distal row of stiff setae separated from setal row of distal margin.

Variation. Individual variation observed in the present material is summarized in Table 1 . The major cheliped exhibits strong sexual dimorphism; the male major cheliped is much heavier and more strongly armed than the female major cheliped. The male major cheliped shows also notable individual variation in the shape of the carpus and palm, and the length and armature of the dactylus, as in other callianassid taxa (e.g., Sakai 1969; Komai 2017). The ischium of the major cheliped is armed with two to five spines on the lower margin in males, while only one spine in female.

The posteromedian spine on the telson is absent, or if present, minute.

The development of the pleopods 1 and 2 in males also shows substantial variation not related to the size. In the male specimen (cl 3.7 mm; CBM-ZC 14140), the pleopod 1 is minute but still bi-articulate, whereas in the other four specimens (cl 3.6–3.7 mm; RUMF-ZC 04571, CBM-ZC 14141–14143), it is greatly reduced to a papilla-like bud. Three conditions are recognized in the development of the pleopod 2. In two specimens (cl 3.7 mm, CBM-ZC 14140; cl 3.7 mm, RUMF-ZC 04571), the pleopod 2 is very small but still distinctly bi-ramous (though basal articulation of each ramus is hardly recognizable); in one specimen (cl 3.7 mm; CBM-ZC 14143), it is greatly reduced to a minute, flattened piece with a bilobed terminus; in the two specimens (cl 3.6 mm, CBM-ZC 14141; cl 3.6 mm, CBM-ZC 14142, the pleopod 2 is completely absent, even a trace being not recognised.

Colour in life. Carapace, pleon and tail fan generally semi-translucent, with tint of pink; cornea of eye black; major cheliped pinkish white; minor cheliped and other pereopods translucent with tint of pink ( Fig. 9 ).

Distribution. Ohura Bay, Okinawa Island, Ryukyu Islands, 3– 23 m.

Remarks. Differences between the new species and all other species in Scallasis are discussed below.

Scallasis amboinae is substantially similar to S. inermis n. sp., but the possession of fewer spines on the lower margin of the major cheliped ischium (one to four versus seven), the absence of a spine on the lower margin of the minor cheliped merus (versus spine present) and the possession of few minute spiniform setae on the uropodal endopod dorsal surface (versus one of spiniform setae rather elongate) distinguish the new species from S. amboinae . Scallasis amboinae is known with certainty from Indonesia and Papua New Guinea ( Bate 1888; Poore et al. 2019; this study).

No information on the minor cheliped is available for S. andamaniensis . Nevertheless, the new species seems to differ from the latter species in the presence of a spiniform seta on the outer margin of the uropodal exopod (versus no spiniform seta present: Sakai 2002: fig. 24C). The chelipeds of the new species are strongly sexually dimorphic: the female major cheliped of the new species is substantially different from the cheliped of the female holotype of S. andamaniensis in the much more slender form with fewer ischium lower spines (one small spine versus four strong spines), the absence of the merus lower spine and the proportionally shorter palm being distinctly shorter than the carpus (versus subequal in length to carpus) ( Fig. 6E versus Sakai 2002: fig. 25B–D). Scallasis andamaniensis is so far known only from the Andaman Sea off Thailand, at depth of 65–70 m ( Sakai 2010).

Ngoc-Ho (1991; as Callianassa ) stated that the chelipeds of Scallasis caledonica were slightly sexually dimorphic. Scallasis inermis n. sp. is distinguished from S. caledonica by the following particulars (cf. Ngoc-Ho 1991): lower spines of the cheliped ischia are fewer in S. inermis n. sp. than in S. caledonica (one to five spines versus six to eight spines in the major side; two or three spines versus six spines in the minor side) ( Figs 5A, C , 6A, E, F versus Ngoc-Ho 1991: fig. 2j, k); the spine on the lower margin of the male major cheliped is bifid in the new species ( Figs 5A , 6A ), rather than simple, acuminate in S. caledonica (cf. Ngoc-Ho 1991: 2 j); the female major cheliped is much less stout in S. inermis n. sp. than in S. caledonica ( Fig. 6E versus Ngoc-Ho 1991: fig. 2j); the minor cheliped merus is unarmed on the lower margin in S. inermis n. sp. ( Figs 5C , 6F ), while there is a conspicuous spine in S. caledonica (cf. Ngoc-Ho 1991: fig. 2k); Scallasis caledonica is known only from New Caledonia, at depth of 21 m ( Ngoc-Ho 1991).

None of available specimens of S. contipes was intact ( Sakai 2002); in particular, no information on the chelipeds is available. Nevertheless, S. inermis n. sp. can be differentiated from S. contipes by the proportionally shorter pleomere 6 (1.1 times as long as wide versus 1.3 times as long) with a lateral notch located at the posterior 1/4 (versus located at the midlength) ( Fig. 4E versus Sakai 2002: fig. 35C) and the possession of one small spine on the uropodal endopod outer margin and of one small spiniform seta on the uropodal exopod outer margin (versus uropodal rami unarmed on outer margin; Sakai 2002: 35 C). Sakai (2002) did not mention on or figure spiniform setae on the uropodal endopod dorsal surface, but these could be easily overlooked. Scallasis contipes is known only from the Andaman Sea, at depths of 20–38 m ( Sakai 2002).

