The Blue Crab Of Christmas Island, Discoplax Celeste, New Species (Crustacea: Decapoda: Brachyura: Gecarcinidae) Author Davie, Peter K. L. Ng Peter J. F. text Raffles Bulletin of Zoology 2012 2012-02-29 60 1 89 100 journal article 10.5281/zenodo.13256684 2345-7600 13256684 Discoplax celeste , new species ( Figs. 1–4 , 6C, 6D , 7F–J ) Cardisoma hirtipes : Balss, 1934: 226 ; Gibson-Hill, 1947: 48 ; Tweedie, 1947: 35 ; Türkay, 1974: 229 (part); George, 1978: 6 ; Hicks et al., 1984: 54; Morgan, 2000: 123 (not Cardisoma hirtipes Dana, 1851 ). Discoplax hirtipes : Ng & Guinot, 2001: 334, 335 (part); Davie, 2002: 185 (not Cardisoma hirtipes Dana, 1851 ). Material examined. — Holotype : male (106.0 × 83.6 mm ) ( QMW29123 ), Waterfall Bay , near Christmas I. Resort , Christmas Island , coll. 30 Jan.2010 . Paratypes : 1 male (41.9 × 36.5 mm ) ( QM-W8258 ), Christmas I., coll. J. Covacevich , Feb.1980 ; 1 male , 3 females ( ZRC 1965.12.1.21–24), Ross Hill , coll. C. A. Gibson-Hill , Aug.–Sep.1932 ; 1 male ( ZRC 2012.0026 ), Ross Hill Spring , coll. M. Orchard , 30 May 2005 ; 7 males , 1 female ( ZRC 1965.12.1.13–20), coll. M. W. F. Tweedie , 1932 ; 1 male (68.3 × 50.2 mm ), 3 females (largest 75.5 × 60.0 mm) ( ZRC 1965.12.1.21–24), coll. M. W. F. Tweedie , Aug.–Sep.1932 ; 3 females ( ZRC 2012.0020 ), station CI 11, Hosnie’s Springs , freshwater spring, uplifted Bruguiera patch, sandy and limestone bedrock, 10°28.650'S , 105°41.491'E , coll. 24 Jan.2010 ; 6 males ( ZRC 2012.0027 ), station CI 11, Hosnie’s Springs , freshwater spring, uplifted Bruguiera patch, sandy and limestone bedrock, 10°28.650'S , 105°41.491'E , coll. 24 Jan.2010 ; 1 male (44.4 × 38.7 mm ), 4 females (15.9 × 14.1 mm , 22.3 × 19.0 mm, 27.5 × 23.5 mm , 39.0 × 34.0 mm), 1 juvenile (7.9 × 7.1 mm ) ( ZRC 2012.0028 ), station CI 32, Hugh’s Dale , freshwater springs, gravel muddy substrate and limestone blocks, 10°28.716'S , 105°33.556'E , coll. 3 Feb.2010 ; 3 young males, 4 young females, 1 juvenile ( ZRC 2012.0019 ) , 2 males ( ZRC 2012.0017 ), station CI 32, Hugh’s Dale , freshwater springs, gravel muddy substrate and limestone blocks, 10°28.716'S , 105°33.556'E , coll. 3 Feb.2010 ; 2 males (larger 103.9 × 77.8 mm ) ( ZRC 2012.0025 ), The Dales , coll. 3 Feb.2010 ; 2 males ( ZRC 2012.0016 ), station CI 14, Whip Cave , along road to Waterfall Bay , in anchialine cave, loamy substrate, limestone bedrock, 10°25.377'S , 105° 42.081'E , coll. 27 Jan.2010 ; 2 females (42.4 × 35.8 mm , 47.1 × 39.2 mm ) ( ZRC 2012.0018 ) , 2 males (99.3 × 80.8 mm , 60.9 × 51.1 mm ), 2 females (75.4 × 61.4 mm , 80.6 × 66.1 mm ) ( QM-W29121 ), station CI 23, Waterfall Bay , near Christmas I. Resort , freshwater stream, sandy beach, limestone base rock, 10°27.54'S , 105°42.30'E , coll. 30 Jan.2010 ; 1 male (84.3 × 65.7 mm ) ( ZRC 2011.0166 ), stream near Waterfall Bay , before entrance of Christmas I. Resort , coll. P. K. L. Ng , Jan.2010 ; 1 female (82.7 × 66.3 mm ) ( ZRC 2011.0167 ), stream near Waterfall Bay , before entrance of Christmas I. Resort , coll. Jan.2010 ; 6 males , 2 females ( ZRC 2011.0169 ), station CI 07, stream near Waterfall Bay , before entrance of Christmas I. Resort , coll. 23 Jan.2010 ; 4 males (largest 103.5 × 80.5 mm ), 1 female (98.0 × 85.0 mm) ( ZRC 2012.0022 ), Waterfall Bay , coll. Feb.2010 ; 1 male (104.9 × 82.7 mm ) ( ZRC 2012.0023 ), stream near Waterfall Bay , before entrance of Christmas I. Resort , coll. Mar.2011 ; 1 female (82.6 × 65.7 mm ) ( ZRC 2012.0024 ), Waterfall Bay , coll. Jan.