Xenograpsus ngatama, Mclay, Colin, 2007

Mclay, Colin, 2007, New crabs from hydrothermal vents of the Kermadec Ridge submarine volcanoes, New Zealand: Gandalfus gen. nov. (Bythograeidae) and Xenograpsus (Varunidae) (Decapoda: Brachyura), Zootaxa 1524, pp. 1-22 : 15-19

Xenograpsus ngatama sp. nov.

( Figs. 6 A–F, 7A–E, 8A–B, 9A–B)

Type material. Holotype: male, 16.0 x 15.6 mm, Macauley Caldera, 30º2.06’S, 181º17.36’E, 161 m, stn KOK0505/43, 15.04.2005 ( NIWA 18013). Paratypes: 1 male, 19.7 x 19.0 mm, Macauley Caldera, 30º2.16’S, 181º17.63’E, 109 m, stn KOK0505/45, 16.04.2005 ( NIWA 18022); 1 female, 12.8 x 11.7 mm, Macauley Caldera, 30º2.01’S, 181º17.37’E, 156 m, stn KOK0505/49, 17.04.2005 ( NIWA 18021).

Other material. 5 males, 11.0 x 10.5, 11.4 x 11.0, 13.0 x 12.1, 14.0 x 13.5, 15.2 x 14.4 mm, Macauley Caldera, 30º2.01’S, 181º17.37’E, 156 m, stn KOK0505/49, 17.04.2005 ( NIWA 18020); 1 male, 27.1 x 25.8 mm, 1 male, missing carapace 44.1 x 42.0 (estimated), 1 female damaged, 34.8 x 33.1 mm, Brothers Seamount, 35º44.22–44.04’S, 178º29.72–29.63’E, 270– 239 m, 21.05.2001 ( NIWA); 2 males 16.8 x 15.3, 17.8 x 16.5 mm, Northwest of Kermadec Ridge, 25º53.4153.61’S, 177º11.10–11.07’E, 139–236 m, stn TAN0411/10, 0 4.10.2004 ( NIWA 18014).

Etymology. The specific name “ ngatama ” is the Maori word for “brother”, in reference to the fact that the new species occurs on a local oceanic feature, the Brothers Seamount. For pronunciation, the “g” is silent but the combination “nga” gives the name a long “aa” sound at the beginning.

Description. Carapace quadrate, wider than long, surface strongly convex, punctate with some areas of coarse granules. Cervical and gastro-cardiac grooves well marked. Epigastric cristae weakly defined, other epigastric and protogastric granules of variable size tending to be arranged in oblique lines; row of granules behind cervical groove and short diagonal granulated ridge near posterolateral corner of carapace. Frontal margin convex on each side of central notch, minutely granulated. Orbital margin granulate, deeply concave, exposing most of orbit to dorsal view; tuberculate swelling above inner orbital angle. External orbital angle continuing as finely granulate anterolateral margin to a small tooth at widest point. Lateral margins parallel until corner where it angles across base of P5 to straight posterior margin. Branchial area with finely granulated oblique striae near posterolateral corner.

Front 0.34CW; interantennular septum complete. Orbit divided in half by angled keel that meets antenna. Antennules folding horizontally; first article filling most of antennal fossa; second and third articles elongate, of similar length; mesial and lateral flagella of similar length. Antenna of five articles plus flagellum; first two articles fixed against strong suborbital projection; distal margin of second article with projecting granulate rim; third to fifth articles mobile, decreasing in size followed by flagellum; length of flagellum and mobile part length 0.1CW.

Eyestalk short, thick, cornea well developed occupying whole of ventro-distal area. Sub-orbital margin begins at postorbital corner, granulate, concave to deep incision terminating in strong suborbital projection, visible dorsally, whose edge is also granulate. Shelf-like granulate ridge beneath suborbital margin, beginning under suborbital projection and extending for short distance beyond postorbital corner beneath anterolateral carapace margin.

Endostome concave; granulated dorsal margin scalloped to form ventral floor of antennal fossa; narrow suture marking attachment to epistome; granulated posterior margin of epistome concave, divided in two by median keel. Epimeral line well-marked; strong granulated shelf at anterior corner of buccal frame. Third maxillipeds closing without median gap, but not reaching epistome anteriorly; ischium smooth, longer than wide ratio 1.35, merus concave cristate granulate especially along distal margin, slightly wider than long, covering end of exopod laterally; palp inserted near disto-lateral corner of merus carpus running across most of distal margin of merus, palp just reaching ischiomerus joint.