Scallasis spinophthalma is characteristic within the genus in having a sub-operculiform ischium-merus of the maxilliped 3 ( Sakai 1970: text-fig. 4a; Komai et al. 2002: 9 F, G). Scallasis inermis n. sp. further differs from S. spinophthalma in the possession of a spiniform seta on the outer margin of the uropodal exopod (versus both unarmed in S. spinophthalma ) ( Fig. 4G versus Fig. 10D ), the absence of spiniform setae on the dorsal midline of the uropodal endopod (versus there are two long spiniform setae on the dorsal midline) ( Fig. 4G versus Fig. 10D ) and the shape and armature of chelipeds. The ischium lower margin of either cheliped is armed with one to five small spines in S. inermis n. sp. ( Figs 5A, C , 6A, E, F ), rather than five to eight spines in S. spinophthalma ( Sakai 1970: text-fig. 3d, e; Komai et al. 2002: fig. 10A–C; Fig 10B, C ); the male major cheliped merus bears a bifid proximal spine on the lower margin in S. inermis n. sp. ( Figs 5A, B , 6A, B ), whereas trifid spine in S. spinophthalma ( Sakai 1970: text-fig. 3d; Komai et al. 2002: fig. 10A, B); the minor cheliped merus is unarmed in S. inermis n. sp. ( Figs 5C , 6F ), whereas armed with one spine in S. spinophthalma (cf. Komai et al. 2002: 10 C; Fig 10C ). Scallasis spinophthalma is known only from the southern area of the Sea of Japan, at depths of 84– 210 m.

The description of Scallasis tonkinae by Grebenyuk (1975) depicts: the eyestalks with a terminally located cornea and lacking the distomesial projection ( Grebenyuk 1975: figs. 3–12, 3–13), relatively long pleomere 6 (1.3 times as long as wide; Grebenyuk 1975: fig. 3–16) and the distinct posteromedian concavity of the telson ( Grebenyuk 1975: fig. 3–16). The unique holotype is a female, having a major cheliped being as stout as that in males of S. inermis n. sp., of which the merus bears a prominent distally bifid spine on the lower margin ( Grebenyuk 1975: fig. 3–8). The female major cheliped of S. inermis n. sp. is much more slender; the merus lower margin bears only one small spine.

Etymology. The specific epithet “ inermis ” (= unarmed) refers to the unarmed lower margin of the minor cheliped merus in the new species, which is characteristic in Scallasis .

Figures

FIGURE 4. Scallasis inermis n. sp. Holotype male (cl 3.7 mm), CBM-ZC 14140: A, carapace and cephalic appendages, lateral view; B, same, dorsal view; C, pleon, telson and pleonal appendages, lateral view; D, pleomeres 1 and 2, dorsal view; E, pleomeres 3–6 and telson, dorsal view; F, telson, dorsal view; G, right uropod, dorsal view (perpendicular; marginal setae on endopod and exopod partially omitted).

FIGURE 5. Scallasis inermis n. sp. Holotype male (cl 3.7 mm), CBM-ZC 14140:A, left major cheliped, lateral view; B, same, merus to chela, mesial view (setae partially omitted); C, right minor cheliped, lateral view.

FIGURE 6. Scallasis inermis n. sp. Paratype male (cl 3.6 mm), RUMF-ZC 04571:A, left major cheliped, lateral view; B, proximal spine on lower margin of merus, lower lateral view; C, fingers of left major cheliped, mesial view (setae omitted). Paratype male (cl 3.7 mm), CBM-ZC 14143: D, carpus and chela of right major cheliped, lateral view (setae omitted). Paratype ovigerous female (cl 3.6 mm), CBM-ZC 14144: E, left major cheliped, lateral view; F, right minor cheliped, lateral view (setae omitted).

FIGURE 7. Scallasis inermis n. sp., Holotype male (cl 3.7 mm), CBM-ZC 14140: A, left maxilliped 3, lateral view; B, same, ischium and merus, mesial view (setae partially omitted); C, left pereopod 2, lateral view; D, left pereopod 3, lateral view; E, same, propodus and dactylus, mesial view (setae omitted); F, left pereopod 4, lateral view; G, right pereopod 5, lateral view; H, same, chela, extensor view (setae omitted).

FIGURE 8. Scallasis inermis n. sp. Holotype male (cl 3.7 mm), CBM-ZC 14140: A, frontal part of carapace and eyestalks, dorsal view; B, shield on thoracomere 7, ventral view; C, left maxilliped 1, external view (setae omitted); D, left maxilliped 2, internal view (setae omitted); E, left pleopod 1, ventral view; G, left pleopod 2, ventral view. Paratype male (cl 3.6 mm), CBM-ZC 14141: F, rudimentary left pleopod 1, lateral view; H, left pleopod 2 le, ventral view. Paratype ovigerous female (cl 3.6 mm), CBM-ZC 14144: I, left pleopod 1, ventral (posterior) view; J, right pleopod 2, ventral (posterior) view; K, left pleopod 3, ventral (posterior) view.

FIGURE 9. Scallasis inermis n. sp., images of entire animals, showing colour in life. A, holotype male (cl 3.7 mm), CBM-ZC 14140, lateral view; B, paratype, male (cl 3.7 mm), RUMF-ZC 04571, dorsal view.

FIGURE 10. Scallasis spinophthalmus (Sakai, 1970). Male (cl 3.3 mm), CBM-ZC 6166: A, telson, dorsal view (setae partially omitted); D, left uropod, dorsal view (marginal setae on rami omitted). Female (cl 3.5 mm), CBM-ZC 6170: B, left major cheliped, lateral view; C, right minor cheliped, lateral view.