–Feb.2010 ; 3 males (largest 112.7 × 88.5 mm ), 1 female ( ZRC 2012.0014 ), stream near Waterfall Bay , before entrance of Christmas I. Resort , coll. March 2011 ; 3 males (99.3 × 80.8 mm , 103.8 × 80.4 mm , 111.2 × 85.6 mm ), 1 female (79.5 × 64.5 mm ) ( QM-W29122 ) , 4 males (largest 106.2 × 83.4 mm ) ( ZRC 2012.0021 ), station CI 08, on path to Dolly Beach , 10°31.272'S , 105°40.512'E , coll. 23 Jan.2010 . All localities in Christmas Island . Comparative material examined: Discoplax hirtipes ( Dana, 1851 ) : Neotype male (64.2 × 53.0 mm) ( ZRC 2010.0415 ), Suva Municipal Market , obtained by D. Huang , Fiji , 11 Dec.2010 . Others : New Caledonia – 1 male (61.4 × 78.0 mm) ( MNHN – B 24813 ), Loyalty Islands , Lifou Island , We Cave , coll. B. Richer de Forges , 14 Jul.1993 ; 1 male (61.0 × 79.0 mm) ( MNHN – B 24811 ), Loyalty Islands . Palau – 1 male (79.5 × 63.1 mm ), 1 female ( ZRC 2000.1087 ), Anggaur Island , coll. S. H. Tan , Sep.1998 ; 1 male (87.4 × 68.3 mm ) ( ZRC 2000.1086 ), Anggaur Island , coll. S. H. Tan , Sep.1998 ; 1 male (86.4 × 68.5 mm ) ( ZRC 2000.1088 ), Anggaur Island , coll. S. H. Tan , Sep.1998 . Solomon Islands – 1 male (54.2 × 45.3 mm ) ( QM-W15247 ), Tulagi Island , coll. J. Covacevich , 15 Dec.1976 ; 1 male (46.5 × 39.2 mm ), 1 female (57.0 × 49.0 mm) ( QM-W15248 ), on road leaving Honiara, Guadalcanal , coll. J. Covacevich , 15 Dec.1976 . Guam – 1 male (93.5 × 73.4 mm ) ( ZRC 2001.2230 ), Pago Bay , coll. Louie , 31 Jul.2001 ; 1 male (66.6 × 54.0 mm) ( ZRC 2001.0704 ), Merizo Bay , coll. P. K. L. Ng , 1 Aug.2001 ; 1 female with zoeae ( ZRC 2001.705 ), Merizo Bay , coll. P. K. L. Ng , 1 Aug.2001 . Philippines – 1 male ( QM-W26690 ), in forest just outside of Virata Cave , Libaong, Panglao, Bohol , coll. P. K. L. Ng et al., 18 Dec.2000 ; 1 male ( ZRC 2001.310 ), Panglao, Bohol , coll. P. K. L. Ng , 17 Dec.2000 ; 1 male ( ZRC 2001.2308 ), Panglao, back beaches, coll. H.-C. Liu , 29 Nov.2001 ; 1 male ( ZRC 2004.0462 ), Hinagdanan Cave vicinity, near Panglao I. Nature Resort , Panglao, Bohol , coll. P. K. L. Ng , 3 Mar.2004 . Taiwan – 1 male (68.5 × 55.5 mm ) ( ZRC 1999.1039 ), 1 female ( ZRC 1998.411 ), Hengchun Peninsula, Pingtung County , coll. H.-C. Liu & S. H. Tan , 22 May 1998 ; 2 males , 2 females ( ZRC 1998.398 , 439 , 452 , 472 ), Hengchun Peninsula, Pingtung , coll. H.-C. Liu et al., 19 May 1998 ; 2 males (larger 76.7 × 59.5 mm ) ( ZRC 1998.424 ), Hengchun Peninsula, Pingtung , coll. H.-C. Liu et al., 19 May 1998 ; 1 female ( ZRC ), Hengchun Peninsula, Pingtung County , coll. May 1998 ; 1 male ( ZRC 1999.203 ), Hengchun Peninsula, Pingtung County , coll. P. K. L. Ng , 30 May 1997 ; 1 male ( ZRC 1998.531 ), Hengchun Peninsula, Pingtung County , coll. P. K. L. Ng , 13 May 1998 . Japan – 1 male ( ZRC 1999.239 ), Yaeyama, Ryukyus, coll. 19 August 1969 ; 1 male (57.4 × 46.8 mm ), 2 females (larger 64.2 × 53.7 mm ), Kumejima Island , Rukyus, coll. P. K. L. Ng , Nov.2009 ; 1 female ( ZRC 2009.0154 ), Funaura jetty and beach, Iriomote Island , Ryukyus, N. K . Ng & Y. Cai , 15 Jun.2000 . Indonesia – 1 male , 1 female ( ZRC 1965.12.1.11–12), Pulau Panjang, South Natuna Island , coll. August 1931 ; 1 male (54.5 × 44.1 mm ) ( ZRC 2003.0582 ), Geropok, Lombok, coll. local fisherman, 12 Feb.2002 . Vanuatu – 3 males , 3 females ( ZRC ), Loganville Market, from around Santo Island , coll. P. K. L. Ng , 14 Sep.2006 ; 1 male ( ZRC ), near rocky areas at Maritime Centre, Santo, Vanuatu, coll. P. K. L. Ng , 10 Sep.2006 . Vietnam – 1 male ( ZRC 1973.10.31.275), Vietnam, coll. Nhathrang Oceanographic Institute , 9 Apr.1971 . Malaysia – 2 males ( ZRC 