Mathildella mclayi, Ahyong, 2008

Ahyong, Shane T., 2008, Deepwater crabs from seamounts and chemosynthetic habitats off eastern New Zealand (Crustacea: Decapoda: Brachyura), Zootaxa 1708 (1), pp. 1-72 : 43-48

Mathildella mclayi sp. nov.

( Figs 19–21 , 29C )

Pilumnoplax sp. — Clark & O’Shea 2001: 15. — Martin & Haney 2005: 451, 495.

Type material. HOLOTYPE: NIWA 6772 , male (cl 13.6 mm, cw 17.5 mm), Mahina Knoll, 37°21.04– 21.26’S, 177°05.96’E, 434–472 m, TAN0413/173, 16 Nov 2004 . PARATYPES: NIWA 6768 , 1 female (cl 6.3 mm, cw 8.2 mm), 37°20.41–20.26’S, 177°06.69–06.81’E, 335– 275 m, TAN0413/129, 14 Nov 2004 ; NIWA 34978 , 1 ovigerous female (cl 13.5 mm, cw 17.9 mm), Mahina Knoll, 37°21.04–21.26’S, 177°05.96’E, 434– 472 m, TAN0413/173, 16 Nov 2004 ; NIWA 6769 , 2 females (cl 4.1–11.9 mm, cw 5.0– 15.3 mm), Mahina Knoll, 37°21.34–21.29’S, 177°05.98–06.22’E, 260–280 m, TAN0413/130, 14 Nov 2004 ; NIWA 6771 , 1 female (cl 10.1 mm, cw 13.1 mm), Mahina Knoll, 37°21.35–21.21’S, 177°06.09–06.08’E, 259–294 m, TAN0413/140, 14 Nov 2004 .

Other material examined. Rumble V seamount , southern Kermadec Ridge: NIWA 27553 , 1 male (cl 10.0 mm, cw 13.5 mm), 36°08.35–08.08’S, 178°11.76–11.95’E, 672– 367 m, TAN0107/235, 24 May 2001 .

Diagnosis. Carapace frontal margin undivided or with indistinct median notch. First (outerorbital) and second anterolateral teeth similar, rounded, fused basally, demarcation varying from distinct U-shaped notch to shallow concavity. Third anterolateral tooth a prominent, anteriorly recurved spine; lateral margin convex. Fourth tooth smaller than second tooth, triangular, broad, apex acute. Fifth tooth minute, blunt, separated from third tooth by shallow notch. Male telson triangular, apex rounded, lateral margins straight; proximal margin wider than distal margin of somite 6. G1 aperture with slightly flared margins; outer proximal margin sinuous but not stepped; distal mesial margin broadly but noticeably convex. Chelipeds fingers black; pigmentation on pollex extending slightly onto manus. P5 dactylus slightly longer than propodus dorsal margin.

Description. Carapace subhexagonal, 1.3 times wider than long; dorsal surface finely granular; regions ill-defined; epigastric, protogastric, hepatic and epibranchial regions slightly swollen; cardiac region flat; with low transverse depression across midlength. Front subtruncate in dorsal view, bimarginate, formed by distinct transverse anterior groove; faintly convex, granular, undivided or with indistinct median notch. Supraorbital margin low; margin with V-shaped notch slightly laterad to midlength. First (outerorbital) and second anterolateral teeth similar, rounded, fused basally, demarcation varying from distinct U-shaped notch to shallow concavity. Third anterolateral tooth a prominent, anteriorly recurved spine; lateral margin convex. Fourth tooth smaller than second tooth, triangular, broad, apex acute. Fifth tooth minute, blunt, separated from fourth tooth by shallow notch. Suborbital margin granular; with blunt inner tooth, visible dorsally. Eyestalks with row of granules on inner distal margin. Suborbital, pterygostomial and sub-branchial regions finely granular. Posterolateral and posterior carapace margins broadly curved to almost straight.