Chelipeds well developed, left and right similar size. Anterior margin of basis-ischium cristate. Merus triangular in cross section; borders granulate; outer and upper surface of carpus granulate; propodus granulate, upper borders marked by row of granules; some granules on outer face of propodus arranged in 3 or 4 rows which can extend on to the fixed finger; granules on upper surface of dactyl arranged as 3 rows converging distally. Fingers curved inward and downward, hollowed out mesially, gaping mid-way but meeting at tips; proximal two-thirds with 8 or 9 granular teeth; distal third margins scissor-like. Tips of fingers horny and covered in dense brush of fine setae.

Walking legs (P2-P5) stout, depressed, margins granulate, without setae; third pair longest. Meri widening distally, surface marked by transverse striae of minute granules; P5 merus length 2.3 times width; carpi, propodi and dactyli with longitudinal rows of larger granules, especially on upper and lower margins. Dactyli short, ending in stout horny claw.

Male abdomen surface smooth, narrow, length 1.3 times width; segments narrowing and becoming longer posteriorly; telson length 1.5 times width, tip narrow, rounded. Abdominal locking mechanism of sternal tubercle on sternite 5 fitting into pit on posterolateral corners of last abdominal segment functional in holotype male, but not functional in smaller male (CW = 17.3 mm) (in this animal the sternal tubercles are present but do not engage with the last abdominal segment). Abdominal cavity extending across two-thirds of cheliped sternite, but abdomen not extending as far. Proximal margin of abdominal cavity raised as finely granulated ridge. Sternal surface minutely punctate, smooth, sutures 4/5 to 7/8 not meeting in midline.

Mature female abdomen covering entire sternal surface; length 0.87 times width; segments becoming longer posteriorly; fifth segment widest, margins fringed with soft setae. Telson length 0.3 times width, tip evenly rounded. Gonopore large, standing out from sternal surface entrance closed by calcified operculum, hinge line not flexible in paratype female. Three well-developed pairs of biramous pleopods.

Male first gonopod stout, almost straight, not twisted, length 5.3 times width; aperture terminal, tip horny concealed by soft setae. Second gonopod curved, much shorter than first (ratio 0.3), length 4.2 times width, tip soft and blunt.

Colour. Pale green-grey ground colour with brown bands on pereopods and brown patches on the carapace. These colours were observed on specimens examined and in photos taken at the time of collection.

Discussion. The three species of Xenograpsus are quite distinct from each other. Xenograpsus novaeinsularis from Japan and Marianas has well developed epigastric cristae, one prominent protogastric tubercle, anterolateral tooth present, oblique granulate crista absent on carapace above base of P5 and cervical groove prominent. Xenograpsus testudinatus Ng, Huang & Ho, 2000 from Taiwan has the epigastric cristae well developed, one prominent protogastric tubercle, anterolateral tooth absent, oblique granulate crista present on carapace above base of P5 and cervical groove not prominent; while X. ngatama from New Zealand has epigastric tubercles not arranged in cristae, many large protogastric tubercles, anterolateral tooth present, oblique granulate crista present on carapace above base of P5 and a prominent cervical groove.

Xenograpsus ngatama has been recorded from five sites on the Kermadec Ridge between 25º53’S – 35º44’S and 181º17’E – 177º11’E. The maximum size for males is 44.1 x 42.0 mm and for females 34.8 x 33.1 mm. The depth range is 109– 270 m. Videos taken at the time of sampling showed X. ngatama crawling over and around the mussels, Gigantidas gladius (Ashley Rowden, pers. com.). Some of the specimens were completely covered with a rusty brown coating. A spectral EDAX analysis in an SEM showed that this coating is mostly composed of iron oxides. Evidently this crab can live very close to actively venting areas, but others, lacking this coating, live further away. Xenograpsus novaeinsularis can live at temperatures around 30ºC, much higher than the surrounding water ( Takeda, Takeuchi & Suganuma 1993; Türkay & Sakai 1995). Jeng et al. (2004) described the peculiar method of feeding by X. testudinatus whereby they swarm near vents, consuming the zooplankton “rain” killed by toxic effluent, catching their prey using their setose chelipeds. These crabs are evidently obligate vent dwellers capable of living in this inimical environment. In contrast to bythograeids these crabs still have fully developed eyes, presumably because they are more recent colonists. Unique larval characters of X. testudinatus suggest that the genus Xenograpsus will probably need to be placed in a separate family or subfamily (Jeng, Clark & Ng 2004).

Figures

FIGURE 6. Xenograpsus ngatama sp. nov., holotype male, 16.0 x 15.6 mm, Macauley Caldera (NIWA 18013). A, dorsal view whole carapace (scale bar = 5 mm). B, magnified right anterolateral margin (scale bar = 2 mm). C, outer face right cheliped (scale bar = 2 mm). D, dorsal view right cheliped (scale bar = 2 mm). E, left third maxilliped (scale bar = 2 mm)., F, posterior view right P 4 and P 5 (scale bar = 5 mm);