2000.2583 ), Pulau Pemanggil, Johor , coll. D. Chia , 11 Sep.1991 . Singapore – 1 female ( ZRC 1965.12.1.10), Paya Lebar, coll. Jul.1938 . Discoplax aff. hirtipes : Indonesia – 2 males ( ZRC 1965.12.1.26–27), West Sumatra , coll. G. E. Rostados , 1897; 1 male (51.1 × 42.5 mm ) ( ZRC 2008.0575 ), Pantai Cerocok, Painan, West Sumatra , coll. J. C. Y. Lai & N. K. Ng , 6 Jul.2003 ; 1 male , 1 female ( ZRC 1965.12.1.28–29), Sumatra , old museum exhibit; 1 male (69.7 × 55.5 mm ) ( ZRC 1965.12.1.25), South Pagi , Mentawei Island , coll. Dec.1902 . Nicobar Islands – 12 males (largest 86.9 × 72.3 mm ), 2 females (larger 66.4 × 55.1 mm ) ( ZRC 1965.12.1.30–39), Nicobar Islands , coll. C. A. Gibson – Hill , Oct.1950 . Christmas Island – 1 female (47.1 × 39.7 mm ) ( ZRC 2012.0003 ), station CI 16, in beach forest, along road towards Dolly Beach , coll. P. K. L. Ng , 23 Jan.2010 ; 1 male (83.1 × 68.1 mm ) ( ZRC 2012.0004 ), stream near Waterfall Bay , before entrance of Christmas I. Resort , coll. P. K. L. Ng , 31 Mar.2011 . Diagnosis. — Adult (carapace width 45 mm or more) blue to bluish-white with completely white cheliped ( Fig. 1 ). Carapace subovate, broader than long; dorsal surface evenly convex tranversely and longitudinally, regions poorly demarcated; epigastric regions swollen but margins poorly defined, separated from frontal margin by transversely narrow concavity, barely separated from postorbital cristae, without distinct furrow or groove ( Figs. 1 , 3A, 3B , 4 ); postorbital region gently swollen without trace of cristae, cervical groove very shallow ( Figs. 1 , 3A, 3B , 4 ); frontal margin almost straight from dorsal and frontal views; margin deflexed inwards to form relatively broad triangular plate touching proepistome ( Figs. 1 , 3A, 3B , 4 ). Male abdomen relatively broad; somite 6 with lateral margin strongly convex broader than long; distal margin gently concave; telson triangular, longer than broad, lateral margins gently concave to almost straight, tip rounded ( Fig. 6C, D ). G1 almost straight, relatively slender; tip bent outwards at an angle of about 90° from vertical; outer surface of distal part deeply concave, forming depression; distal part of inner margin prominently convex, forming hump-like structure; subdistal part of dorsal (sternal) surface with well developed elongate, subovate flap appressed to main structure, distal-most part pectinated, beak-like, upper part sharply tapering to acute tip, lower part subtruncate ( Fig. 7F–J ). Fig. 1. Discoplax celeste , new species . Colour in life. Holotype male (106.0 × 83.6 mm) (QMW-29123), Waterfall Bay, Christmas Island. A, dorsal view; B, frontal view. Description of adult male. — In life, carapace blue to bluish-white with completely white cheliped ( Fig. 1 ). Carapace subovate, broader than long; dorsal surface evenly convex tranversely and longitudinally, regions not well demarcated; H-shaped (gastro-cardiac) median depression prominent ( Figs. 1 , 3A, 3B , 4 ). Epigastric regions swollen but margins poorly defined, separated from each other by deep, short, median longitudinal furrow, separated from frontal margin by transversely narrow concavity, barely separated from postorbital cristae, no furrow or groove discernible ( Figs. 1 , 3A, 3B , 4 ). Postorbital region gently swollen without trace of cristae, gently merging with other surfaces, in 2 parts, separated by short, very shallow longitudinal furrow, proximal part almost reaching very shallow cervical groove ( Figs. 1 , 3A, 3B , 4 ). Entire dorsal surface of carapace smooth ( Figs. 