Thoracic sternum surfaces finely punctate. Sternites 2 and 3 well demarcated. Sternites 3 and 4 fused, sutures visible laterally. Press-button of abdominal locking mechanism positioned near posterior margin of sternite 5 occluding with socket on abdominal somite 6. Anterior end of sternoabdominal cavity reaching beyond midlength of sternite 4. Sutures 4/5 and 5/6 medially interrupted. Suture 6/7 and 7/8 entire. Sternites 7 and 8 with median groove. Female gonopore on sternite 5; orifice simple.

Male abdomen with 6 free somites and telson; surface sparsely pitted; lateral margins concave. Somite 1 broad, slender, shortest medially, lateral margins reaching base of P5 coxae. Somite 2 subrectangular, narrower than somite 1. Somite 3 tranversely subtrapezoid, lateral margins angular. Somites 2 and 3 completely filling space between P5 coxae, concealing sternite 8. Somites 4–6 becoming progressively narrower and longer. Telson triangular, broader than long, apex rounded, lateral margins straight; proximal margin wider than distal margin of somite 6. G1 with stout, swollen proximal portion and slender distal portion, medially curving gently outwards towards apex; aperture with slightly flared margins; outer proximal margin sinuous but not stepped; margins of distal one-fifth lined with small spinules; distomesial margin broadly convex. G2 sinuous, longer than G1, apex tapering to sharp point, distal portion shorter than proximal portion.

Female abdomen broad; of 6 free somites and telson; lateral margins convex; surface sparsely pitted. Somite 1 broad, slender, shortest medially, lateral margins reaching base of coxae of fifth ambulatory legs. Somite 2 subrectangular, narrower than somite 1; lateral margins straight. Somite 3 tranversely similar to somite 2 but lateral margins bluntly angular. Somites 1–3 entirely covering space between P5 coxae, sternite 8 not visible. Somites 4–6 becoming progressively narrower and longer. Telson broader than long, rounded.

Third maxilliped minutely punctate; merus subpentagonal, anteroexternal angle rounded; ischium subrectangular, with submedian sulcus; exopod stout, apex with small distal tooth, reaching to distal edge of merus. Lateral surfaces of endostome each with single longitudinal carina.

Chelipeds (P1) unequal. Merus relatively short, trigonal; dorsal margin with convex granular ridge bearing small subdistal spine or tubercle; inner distal margin with flat, rounded lobe at articulation with carpus; distoventral margin with blunt tooth or tubercle; surfaces otherwise appearing smooth but microscopically granular. Carpus with 2 distinct spines on inner distal margin; surface granular; with shallow transverse subdistal sulcus between articular condyles. Major palm stout, unarmed, dorsal surface granular, other surfaces smooth; dorsal margin as long as fingers; fingers with slight gape; occlusal margins bluntly dentate, apices simple, crossing; fingers pigmented black throughout length; pigmentation on pollex extending slightly onto manus. Minor palm rugose to finely granular, more pronounced dorsally; fingers about 1.5 times as long as dorsal margin of palm; occlusal margins bluntly dentate, apices simple, crossing; fingers pigmented black throughout length; pigmentation on pollex extending slightly onto manus; dactylus with dorsal ridge; pollex with shallow submarginal sulcus.

Ambulatory legs (P2–5) relatively long, slender, relative lengths P4>P3>P5>P2; P4 longest (2.65–2.75 cl); P4 ischiomerus slightly longer than cl (1.08–1.14 cl); segments sparsely setose, dorsal margins finely granular (more prominent with increasing size). Dactyli sparsely setose, ventral margins with corneous robust setae, dorsal margin with few small corneous setae, apex corneous. P2–5 dactyli elongate, flattened, almost straight; relative lengths P4>P3>P2>P5. P5 dactylus slightly longer than propodus dorsal margin; 2/3 as long as P4 dactylus.

Etymology. Named for Colin McLay, for his contributions to Indo-Pacific carcinology.

