PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 118(3):539–542. 2005.
A new species of Hexagonalia (Crustacea: Brachyura: Trapeziidae) from the Solomon Islands Peter Castro Biological Sciences Department, California State Polytechnic University, Pomona, California 91768, U.S.A., e-mail:
[email protected]
Abstract.—A new trapeziid crab species of the genus Hexagonalia Galil, 1986, whose congeners are typically symbiotic with stylasterid corals, is described from deep water off the Solomon Islands. Its association with a host remains unknown.
Hexagonalia was created by Galil (1986) for Quadrella brucei Sere`ne, 1973, on account of several unique characters, the most outstanding of which are the presence of two conspicuous, slightly incurved teeth along each anterolateral margin of the carapace (only one small pointed tooth, very rarely a second, in adults of Quadrella Dana, 1851 sensu stricto), presence of paired teeth on the posterior margin of the dactyli of ambulatory legs, and the presence of a row of pinnate appendages on the inner, distal margin of each of the first male pleopods. Hexagonalia brucei is known only from the western Indian Ocean as an associate of stylasterid (hydrozoan) corals from depths of 109–146 m (Sere`ne 1984) and what appeared to be a gorgonian dredged from depths of 50–170 m (Castro 1999). A second species, H. laboutei Galil, 1997, was described from three specimens collected from a stylasterid coral at a depth of 50–60 m in the Tuamotu Archipelago, French Polynesia (Galil 1997). Three specimens belonging to the genus Hexagonalia, in the family Trapeziidae Miers, 1886 (see Castro et al. 2004), collected by the SALOMON 1 expedition to the Solomon Islands in 2001 proved to belong to an undescribed species and are described herein. Methods The measurements provided are of the carapace length (cl) and width (cw) in mil-
limeters respectively. Carapace length was measured along the carapace midline from the frontal margin to the posterior margin. The width of the carapace was measured across the widest breadth of the carapace. The type material is deposited in the Muse´um national d’Histoire naturelle, Paris, France (MNHN). Class Malacostraca Latreille, 1802 Order Decapoda Latreille, 1802 Infraorder Brachyura Latreille, 1802 Family Trapeziidae Miers, 1886 Genus Hexagonalia Galil, 1986 Hexagonalia unidentata, sp. nov. Type material.—Indispensable Strait, west of Malaita I., Solomon Islands, 8832.69S, 160838.69E, SALOMON 1 Expedition, station DW1782, 661–744 m, 29 September 2001: male holotype (cl 7.2 mm, cw 9.4 mm) (MNHN-B 29165), 2 female paratypes (cl 6.4, 5.5 mm, cw 8.3, 7.5 mm) (MNHN-B 292166). Description.—Carapace (Fig. 1) hexagonal, dorsal surface slightly convex, microscopically punctate. Frontal margin cut into two broadly rounded lobes, margins finely serrated, separated by slight emargination. Anterolateral margins convex, each with two anteriorly pointed (left one less pointed in holotype) epibranchial teeth. Posterolateral borders convex, longer than anterolateral. Orbits large, wide, oblique. Eyes large,
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Fig. 1. Hexagonalia unidentata, holotype (MNHN-B 29165), dorsal surface of carapace and chelipeds; scale 5 5.0 mm.
