Discoplax celeste, Davie, 2012

Davie, Peter K. L. Ng Peter J. F., 2012, The Blue Crab Of Christmas Island, Discoplax Celeste, New Species (Crustacea: Decapoda: Brachyura: Gecarcinidae), Raffles Bulletin of Zoology 60 (1), pp. 89-100 : 90-98

publication ID	https://doi.org/ 10.5281/zenodo.13256684
persistent identifier	https://treatment.plazi.org/id/03E42551-E115-FFDB-92C0-FC9AFDD5158C
treatment provided by	Felipe
scientific name	Discoplax celeste
status	sp. nov.

Treatment

Discoplax celeste View in CoL , new species

( Figs. 1–4 View Fig View Fig View Fig View Fig , 6C, 6D View Fig , 7F–J View Fig )

Cardisoma hirtipes View in CoL : 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 View in CoL ).

Discoplax hirtipes View in CoL : Ng & Guinot, 2001: 334, 335 (part); Davie, 2002: 185 (not Cardisoma hirtipes Dana, 1851 View in CoL ).

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 View Materials ), 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 View Materials ), 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 GoogleMaps ; 6 males ( ZRC 2012.0027 View Materials ), 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 GoogleMaps ; 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 View Materials ), 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 GoogleMaps ; 3 young males, 4 young females, 1 juvenile ( ZRC 2012.0019 View Materials ) , 2 males ( ZRC 2012.0017 View Materials ), 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 GoogleMaps ; 2 males (larger 103.9 × 77.8 mm) ( ZRC 2012.0025 View Materials ), The Dales, coll. 3 Feb.2010 ; 2 males ( ZRC 2012.0016 View Materials ), 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 GoogleMaps ; 2 females (42.4 × 35.8 mm, 47.1 × 39.2 mm) ( ZRC 2012.0018 View Materials ) , 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 GoogleMaps ; 1 male (84.3 × 65.7 mm) ( ZRC 2011.0166 View Materials ), 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 View Materials ), stream near Waterfall Bay , before entrance of Christmas I. Resort, coll. Jan.2010 ; 6 males, 2 females ( ZRC 2011.0169 View Materials ), 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 View Materials ), Waterfall Bay , coll. Feb.2010 ; 1 male (104.9 × 82.7 mm) ( ZRC 2012.0023 View Materials ), stream near Waterfall Bay , before entrance of Christmas I. Resort, coll. Mar.2011 ; 1 female (82.6 × 65.7 mm) ( ZRC 2012.0024 View Materials ), Waterfall Bay , coll. Jan.–Feb.2010 ; 3 males (largest 112.7 × 88.5 mm), 1 female ( ZRC 2012.0014 View Materials ), 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 View Materials ), station CI 08, on path to Dolly Beach, 10°31.272'S, 105°40.512'E, coll. 23 Jan.2010 GoogleMaps . All localities in Christmas Island .

Comparative material examined: Discoplax hirtipes ( Dana, 1851) : Neotype male (64.2 × 53.0 mm) ( ZRC 2010.0415 View Materials ), Suva Municipal Market, obtained by D. Huang, Fiji, 11 Dec.2010. Others : New Caledonia – 1 male (61.4 × 78.0 mm) ( MNHN – B 24813 About MNHN ), Loyalty Islands, Lifou Island , We Cave , coll. B. Richer de Forges , 14 Jul.1993; 1 male (61.0 × 79.0 mm) ( MNHN – B 24811 About MNHN ), Loyalty Islands. Palau – 1 male (79.5 × 63.1 mm), 1 female ( ZRC 2000.1087 View Materials ), Anggaur Island , coll. S. H. Tan, Sep.1998; 1 male (87.4 × 68.3 mm) ( ZRC 2000.1086 View Materials ), Anggaur Island , coll. S. H. Tan, Sep.1998; 1 male (86.4 × 68.5 mm) ( ZRC 2000.1088 View Materials ), 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 View Materials ), Pago Bay , coll. Louie, 31 Jul.2001; 1 male (66.6 × 54.0 mm) ( ZRC 2001.0704 View Materials ), Merizo Bay , coll. P. K. L. Ng, 1 Aug.2001; 1 female with zoeae ( ZRC 2001.705 View Materials ), 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 View Materials ), Panglao, Bohol, coll. P. K. L. Ng, 17 Dec.2000; 1 male ( ZRC 2001.2308 View Materials ), Panglao, back beaches, coll. H.-C. Liu, 29 Nov.2001; 1 male ( ZRC 2004.0462 View Materials ), 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 View Materials ), 1 female ( ZRC 1998.411 View Materials ), Hengchun Peninsula, Pingtung County, coll. H.-C. Liu & S. H. Tan, 22 May 1998; 2 males, 2 females ( ZRC 1998.398 View Materials , 439 View Materials , 452 View Materials , 472 View Materials ), Hengchun Peninsula, Pingtung, coll. H.-C. Liu et al., 19 May 1998; 2 males (larger 76.7 × 59.5 mm) ( ZRC 1998.424 View Materials ), 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 View Materials ), Hengchun Peninsula, Pingtung County, coll. P. K. L. Ng, 30 May 1997; 1 male ( ZRC 1998.531 View Materials ), Hengchun Peninsula, Pingtung County, coll. P. K. L. Ng, 13 May 1998. Japan – 1 male ( ZRC 1999.239 View Materials ), 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 View Materials ), 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 View Materials ), 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 View Materials ), 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 View Materials ), 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 View Materials ), 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 View Materials ), 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 View Fig ). 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 View Fig , 3A, 3B View Fig , 4 View Fig ); postorbital region gently swollen without trace of cristae, cervical groove very shallow ( Figs. 1 View Fig , 3A, 3B View Fig , 4 View Fig ); frontal margin almost straight from dorsal and frontal views; margin deflexed inwards to form relatively broad triangular plate touching proepistome ( Figs. 1 View Fig , 3A, 3B View Fig , 4 View Fig ). 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 View Fig ). 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 View Fig ).