Remarks. Mathildella mclayi sp. nov. is the first representative of the genus from New Zealand waters. It most closely resembles M. maxima Guinot & Richer de Forges, 1981b ( Japan, Loyalty Islands, New Caledonia, French Polynesia) in the combination of the broadly convex versus straight frontal margin with at most a shallow rather than distinct median notch, and in the sinuous rather than stepped mid-outer margin of the G1 ( Fig. 21G ). Mathildella mclayi differs from M. maxima in the extent of black pigmentation of the cheliped pollex ( Fig. 20 D–E ), which does not extend onto the manus (versus extending well onto the manus in M. maxima ), in having a more distinctly triangular male telson (with straight rather than convex lateral margins) that is wider rather than narrower than the distal margin of the sixth abdominal somite ( Fig. 20B , 21F ), in the length of the P5 dactyli (longer rather than shorter than the propodus dorsal margin; Fig. 21E ) and in the morphology of the male G1 ( Fig. 21G ). In the M. maxima , the distomedial margin is relatively straight and the margin of the G1 aperture is distinctly flared forming a prominent lip. The G1 of M. mclayi has instead a low convex ‘lobe’ on the mesiodistal margin and lacks the distinct distal flaring of the G 1 in M. maxima . Like M. rubra Ng & Ho, 2003 ( Philippines) , M. mclayi is readily distinguished from M. serrata ( Sakai, 1974) ( Japan, Taiwan, Philippines) and M. kyushupalauensis Takeda & Watabe, 2004 ( Japan) , in having the black pigmentation of the cheliped pollex extending only slightly, rather than prominently onto the manus. Mathildella mclayi is readily distinguished from M. rubra by the sharper, more prominently falcate, third and fourth anterolateral carapace teeth and proportionally longer P5 dactyli, which are slightly longer rather than shorter than the dorsal margin of the respective propodi.

Ng & Ho (2003) emphasised the degree of demarcation between the first and second anterolateral teeth in distinguishing species of Mathildella , whether fused or distinctly separated. The degree of fusion/separation appears to be stable in most species of the genus ( M. serrata , M. rubra , M. kyushupulauensis ), but is apparently variable in M. maxima (see Takeda & Watabe 2004), as it is in M. mclayi . In M. mclayi , the first and second anterolateral teeth are widely separated in the holotype ( Fig. 21A ), shallowly demarcated in one paratype (female, NIWA 6769), and intermediate in the remaining paratypes ( Fig. 21K–N ). The third anterolateral tooth is always the most prominent with a convex outer margin and sharp apex, but the size of the fourth anterolateral tooth is apparently size related, ranging from a low but distinct tooth in the smallest specimens (NIWA 6771, 27553) to a prominent tooth of similar size to the preceding tooth in the largest specimen (holotype, NIWA 6772). As observed by Ng & Chan (2000), the granulation of the dorsal margins of the ambulatory legs is size related, becoming more pronounced with increasing size.

Of the known deepwater Brachyura from New Zealand, M. mclayi could be confused with Neopilumnoplax nieli sp. nov., Pycnoplax meridionalis ( Rathbun, 1923) and P. victoriensis ( Rathbun, 1923) . The two species of Pycnoplax are readily distinguishable from M. mclayi by the presence of only two rather than four anterolateral teeth behind the outerorbital tooth. Additionally, the most common Pycnoplax in New Zealand waters, P. victoriensis , has pale rather than black cheliped fingers ( Fig. 15B , 29B ). The strong similarity between Mathildella and Neoplimunoplax means that M. mclayi is most likely to be confused with N. nieli . The two species are readily distinguished by the epibranchial ornamentation of the carapace (a slight swelling in M. mclayi ; a distinct ridge in N. nieli ; Fig. 19B , 22B ); the carination of the endostome (one lateral carinae in M. mclayi ; two in N. nieli ; Fig. 21C , 24C ); and overall colouration in which M. mclayi is uniformly deep orange rather than diffusely pale orange ( Fig. 29C–D ).

Distribution. Presently known only from seamounts on the southern Kermadec Ridge, northeastern New

Zealand; 259–672 m.