with short peduncle. Outer orbital angles spiniform, pointed outwardly. Inner suborbital teeth short, acute. Thoracic sternum oval. Suture between second and third sternites (sternal suture 2/ 3) absent. Meri of endognaths of third maxillipeds subrectangular, ischia subquadrate; surface of meri and ischia granular, margins entire. Palp with three articles, setose. Exognaths columnar, tapering. Chelipeds (Fig. 1) massive, slender, subcylindrical, unequal in male holotype, slightly unequal in female paratypes. Fingers slender, slightly curved. Dactylus of larger cheliped serrate, with few pointed teeth, broad cutting edges in smaller; immovable fingers smooth, with few long setae along upper margins. Upper and lower margins of propodi slightly keeled, few long setae on upper margin; lower margins entire. Carpi globose, each with one small, acute tooth on inner angle of anterior margin. Meri with 7–10 conspicuous, acute
teeth that typically increase in size distally, about ¾ carapace width. Ambulatory legs long, slender. Meri laterally flattened, with cristate dorsal margins. Upper margins of carpi, propodi, dactyli with many long, simple setae; fewer setae along lower margins. Dactyli with two rows of triangular teeth along lower margins; thick, curved spines at base of teeth. Tip of each dactylus with apical tooth; thick, curved spines on upper margin of tip. Somite 6 of male abdomen as long as wide, not elongated. Male first pleopods (Fig. 2) long, slender, slightly sinuous; tip simple, without pinnate appendages. Etymology.—From uni, Latin for one, and dentis, Latin for tooth, in reference to the diagnostic single tooth on the inner border of the anterior margin of the carpus of each cheliped. Remarks.—Hexagonalia unidentata can be easily distinguished from the other two known species of the genus. The frontal
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carapace (Fig. 1), whereas they are longer than or as long as the width of the carapace in H. brucei (Sere`ne 1973: pl. 2, fig. A; 1984: pl. 41, fig. B) and H. laboutei (Galil 1997: 452). Very significant is the absence of pinnate appendages on the inner margins of the tips of the male first pleopods, a character present in H. brucei (Galil 1986: fig. 2) and H. laboutei (Galil 1997: fig. 3). Although the appendages are similarly absent in Quadrella and the character was one of several originally used by Galil (1986) to define Hexagonalia, their absence alone does not warrant the inclusion of H. unidentata in Quadrella. The shape and ornamentation of the carapace and chelipeds are closer to the two species of Hexagonalia than to any of the six known species of Quadrella. The microhabitat of H. unidentata remains unknown. Data from station DW1782 of SALOMON 1 do not indicate the presence of stylasterid corals or of any other colonial cnidarians. It was collected from a depth of 661–744 m, which is much greater that the depth range of the other two known species of Hexagonalia (50–170 m). Acknowledgments Fig. 2. Hexagonalia unidentata, male first pleopod. Scale 5 1.0 mm.
margin of its carapace consists of two rounded lobes (Fig. 1) in contrast to two flat lobes in H. brucei (Sere`ne 1973: fig. 8, pl. 2, figs. A–C; 1984: pl. 41, fig. C; Galil 1986: fig. 1A; 1997: fig. 2A) and four teeth in H. laboutei (Galil 1997: figs. 1A, 1C). The carpus of the chelipeds in H. unidentata has one tooth on the inner border of the anterior margin (Fig. 1) rather than two as in H. brucei (Galil 1986: fig. 1C; 1997: fig. 2B) and H. laboutei (Galil 1997: fig. 1D). The last somite of the male abdomen is short as in Quadrella and not elongated as in H. brucei (Galil 1986: fig. 1E) and H. laboutei (Galil 1997: 452). The meri of the chelipeds are shorter than the width of the
Hearty thanks to A. Crosnier (MNHN) for making available the material used in the description and to J.-F. Dejouannet (MNHN) for the line drawings. Literature Cited Castro, P. 1999. Trapeziid crabs (Crustacea, Brachyura, Trapeziidae) of the Indian Ocean and the Red Sea.—Zoosystema 21(1):93–120. , P. K. L. Ng, & S. T. Ahyong. 2004. Phylogeny and systematics of the Trapeziidae Miers, 1886 (Crustacea: Brachyura), with the description of a new family.—Zootaxa 643:1–70. Dana, J. D. 1851. Conspectus Crustaceorum quae in Orbis Terrarum circumnavigatione, Carolo Wilkes e Classe Republicae Foederatae Duce, lexit et descripsit J.D. Dana.—Pars VI.—American Journal of Science and Arts, ser. 2, 11(32): 268–274. Galil, B. 1986. Quadrella (Brachyura: Xanthoidea:
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Trapeziidae)—review and revision.—Journal of Crustacean Biology 6:275–293. . 1997. A new Hexagonalia (Brachyura, Xanthoidea, Trapeziidae), obligate commensal on Stylasteridae.—Zoosystema 19(2/3):449–452. Miers, E. J. 1886. Report on the Brachyura collected by H.M.S. Challenger during the years 1873– 76. Report on the Scientific Results of the Voyage of H.M.S. Challenger During the years1873–76, Zoology, 17(49). Eyre & Spottiswoode, London, L 1 362 pp., pls. 1–29.
Sere`ne, R. 1973. Observations sur les espe`ces des genres Quadrella Dana 1851 et Sphenomerides Rathbun 1898.—Bulletin de la Societe´ Zoologique de France 98(1):191–209. . 1984. Crustace´s de´capodes brachyoures de l’oce´an Indien occidental et de la Mer Rouge. Xanthoidea: Xanthidae et Trapeziidae. Addendum: Carpillidae et Menippidae par A. Crosnier.—Faune Tropicale (ORSTOM, Paris) 24:1– 400, pls. 1–48. Associate Editor: Christopher B. Boyko