1 , 3A, 3B , 4 ). Pterygostomial region covered with dense short setae which completely obscure surface, setae partly reaching suborbital surface ( Figs. 1B , 3B ). Fig. 2. Discoplax celeste , new species . Colour changes with size. A, juvenile (7.9 × 7.1 mm) (ZRC 2012.0028a); B, female (15.9 × 14.1 mm) (ZRC 2012.0028b); C, female (22.3 × 19.0 mm) (ZRC 2012.0028c); D, female (27.5 × 23.5 mm) (ZRC 2012.0028d); E, female (39.0 × 34.0 mm) (ZRC 2012.0028e); F, male (44.4 × 38.7 mm) (ZRC 2012.0028f); G, female (42.4 × 35.8 mm) (ZRC 2012.0018a); H, female (47.1 × 39.2 mm) (ZRC 2012.0018b). A–F from station CI 32, Hugh’s Dale, Christmas Island; G, H, from station CI 23, Waterfall Bay, Christmas Island. Frontal region gently upturned to form shallow shelf between it and epigastric regions; front deflexed downwards; frontal margin almost straight from dorsal and frontal views; margin deflexed inwards to form relatively broad triangular plate touching proepistome ( Figs. 1 , 3 , 4 ). Supraorbital margin gently sinuous, smooth ( Figs. 1 , 3 , 4 ). External orbital tooth broadly triangular, very low, tip not over-reaching orbit, outer margin almost straight, confluent with anterolateral margin ( Figs. 1 , 3 , 4 ). Anterolateral margin strongly convex, rounded, smooth, without trace of cristae or ridges, gradually curving to meet posterolateral margin ( Figs. 1 , 3 , 4 ). Posterolateral margin gently convex, smooth, rounded, converging sharply towards gently convex posterior carapace margin ( Figs. 1 , 3 , 4 ). Suborbital margin weakly granulated, shelf-like, appearing gently sinuous in frontal view, gently arcuate in dorsal view; outer edge reaching point directly beyond end of external orbital tooth, not directly confluent with tooth but separated by gently convex surface ( Figs. 1B , 3B ). Eyes well developed, filling most of orbital cavity; eyestalks relatively short ( Fig. 1B , 3B ). Basal article of antenna subquadrate, separated from frontal margin by prominent gap; article 4 small, not closing hiatus between front and ocular peduncle. Antennules folding sub-obliquely ( Figs. 1B , 3B ). Epistome relatively narrow; posterior margin with median triangular lobe, lateral margins concave, lined with small rounded granules ( Figs. 1 , 3 , 4 ). Buccal cavity relatively broad, third maxillipeds not closing anterior part which appears as a prominent transverse gap even with palp not fully folded ( Figs. 1 , 3 , 4 ). Third maxillipeds relatively elongate, meri and ischia forming rhomboidal gape when maxillipeds closed; merus subquadrate, anterior and outer lateral margins concave, median surface depressed; ischium subquadrate with prominent submedian sulcus; exopod slender, tip reaching to about half length of merus, with well developed flagellum ( Figs. 1 , 3 , 4 ). Chelipeds subequal; surfaces smooth ( Figs. 1 , 3C , 4 ). Merus with dorsal margin rugose or uneven but not prominently serrated or granular; ventral margin lined with low granules, otherwise unarmed. Carpus with large, well developed triangular inner subdistal tooth ( Figs. 1 , 3A , 4 ). Chela large, surfaces smooth; lower margin of palm sinuous; fingers slender, curved, longer than palm; cutting margins with low denticles along length, distal-most part pectinated, tip gently recurved; fingers forming large gape between them when closed ( Figs. 1B , 3B ). Ambulatory legs with second pair longest, last pair shortest; surfaces smooth to slightly rugose ( Figs. 1A , 2 , 3A , 4 ). Merus laterally flattened, cross-section ovate, relatively stout; dorsal margin