Arxellia thaumasta, Vilvens, Claude, Williams, Suzanne T. & Herbert, David G., 2014

Vilvens, Claude, Williams, Suzanne T. & Herbert, David G., 2014, New genus Arxellia with new species of Solariellidae (Gastropoda: Trochoidea) from New Caledonia, Papua New Guinea, Philippines, Western Australia, Vanuatu and Tonga, Zootaxa 3826 (1), pp. 255-281 : 274-276

Arxellia thaumasta n. sp.

( Figs 8 , 75–82 , Table 6 )

Clade A, sp. 6— Williams et al., 2013.

Type material. Holotype (7.2 x 6.2 mm) WAM S25773 . Paratypes: 1 WAM S32251 , 1 NHMUK 20140010.

Type locality. Western Australia, Perth Canyon, CSIRO RV 'Southern Surveyor', stn SS1005/012, 31.92– 31.92°S, 115.01– 115.02 °E, 479– 484 m.

Material examined. Western Australia. CSIRO RV 'Southern Surveyor': stn SS1005/012, 31.92°S, 115.02°E, 479–484 m, 1 lv (holotype WAM S25773 ).—Stn SS1005/095, 28.49°S – 28.50°S, 113.42°E – 113.43°E, 416–431 m, 2 dd (paratypes WAM S32251 and NHMUK 20140010).

Distribution. Western Australia, 431–479 m, living at 479– 484 m.

Description. Shell: Size moderate for genus (height 7.2 mm, width 6.3mm), conical, slightly higher than wide; height 1.2x width, 3.9x aperture height; periphery subangular. Protoconch paucispiral, diameter approx. 320–330 µm, rounded, with minute, crisp granules on first quarter and 5 thin spiral threads; terminal lip straight, without varix. Teleoconch of up to 5.5 convex whorls; shoulder oblique with angulate rim on 3 first whorls, weaker and horizontal on later whorls; early whorls with up to 7 smooth to subgranular spiral cords, last whorl smooth. Suture canaliculate on first two whorls, not so on subsequent whorls. First whorl sculptured by 5 spiral cords present from start, the most abapical one close to suture; P1 forming rim of shoulder; microscopic axial threads between cords; intervals between threads approximately 1.5–2.0x width of threads. On second whorl, P1 slightly stronger than other cords; secondary cord S2 arising between middle (holotype) and end (paratype) of whorl; P5 level with suture. On third whorl, P1 and P4 strongest of which P1 clearly stronger than P4; P4 forming suprasutural carina; secondary S1 and/or S3 possibly arising between start and middle of whorl; all cords except P1 and P4 evanescing and hard to distinguish at end of whorl; prosocline axial folds developing at beginning of whorl, stronger on subsutural area and adapical part, rendering P1 beaded and P4 subgranular; axial threads fading. On fourth whorl, all cords evanescing, P1 the last visible. Last whorl smooth. Aperture subelliptic to almost rounded, sometimes fractionally oblique; peristome incomplete; outer and inner lips moderately thick at rim; inner lip slightly projecting into umbilicus. Columella concave, its edge slightly reflected and overlapping umbilicus; a small flange at its base. Base moderately convex, essentially smooth, but with faint, thin axial threads, stronger nearer to umbilicus and producing beads on a strong spiral cord at rim of umbilicus. Umbilicus deep, relatively wide (diameter ca. 18% of shell width), its rim angulate; internal wall weakly convex, lacking spiral cords, but with numerous, very thin axial threads.

Colour: Early teleoconch whorls nacreous grey to whitish-grey; subsequent whorls greyish-blue to greyishwhite; protoconch white.

Remarks. Molecular data clearly show that this species belongs to Arxellia ( Williams et al. 2013) , but given the morphology of the shell this seems surprising, particularly in view of the reduced sculpture and almost smooth last adult whorl. However, the ontogeny of the spiral cords, with a strong abapical peripheral cord (P4) and a strong subsutural spiral cord (P1) are in fact features consistent with those of Arxellia .

In the abovementioned molecular phylogeny, A. thaumasta was shown to be genetically very similar to a species known only from a single specimen from the Philippines. We have kept the two as separate species given that they differ morphologically and come from widely disjunct localities. The other species (Clade A, sp. 5 in Williams et al. 2013) has the beaded spiral cords typical of other Arxellia species. This species is not described here, as it is known only from a single specimen, which has been misplaced. In molecular analyses A. thaumasta and Clade A sp. 5 form the sister group to all other Arxellia species ( Williams et al. 2013).

Another specimen (WAM S25779 , Figs 83–84), from off Point Hillier, Western Australia (CSIRO RV 'Southern Surveyor', stn SS1005/020, 35.3818°S, 117.2030°E to 35.3822°S, 117.1920°E, 419–460 m), is conchologically close to A. thaumasta , but differs most notably in that P1 remains present on the last whorl as a beaded subsutural cord. In addition, S2 arises much earlier (middle of first whorl), and the protoconch is slightly larger (diameter 380 µm). We have no molecular data for this specimen. Thus, it is impossible, based on a single specimen, to assert whether this is an aberrant form of A. thaumasta or a distinct species and therefore we refer to it here as A. cf. thaumasta .

Etymology. Surprising, unexpected, extraordinary (Greek: θυΜστζ, ον, adjective)—with reference to the smooth whorls of this unusual Arxellia species.

Figures

FIGURES 1 – 9. Arxellia new species, all holotypes except A. tenorioi. 1, A. trochos, n. sp., type species of Arxellia n. gen. 2, A. boucheti n. sp. 3, A. herosae n. sp. 4, A. helicoides n. sp. 5, A. tenorioi n. sp. 6, A. tracheia n. sp. 7, A. maestratii n. sp. 8, A. thaumasta n. sp. 9, A. erythrea n. sp. Scale bar = 5 mm.

FIGURES 75 – 82. Arxellia thaumasta n. sp., Western Australia. 75 – 79, Perth Canyon, holotype WAM S 25773, 479 – 484 m [CSIRO RV ' Southern Surveyor', stn SS 1005 / 012]. 75 – 77, shell, 7.2 x 6.2 mm. 78, details of protoconch and early whorls. 79, details of columella, umbilicus and aperture. 80 – 82, paratype WAM S 32251, 416 – 431 m [CSIRO RV ' Southern Surveyor', stn SS 1005 / 095], 6.9 x 6.3 mm. 83 – 84, Arxellia cf. thaumasta n. sp., WAM S 25779, Western Australia, off Point Hillier, 419 – 460 m [CSIRO RV ' Southern Surveyor', stn SS 1005 / 020], 6.8 x 6.4 mm. Scale bar = 5 mm, except fig. 78: 100 µm, fig. 79: 200 µm.

Tables

TABLE 6. Arxellia thaumasta n. sp.: shell dimensions for type specimens.

	TW	H	W	HA	H/W	H/HA H/TW
holotype WAM S25773	5.5	7.2	6.2	2.4	1.16	3.00 1.31
paratype WAM S32251	5.5	6.9	6.3	1.8	1.10	3.83 1.25
paratype NHMUK 20140010	5.4	6.3	5.7	1.6	1.11	3.94 1.17