Description of adult male. — In life, carapace blue to bluish-white with completely white cheliped ( Fig. 1 View Fig ). 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 View Fig , 3A, 3B View Fig , 4 View Fig ). 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 View Fig , 3A, 3B View Fig , 4 View Fig ). 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 View Fig , 3A, 3B View Fig , 4 View Fig ). Entire dorsal surface of carapace smooth ( Figs. 1 View Fig , 3A, 3B View Fig , 4 View Fig ). Pterygostomial region covered with dense short setae which completely obscure surface, setae partly reaching suborbital surface ( Figs. 1B View Fig , 3B View Fig ).

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 View Fig , 3 View Fig , 4 View Fig ). Supraorbital margin gently sinuous, smooth ( Figs. 1 View Fig , 3 View Fig , 4 View Fig ). External orbital tooth broadly triangular, very low, tip not over-reaching orbit, outer margin almost straight, confluent with anterolateral margin ( Figs. 1 View Fig , 3 View Fig , 4 View Fig ). Anterolateral margin strongly convex, rounded, smooth, without trace of cristae or ridges, gradually curving to meet posterolateral margin ( Figs. 1 View Fig , 3 View Fig , 4 View Fig ). Posterolateral margin gently convex, smooth, rounded, converging sharply towards gently convex posterior carapace margin ( Figs. 1 View Fig , 3 View Fig , 4 View Fig ). 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 View Fig , 3B View Fig ). Eyes well developed, filling most of orbital cavity; eyestalks relatively short ( Fig. 1B View Fig , 3B View Fig ). 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 View Fig , 3B View Fig ). Epistome relatively narrow; posterior margin with median triangular lobe, lateral margins concave, lined with small rounded granules ( Figs. 1 View Fig , 3 View Fig , 4 View Fig ). 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 View Fig , 3 View Fig , 4 View Fig ). 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 View Fig , 3 View Fig , 4 View Fig ).

Chelipeds subequal; surfaces smooth ( Figs. 1 View Fig , 3C View Fig , 4 View Fig ). 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 View Fig , 3A View Fig , 4 View Fig ). 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 View Fig , 3B View Fig ).

Ambulatory legs with second pair longest, last pair shortest; surfaces smooth to slightly rugose ( Figs. 1A View Fig , 2 View Fig , 3A View Fig , 4 View Fig ). 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 View Fig , 2 View Fig , 3A View Fig , 4 View Fig ).

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 View Fig ). 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 View Fig ).

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 View Fig ).

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 View Fig ). 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 View Fig ), becoming increasingly rounded in medium-sized specimens ( Figs. 2D–H View Fig , 4A View Fig ). In large males and females, the carapace appears more transversely subovate ( Figs. 1A View Fig , 3A View Fig , 4B View Fig ). 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 View Fig , 4A View Fig ). 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 View Fig , 2A View Fig , 4B View Fig ).

chelipeds become white, with the carpus and chela losing almost all trace of yellow or orange ( Fig. 2E–G View Fig ). At carapace widths of 46–52 mm, the entire carapace and chelipeds are already bluish white to powder blue ( Fig. 2H View Fig ), with larger specimens always completely white or blue ( Fig. 1 View Fig ) (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 View Fig ). 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 ).

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 View Fig , 7A–E View Fig ) 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 View Fig ). The cervical grooves are also relatively deeper ( Fig. 5 View Fig ). 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 View Fig ) when compared to that of D. celeste ( Fig. 3B View Fig ). In D. celeste , the postorbital and branchial regions are less markedly separated ( Figs. 1–4 View Fig View Fig View Fig View Fig ).

The male abdomens of relatively smaller D. celeste are very similar to those of D. hirtipes ( Fig. 6 View Fig ) but in adults of D. celeste , somite 6 ( Fig. 6C, D View Fig ) is relatively longer than that of D. hirtipes ( Fig. 6A, B View Fig ). 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 View Fig ) compared to that of D. hirtipes ( Fig. 7A–E View Fig ), 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 View Fig ) compared to that of D. celeste ( Fig. 7F–I View Fig ).

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 View Fig , 9 View Fig ), 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 View Fig ). 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 View Fig . 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.

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.