uneven but not granulated or serrate, subdistal tooth low; margins with dense, long, stiff setae that partially obscure margins. Carpus slender; outer surface with 2 low, subparallel carinae; margins and carinae lined with dense stiff setae. Propodus quadrate; lateral margins generally subparallel, lined with dense stiff setae, outer median surface with longitudinal row of short stiff setae; anterior distal margin with prominent rounded lobe into which base of dactylus fits. Dactylus elongate, styliform, gently curving, subquadrate in cross–section, margins lined with strong short spines; tip corneous ( Figs. 1A , 2 , 3A , 4 ). Thoracic sternites 1 and 2 completely fused, forming small semi-circular plate, separated from sternite 3 by gently convex suture; sternites 3 and 4 completely fused without trace of suture ( Fig. 6C, D ). Sternite 4 with relatively broad sternoabdominal cavity reaching almost to approximate position between sternites 3 and 4, level with imaginary line joining anterior edges of coxae of chelipeds ( Fig. 6C, D ). Male abdomen relatively broad, all segments and telson freely articulating; lateral margins of somites lined with short setae. Somite 1 longitudinally very narrow. Somite 2 similar to somite 1 but relatively broader longitudinally. Somites 3–5 increasingly trapezoidal in shape, all lateral margins gently convex. Somite 6 with lateral margin strongly convex especially along distal part, transversely broad, broader than long; distal margin gently concave. Telson triangular, longer than broad, lateral margins gently concave to almost straight, tip rounded ( Fig. 6C, D ). Fig. 3. Discoplax celeste , new species . Holotype male (106.0 × 83.6 mm) (QMW-29123), Waterfall Bay, Christmas Island. A, dorsal view; B, frontal view of carapace; C, outer view of chelae. G1 almost straight, relatively slender; distal surfaces adjacent to pectinated tip densely lined with long, stiff setae which almost completely obscure structure and margins, rest of surface with scattered long and short soft setae; tip bent outwards at an angle of about 90° from vertical; outer surface of distal part deeply concave, forming depression; distal part of inner margin prominently convex, forming hump-like structure; subdistal part of dorsal (sternal) surface with well developed elongate, subovate flap which is appressed to main structure, distal margin convex, lined with dense stiff setae; distal-most part pectinated, beak-like, upper part sharply tapering to sharp tip, lower part subtruncate ( Fig. 7F–J ). G2 short, ca. 0.2 times length of G1; tip spatulate. Morphological variation. — The carapace shape is more quadrate in smaller specimens ( Fig. 2A–C ), becoming increasingly rounded in medium-sized specimens ( Figs. 2D–H , 4A ). In large males and females, the carapace appears more transversely subovate ( Figs. 1A , 3A , 4B ). The carapace of specimens below about 60 mm carapace width have the anterolateral margins cristate, with striae and flattened granules, these being relatively weaker in larger specimens ( Figs. 2 , 4A ). Once they reach carapace widths in excess of 60 mm , these cristae disappear, with the margins becoming rounded; and the carapace surface generally very smooth ( Figs. 1 , 2A , 4B ). chelipeds become white, with the carpus and chela losing almost all trace of yellow or orange ( Fig. 2E–G ). At carapace widths of 46–52 mm , the entire carapace and chelipeds are already bluish white to powder blue ( Fig. 2H ), with larger specimens always completely white or blue ( Fig. 1 ) (see later for very rare exceptions). Etymology. — The name is derived from the French céleste meaning sky or heaven and alludes to the sky or powder blue live colour of adults of this species. The name is used as a noun in apposition. Remarks. — A preliminary genetic study of Discoplax species by H.