Euxanthus sp.

Iwasa-Arai, Tammy, McCallum, Anna W. & Taylor, Joanne, 2015, Oceanic Shoals Commonwealth Marine Reserve survey reveals new records of xanthid crabs (Crustacea: Brachyura: Xanthidae) from northern Australia, Memoirs of Museum Victoria 73, pp. 1-11 : 2-4

Euxanthus sp.

( Figures 1C , 2A–F )

Material examined. NMV J46923 , 1 juvenile male (Cb= 7.9 mm, Cl= 5.6 mm), northern Australia, Timor Sea , Oceanic Shoals Commonwealth Marine Reserve, Survey SOL5650 station unknown (acq. 20052), 12 Sep–6 Oct 2012 .

Description. Carapace ( Figs. 2A; 2B ) about 1.4 times as broad as long, regions well defined, convex, strongly areolated, smooth; 2M entirely divided longitudinally; 3M and 3L distinct, entire; 1L indistinct; 4M fused with 3M; 4L merging with third anterolateral tooth; 2L partially divided transversely with shallow groove on medial margin; 2P distinct; 1R separated from 2R by indistinct transverse granular ridge, 2R, 3R indistinct. Pterygostomial region setose, tuberculate. Front ( Fig. 2C ) about 0.3 times carapace width, bilobed, distinctly produced beyond internal orbital angle; lobes separated by V-shaped cleft, which continues on frontal region as shallow groove. Supraorbital margin indistinctly granular, with no clear external orbital tooth. Orbits relatively small, width about 0.2 times carapace width. Eyes with short stalks, smooth; corneas well developed. Anterolateral margin with 4 broadly triangular teeth: first rounded, apices of third tooth at the point of maximum carapace width. Posterolateral margin concave, convergent posteriorly. Median part of posterior carapace margin almost straight.

Antennules folding transversely. Basal antennal segment large, smooth, subrectangular, occupying entire space between antennular fossa, internal orbital angle, filling orbital hiatus; flagellum arising from distal margin, not reaching outer edge of orbit. Posterior margin of epistome with median projection.

Outer surface of third maxillipeds smooth ( Fig. 2D ). Merus subquadrate, median length about half that of ischium, with 2 shallow depressions on either side of low, submedian, smooth ridge. Margins smooth, anterior, internal and external margins slightly concave. Ischium subrectangular, inner margin with short, stiff setae; with shallow, longitudinal submedian groove. Exopod smooth, tapering toward distal end, falling short of anterior edge of merus; flagellum long.

Surface of thoracic sternum smooth, anterior region elongate. Sternites 1–2 completely fused to form triangular plate. Sternites 3–4 completely fused, with suture visible at sternite edges and represented by a transverse furrow; sternite 4 with oblique depression on either side of median. Intersternal sutures depressed, giving sternites 5–7 a raised appearance. Sterno-abdominal cavity deep; anterior limit reaching to imaginary line joining posterior edges of cheliped coxae; tubercle for abdominal locking mechanism on sternite 5 slightly nearer to suture with sternite 4.

Chelipeds similar, subequal. Fingers ( Fig. 2F ) shorter than palm, cutting edges with 4 teeth, brown, tips pointed. Dactylus slightly curved, with 3 granulate ridges and deep submarginal groove along length, stiff short setae on upper margin. Fixed finger slightly deflexed with 3 smooth ridges and broad submarginal groove continuing from palm. Palm outer surface with 4 large nodules, with 2 irregular rows of granules near convex proximal-lower margin; inner surface relatively smooth. Carpus short, dorsal and ventral surface smooth, outer surface with 2 prominent round expansions, upper margin with 2 small nodules. Inner surface of fingers, palm and carpus coapted against pterygostomial/hepatic region of carapace. Merus rugose, ventral surface tuberculose, slightly longer than carpus, with rectangular, ventro-distal tooth apposed against carpus.

Ambulatory legs ( Fig. 2E ) smooth, edges with setae; second leg longest, coxa-to-dactylus length about 0.8 times carapace width. Merus subrectangular and flattened in crosssection. Dorsal surface of carpus with one large nodule and serrated anterior edge. Propodus subquadrate, with two smaller nodules on the dorsal surface, short setae; terminates distally in curved chitinous claw.

External surface of male abdomen, telson smooth.

Abdominal somites 1–2 subtrapezoidal, with two shallow longitudinal grooves on either side of central raised region. Somites 3–5 immovably fused, with a transverse depression between somites 3 and 4; lateral margins slightly concave. Somite 6 quadrangular, central region slightly raised, lateral margins slightly concave. Telson subtriangular with rounded tip, lateral margins relatively straight, median length about 0.7 times basal width, about 0.7 times shorter than penultimate somite.

G1 not observable, G2 very reduced in size, without setae or spines.

Colour in preservative. Carapace pale rose with pale orange patches in lateral part of 5L, 1R, bottom of 4M, medial part of 2M and 2L, and the anterolateral edges of anterolateral teeth; ambulatory legs, chelipeds and abdomen pale rose.

Remarks. As the material described herein is limited to a single small juvenile male, we hesitate to describe it as a new species at this stage. Euxanthus sp. is similar to Euxanthus ruali Guinot, 1971 and Euxanthus herdmani Laurie, 1906 which both have four teeth on the anterolateral margin of the carapace and a smooth dorsal surface. The material is most similar to E. herdmani described from the Philippines (see figure 1D, Mendoza and Ng, 2010), but can be distinguished by: the carapace shape which is narrower in E. herdmani (Cb/Cl holotype = 1.33) than Euxanthus sp. (Cb/Cl= 1.41); anterolateral teeth that are acute and noticeably larger in Euxanthus sp. than E. herdmani ; the frontal part of the carapace, in which the frontal and orbital lobes are of similar size in E. herdmani , while in Euxanthus sp. the frontal lobes are distinctly larger than the orbital lobes. Euxanthus sp. can be distinguished from E. ruali from New Caledonia and Japan, by the anterolateral teeth which are unequal in size (last two larger) in E. ruali , and subequal in Euxanthus sp.

Figures

Figure 1. A, Actaeodes mutatus Guinot 1976, female, 18.1 X 11.5 mm (NMV J46921); B, Paractaea rufopunctata plumosa Guinot, in Sakai, 1976, male, 29.1 X 19.7 mm (NMV J46922); C, Euxanthus sp., male, 8.12 X 5.83 mm (NMV J46923); D, Liomera edwardsi Kossmann, 1877, male, 9.37 X 5.83 mm (NMV J46927); E, Liomera margaritata, male, 16.3 X 9.8 mm (NMV J46928); F, Atergatopsis granulata A. Milne Edwards, 1865, female, 19.6 X 12.9 mm (NMV J46929); G, Atergatopsis alcocki, male, 32.4 X 22.4 mm (NMV J46930); H, Lophozozymus dodone (Herbst, 1801), male, 16.7 X 10.4mm (NMV J46931). Photos: T. Iwasa-Arai.

Figure 2. Euxanthus sp., male (NMV J46923) (7.9 X 5.6mm): A, carapace, dorsal view; B, carapace, ventral view; C, frontal view; D, third maxilliped; E, fourth right leg; F, right cheliped.