Figures

FIGURE 15. A, Pycnoplax meridionalis (Rathbun, 1923), male, cl 13.2 mm (NIWA 6643). B, Pycnoplax victoriensis (Rathbun, 1923), female, cl 10.5 mm (NIWA 29349). C–G, Intesius richeri Crosnier & Ng, 2004, female, cl 15.5 mm (NIWA 27549). C, carapace. D, ventral. E, right chela. F, right cheliped carpus. G, right cheliped merus and carpus.

FIGURE 19. Mathildella mclayi sp. nov., male, cl 13.6 mm, cw 17.5 mm (NIWA 6772). A, dorsal habitus. B, carapace, dorsal view.

FIGURE 20. Mathildella mclayi sp. nov., male, cl 13.6 mm, cw 17.5 mm (NIWA 6772). A, anterior. B, ventral. C, right chela. D, right cheliped carpus and merus. E, left chela.

FIGURE 21. Mathildella mclayi sp. nov. A–H, male holotype, cl 13.6 mm, cw 17.5 mm (NIWA 6772). I–K, ovigerous female paratype, cl 13.5 mm, cw 17.9 mm (NIWA 34978). L, female paratype, cl 11.9 mm, cw 15.3 mm (NIWA 6769). M, female paratype, cl 10.1 mm, cw 13.1 mm (NIWA 6771). N, female paratype, cl 6.3 mm, cw 8.2 mm (NIWA 6768). A, K–N, carapace outline, right anterior and anterolateral margin. B, suborbital margin, ventral. C, endostome, right half. D, right third maxilliped. E, right P5. F, I, abdomen. G, right G1, abdominal view. H, right G2, abdominal view. J, female gonopores. Scale A–B, D–F, I–N = 2.5 mm, C = 1.6, G–H = 1.3 mm.

FIGURE 22. Neopilumnoplax nieli sp. nov., male holotype, cl 14.7 mm, cw 19.0 mm (NIWA 29364). A, dorsal habitus. B, carapace, dorsal view.

FIGURE 24. Neopilumnoplax nieli sp. nov. A–G, male holotype, cl 14.7 mm, cw 19.0 mm (NIWA 29364). H–I, female paratype, cl 11.8 mm, cw 15.8 mm (NIWA 29367). J, female paratype, cl 5.9 mm, cw 7.5 mm (NIWA 29371). K, female paratype, cl 7.0 mm, cw 9.1 mm (NIWA 29366). L, male paratype, cl 8.3 mm, cw 10.9 mm (NIWA 29366). M, ovigerous female paratype, cl 10.0 mm, cw 13.1 mm (NIWA 29366). N, ovigerous female, cl 10.3 mm, cw 13.5 mm (NIWA 29360). O, male, cl 15.0 mm, cw 19.7 mm (NIWA 29651). A, J–O, carapace outline, right anterior and anterolateral margin. B, right third maxilliped. C, endostome, right half. D, H, abdomen. E–F, right G1, abdominal view and sternal view of apex. G, right G2, abdominal view. I, female gonopores. Scale A, D, H–O = 2.5 mm; B, C = 2.0 mm; E, G = 0.8 mm. CX5 = coxa of P5. ST8 = sternite 8.

FIGURE 29. A, Pycnoplax meridionalis (Rathbun, 1923), male, cl 20.9 mm (NIWA 29658). B, Pycnoplax victoriensis (Rathbun, 1923), male, cl 10.4 mm (NIWA 6736). C, Mathildella mclayi sp. nov., female paratype, cl 11.9 mm (NIWA 6769). D, Neopilumnoplax nieli sp. nov., male paratype, cl 15.0 mm (NIWA 29651). E, Miersiella haswelli (Miers, 1886), male, cl 7.8 mm (NIWA 6760). F, Miersiella haswelli (Miers, 1886), ovigerous female, cl 7.1 mm (NIWA 6760). Photo credits: C. Marriot (A, D), R. Stewart (B–C, E–F).