-T. Shih shows that Discoplax hirtipes sensu lato is split into two major clades. One major clade contains all the material from West Pacific (including Sundaic Southeast Asia). Discoplax hirtipes was originally described from Fiji , and thus the true D. hirtipes belongs genetically to the West Pacific clade. The second major clade contains the Indian Ocean material. The Indian Ocean clade, however, has two well-defined subclades: one which contains the Blue Crab from Christmas Island (i.e. Discoplax celeste ), while the second includes the purple carapace and orange-red cheliped specimens from western Sumatra, Nicobar Islands The live colour of D. celeste varies with maturity but is generally consistent within size classes. In small individuals (carapace widths ca. 40 mm or less), the carapace is invariably brown or purplish-brown, with the chelae yellow or orange ( Fig 2A–D ). When they reach carapace widths of between 41–51 mm , the carapace remains purplish-brown but the and Christmas Island (here referred to as Discoplax aff. hirtipes ). Fig. 4. Discoplax celeste , new species . A, paratype male (52.0 × 44.0 mm) (ZRC 2012.0027), station CI 11, Waterfall Bay, Christmas Island; B, paratype male (84.3 × 65.7 mm) (ZRC 2011.0166), Waterfall Bay, Christmas Island. Fig. 5. Discoplax hirtipes ( Dana, 1851 ) s. str. A, neotype male (64.2 × 53.0 mm) (ZRC 2010.0415), Suva, Fiji; B, C, male (93.5 × 73.4 mm) (ZRC 2001.2230), Pago Bay, Guam. A, B, dorsal overall views; C, frontal view of carapace. As with most of Dana’s material, the type specimens of Discoplax hirtipes are lost (see Evans, 1967 ; Deiss & Manning, 1981). In view of the present recognition of the Christmas Island population as a separate species, it is consequently important to designate a neotype for Cardisoma hirtipes Dana, 1851 , to fix the identity of that species. The specimen we have on hand from Fiji agrees very well with the type description and figures by Dana (1851) , and we are confident it is conspecific with D. hirtipes s. str. Thus we here designate a male (64.2 × 53.0 mm) (ZRC 2010.0415) collected from Fiji ( Figs. 5A , 7A–E ) as the neotype . It was collected alive, with colour photographs obtained; tissue was preserved in 95% alcohol for genetic studies. Apart from colour, there are some morphological differences between D. celeste and the Western Pacific D. hirtipes , most obviously in the degree of development of the epigastric and postorbital regions. In D. hirtipes s. str. , these regions are sharply defined, and separated from each other by relatively deep grooves or notches ( Fig. 5 ). The cervical grooves are also relatively deeper ( Fig. 5 ). As a result, when viewed from the front, the postorbital and branchial regions appear more swollen and more clearly demarcated from the lower gastric regions ( Fig. 5C ) when compared to that of D. celeste ( Fig. 3B ). In D. celeste , the postorbital and branchial regions are less markedly separated ( Figs. 1–4 ). Fig. 6. Male abdomens and anterior thoracic sternums. A, Discoplax hirtipes ( Dana, 1851 ) , neotype male (64.2 × 53.0 mm) (ZRC 2010.0415), Suva, Fiji; B, D. hirtipes ( Dana, 1851 ) , male (73.5 × 58.4 mm) (ZRC 2001.0706), Merizo Bay, Guam; C, D. celeste , new species , paratype male (52.0 × 44.0 