Euryxanthops chiltoni Ng & Mclay, 2007, new species

Euryxanthops chiltoni, new species

Figs. 1–3

Material examined: Holotype: female, 31.2 by 22.4 mm, station DR02A/16, McCauley Caldera, Kermadec Islands, New Zealand, 30º12.8’S, 178º02’W, 200 m, coll. R.V. Tangaroa, 10 May 2002. Paratype: 1 ovigerous female, 31.4 by 21.3 mm, same data as holotype.

Description of holotype female. Carapace distinctly broader than long, ratio 1.4 – 1.5, regions distinct, with grooves demarcating gastric regions relatively deeper, distinct, 1–4M well defined, 2M entire, without longitudinal or transverse grooves; anterior part of 3M elongate, distinct; 1–4L discernible; 5L, 6L indistinct; 1–3R demarcated as one region; 1P, 2P one region; dorsal surface distinctly granular, with those on anterior parts relatively larger; fronto-orbital region (1F, 2F) marked by shallow depression. Front produced slightly beyond orbits; weakly bilobed, truncate lobes separated by small, shallow cleft; margin gently convex, granular; lateral frontal lobe small, triangular, separated from frontal lobes by deep notch. Orbits oblique, small; cornea well developed; ocular peduncle with several tubercles. Supraorbital margin concave, granular with distinct fissure on outer two-thirds of margin; external orbital tooth very low, lobiform, broadly triangular, not sharp, inner margin gently concave, outer margin gently convex, margins granulated. Infraorbital margin granular, with low proximal tooth, just visible from dorsal view; rest of margin concave, granulated. Anterolateral margin convex, with numerous granules; with four lobiform teeth (first extends from external orbital corner), first to third progressively more acute, smaller, directed laterally; margins sub-cristate. Posterolateral margin granular, gently concave, converging towards gently sinuous posterior margin of carapace. Antennules folding transversely. Antennal flagellum short, passes through open orbital hiatus; basal article relatively short, subrectangular, surface granular. Posterior margin of epistome sinuous, with median fissure. Endostome without discernible ridges. Sub-orbital, sub-hepatic, pterygostomial, sub-branchial regions prominently granular.

Outer surface of third maxilliped finely granulose. Ischium subrectangular, with shallow submedian longitudinal sulcus, separated from basis by shallow suture. Merus quadrate, medially depressed, antero-external margin angular but not prominently produced outwards, not covering exopod. Exopod relatively stout proximally, distal part reaching to about three-quarters length of merus; flagellum distinct.

Chelipeds asymmetrical; outer surfaces of merus, carpus, chelae granular to rugose. Carpus sub-hexagonal, with uneven granular ridge on outer surface; inner distal angle dentiform, prominent. Merus short, trigonal in cross-section; dorsal margin with low but prominent sub-distal tooth. Chelae stout; fingers two-thirds length of palm, pigmented black; with uneven, low teeth on cutting edges. On larger right chela, outer surface of dactylus with two longitudinal pitted grooves; pollex with a deep sub-marginal groove which runs along length of finger, reaching partially into palm. Grooves on outer surface of fingers of smaller chela similar to that of larger chela but grooves distinctly deeper, more prominent.

Ambulatory legs with second pair longest. Margins of all segments smooth or gently rugose. Merus glabrous, with dorsal margin cristate, distal angle dentiform but not sharp, sub-distal angle present, sharply demarcated from rest of margin by deep notch; ventral margins angular to rounded, not cristate. Carpus relatively short, with distinct shallow longitudinal sub-medial groove, sparsely setose. Propodus ovate to subovate, laterally flattened, setose, unarmed. Dactylus covered with dense short setae, appears velvet-like; straight for most of length except for curved distal corneous part.

Thoracic sternum broad, entire surface finely granular; sternites 1, 2 fused; suture between sternites 3, 4 deep, complete; suture between sternites 3, 4 medially interrupted, only visible laterally. Gonopore without operculum, lunate (1.5 x 0.6mm), scarcely visible in ventral view, only accessible by gonopods that are strongly curved laterally. Abdomen with all segments, telson freely moveable; broadly ovate, covering most of thoracic sternum; margins prominently setose; telson semicircular.

Paratype female. The paratype female is ovigerous but otherwise agrees very well with the holotype female in all key features.

Etymology. The name of the new species of Euryxanthops recognizes the contribution of Charles Chilton, Canterbury College (now Canterbury University), to the study of New Zealand Crustacea as a whole and the Kermadec Is. in particular. Chilton (1911) compiled the first list of the Brachyura from the islands.

Remarks. Although it is unfortunate that E. chiltoni is known only from two adult females, it is nevertheless a distinct taxon, easily distinguished from all congeners. In terms of the anterolateral carapace margin E. chiltoni is closest to E. flexidentatus and E. latifrons by having four well developed and distinct marginal teeth. E. chiltoni most closely resembles E. dorsiconvexus from the Philippines in terms of its granular carapace and chelipeds. Euryxanthops chiltoni, however, has a far more granular carapace and chelipeds, with the granules (and rugae) prominent even to the naked eye, being denser and larger. In addition the anterior margins of the gastric regions of E. chiltoni are marked by a low but prominent crest of granules, absent in E. dorsiconvexus. The carapace of E. chiltoni is also more hexagonal in shape than E. dorsiconvexus, due primarily to the more prominent anterolateral teeth and stronger front. The carpus of the cheliped of E. chiltoni also bears a low but distinct ridge sub-marginally on the dorsal surface, a structure absent on that of E. dorsiconvexus. In addition, the ambulatory legs of E. chiltoni are proportionately stouter and shorter than those of E. dorsiconvexus (see Garth & Kim 1983: Fig. 2).

The size, depth and distribution of the species in this genus are summarized in Table 1.

1. Anterolateral carapace margins evenly convex, with lobes separated only by small notches....................... ................................................................................................................. Euryxanthops cepros Davie, 1997

– Anterolateral lobes well defined, more-or-less distinct............................................................................... 2

2. Front markedly produced beyond carapace outline, a distinct median notch separating two lobes, with margins laterally sloping; 2M anteriorly divided ............................. Euryxanthops orientalis (Sakai, 1939)

– Front not markedly produced, composed of two straight to almost straight lobes, without strong median incision; 2M entire....................................................................................................................................... 3

3. Front relatively broad about c. 0.34 times CW; anterolateral margin behind external orbital angle sloping backwards ............................................................................................. Euryxanthops latifrons Davie, 1997

– Front narrower less than 0.3 times CW; anterolateral margin behind external orbital angle only moderately sloping backwards, nearly horizontal or even projecting slightly forwards................................................ 4

4. Apex of first anterolateral tooth level with, or slightly in advance of rear margin of orbit........................... ........................................................................................... Euryxanthops flexidentatus Garth & Kim, 1983

– Apex of first anterolateral tooth distinctly posterior to rear margin of orbit............................................... 5

5. Anterolateral margin behind external orbital angle moderately sloping backwards..................................... ......................................................................................... Euryxanthops dorsiconvexus Garth & Kim, 1983

– Anterolateral margin behind external orbital angle nearly horizontal to projecting slightly forwards...................................................................................................................... Euryxanthops chiltoni, new species.