mm) (ZRC 2012.0027), station CI 11, Hosnie’s Spring, Christmas Island; D, D. celeste , new species , holotype male (106.0 × 83.6 mm) (QMW-29123), Waterfall Bay, Christmas Island. Fig. 7. G1s. A–E, Discoplax hirtipes ( Dana, 1851 ) , neotype male (64.2 × 53.0 mm) (ZRC 2010.0415), Suva, Fiji; F–J, D. celeste , new species , holotype male (106.0 × 83.6 mm) (QMW-29123 ex ZRC 2011.0168), Waterfall Bay, Christmas Island. The male abdomens of relatively smaller D. celeste are very similar to those of D. hirtipes ( Fig. 6 ) but in adults of D. celeste , somite 6 ( Fig. 6C, D ) is relatively longer than that of D. hirtipes ( Fig. 6A, B ). Most markedly, the G1 structures of the two species are different. The G1 of D. celeste is relatively more slender and proportionately longer ( Fig. 7F–J ) compared to that of D. hirtipes ( Fig. 7A–E ), the difference been all the more apparent when similar sized specimens are compared. In addition, the outer margin of the subdistal part is relatively more produced in D. hirtipes ( Fig. 7A–D ) compared to that of D. celeste ( Fig. 7F–I ). The colour of the adult crabs is very important. The authors have collected or observed numerous specimens of D. hirtipes s. lato from throughout the western Pacific and have never seen crabs with the sky-blue carapace and uniformly pale white claws anywhere other than Christmas Island . Adults of D. hirtipes invariably have brown to purplish-brown carapaces, and while very large males have most of the chelae white, fringes and patches of purple or orange always remain on the dorsal parts, and the carpus retains most of the purple or orange colour (see Chia & Ng, 1994). This is also true of all photographs of D. hirtipes we have seen. While as described earlier, smaller specimens of D. celeste closely resemble D. hirtipes , but once they reach larger sizes (carapace widths ca. 50 mm ), their colour differences are unmistakable. There remains the question of the population of D. aff. hirtipes from the Andaman and Nicobar Islands which has dark violet carapaces and cinnabar red chelae, as reported by Alcock (1900: 448) . The specimens in the ZRC from the Nicobars collected by the late Carl Gibson-Hill still retain some of the original colours. We have also specimens of D. aff. hirtipes from the western shores of Sumatra and Mentawei Islands, and their morphologies agree with those from the Nicobars in most aspects. One of these Sumatran specimens (ZRC 2008.0575) was collected relatively recently, and the colour of its carapace was purplish-brown with bright reddish-orange chelae when alive (J. Lai, pers. comm.). Morphologically, the Sumatran and Nicobar specimens most closely resemble D. celeste in that the epigastric and postorbital regions are relatively less inflated and more rounded, with the G1 relatively slender and elongate. However, the G1 of D. aff. hirtipes from the Indian Ocean is proportionately even more slender than that of D. celeste . In addition, the male abdomen of these specimens is relatively more slender with somite 6 proportionately longer. We believe they should be also referred to a separate species, but this will be undertaken later as part of the larger revision of the genus. Interestingly, we observed three medium sized individuals on Christmas Island (carapace lengths ca. 50–80 mm ) that had the purplish-brown carapace and orange chelipeds ( Figs. 8 , 9 ), of the more northern Indian Ocean Discoplax aff. hirtipes from the Nicobars and Sumatra. As discussed earlier, at these sizes the carapace of D. celeste may still be purplish but the chelipeds should already be almost completely white ( Fig. 2H ). A male and a female specimen were collected. The male specimen (ZRC 2012.0004) was morphologically typical in all respects with the Nicobar and Sumatran crabs, including the form of the G1. Its DNA also clusters with the Sumatran specimens rather than with Christmas Island D. celeste (H.-T. Shih, unpublished data). A photograph of the female specimen (ZRC 2012.0003) is presented in Fig. 9B . These two specimens are tentatively referred to Discoplax aff. hirtipes . The presence of both species on Christmas Island is not surprising. Although the dominant species of Gecarcoidea on Christmas Island is the Red Land Crab, G. natalis , specimens of the closely related G. lalandii H. Milne Edwards, 1837 , have also been found there, albeit very rarely (see Hicks et al., 1984). With Indonesia relatively near and the pattern of currents known, it can be expected that some larvae of Gecarcoidea lalandii and Discoplax aff. hirtipes would occasionally reach Christmas Island , but apparently few survive and even fewer reach maturity as the ecological niches are dominated by the indigenous species. Fig. 9. Discoplax aff. hirtipes from Christmas Island. A, probably male, ca. 80 mm carapace width (not collected), Waterfall Bay (Photograph by: H. H. Tan, 2007); B, female (47.1 × 39.7 mm) (ZRC 2012.0003), station CI 16. Fig. 8. Top, Discoplax celeste , new species ; bottom, Discoplax aff. hirtipes . Waterfall Bay, Christmas Island (specimens not collected), all ca. 80 mm carapace width (Photograph by: H. H. Tan, 2007). Biology. — Hicks et al. (1984) give an excellent account of the ecology and habits of Discoplax celeste , new species (as Cardisoma hirtipes ) (see also Gibson-Hill, 1947 ; Tweedie, 1947 ). It has occasionally been found entering the mouth of cave systems (Humphreys & Eberhard, 2001). The preferred habitat of Discoplax celeste is quite different from that of D. hirtipes s. str. and Discoplax aff. hirtipes . As discussed earlier, D. celeste is almost always found next to streams or swampy areas, digging burrows in the soft soil adjacent to the stream, or inbetween the roots of the trees growing next to the water. It is also frequently found in the water itself (see also Gibson-Hill, 1947 ; Tweedie, 1947 ; Gray, 1981 , 1995 ; Hicks et al., 1984). Discoplax hirtipes s. str. and Discoplax aff. hirtipes on the other hand are found in much drier karstic habitats, often far from surface water or freshwater swamps, and their burrows are often dug next to rocks (unpublished data). The general biology of this species (as Cardisoma hirtipes or Discoplax hirtipes ) has also been relatively well studied, with many papers on its respiratory and digestive physiology (e.g., Adamczewska & Morris, 1996, 2000; Dela-Cruz & Morris, 1997a, 1997b; Farrelly & Greenaway, 1994; Greenaway, 1989 , 1993 , 1998 ; Linton & Greenaway, 2004; Linton et al., 2006 ; Morris, 2005 ; Morris & Dela-Cruz, 1998), gill ultrastructure (Farrelly & Greenaway, 1992) and population structure ( Turner et al., 2011 ). The population of the Red Land Crab ( Gecarcoidea natalis ) has been substantially impacted by the invasive yellow crazy ant ( Anoplolepis gracilipes ) (see Abbott, 2005 ; O’Dowd et al., 2003 ; Green et al., 2004 ). However, the Blue Crab seems to have been less markedly affected because of its more aquatic habits and typically water-filled burrows (see Greenaway, 1989 ; Turner et al., 2011 ; M. Orchard, pers. comm.) that are unsuitable for the fully terrestrial ants.