Mikro crassus, Herbert, 2024

Herbert, David G., 2024, The Vetigastropoda (Mollusca) of Walters Shoal, with descriptions of two new genera and thirty new species, European Journal of Taxonomy 923, pp. 1-119 : 81-83

Mikro crassus sp. nov.

urn:lsid:zoobank.org:act:3CE83169-0B5E-4678-85EA-1150A6EA8C60

Fig. 33

Diagnosis

Shell globose-turbiniform, white, thick, H/D ± 1.0, whorls rounded with distinct shoulder delineated by rounded spiral cord, beginning immediately after protoconch, becoming less prominent on last adult whorl; umbilicus narrow, bordered by two well-defined spiral cords, with two finer spiral cords within it; whorls otherwise smooth and glossy; aperture subcircular; basal lip weakly notched by cords bordering umbilicus; outer lip shallowly concave between suture and shoulder; interior of aperture somewhat thickened with opaque, chalky layer (?nacre); protoconch smooth.

Etymology

From the Latin ” crassus ” – “thick”; in reference to the thickness of the shell.

Material examined

Holotype

WALTERS SHOAL • empty shell; south plain, stn CP4913; 33°52′ S, 44°05′ E; depth 1539–1615 m; 11 May 2017; MNHN-IM-2000-38066 .

Paratypes

WALTERS SHOAL • 14 empty shells; same collection data as for holotype; SEM shell; MNHN- IM-2000-38067 .

Description

SHELL. Small (diameter up to 3.5 mm), relatively thick, globose-turbiniform, height and width similar (apex missing in most specimens); whorls rounded with distinct shoulder delineated by a rounded spiral cord, middle of shoulder shallowly sunken; teleoconch of up to 3.5 whorls; shoulder cord begins immediately after protoconch as low, broad rib and persists throughout, but becomes broader and less prominent on last adult whorl; periphery and base evenly rounded, periphery at mid-whorl; umbilicus narrow, bordered by two well-defined spiral cords, inner of which forms umbilical rim and ends at junction of columella and basal lips; outer one peri-umbilical, ending at lowest point of basal lip; interval between these cords distinctly concave; two additional, finer spiral cords descending steeply within umbilicus, ending on columella lip, inner one weaker; whorls otherwise smooth and glossy, sculptured only by extremely fine collabral growth-lines. Aperture subcircular, prosocline, peristome interrupted in parietal region; columella lip duplex, with thin outer layer protruding slightly beyond thicker inner layer; basal lip thickened and weakly notched at termination of outer two cords bordering umbilicus; outer lip simple, notched at end of shoulder cord and shallowly concave between this and its insertion on penultimate whorl; interior somewhat thickened, inner layer opaque and chalky.

PROTOCONCH ( Fig. 33F ). Comprising just less than one whorl, globose and slightly exsert, diameter ± 330 μm; smooth save for an extremely fine superficial granulation; terminal lip straight, slightly flared.

COLOUR. Uniformly milky-white; inner shell layer opaque and chalky. Some individuals partly or almost completely covered with lustreless dark brown coating ( Fig. 33D–E ).

DIMENSIONS. Holotype, height 2.6 mm (apex missing), diameter 2.9 mm; largest specimen, diameter 3.5 mm.

Distribution

Known only from the plain surrounding Walters Shoal, at a depth of 1539–1615 m; dredged on coarse sandy substrata; living specimens unknown.

Remarks

Mikro crassus sp. nov. is considerably larger than any of the other species currently referred to the genus (most with height <1.5 mm). The most similar species is M. hattonensis Hoffman, Van Heugten & Lavaleye, 2010 from the north Atlantic, but that species has deeper spire whorls and thus a more elevated spire, and it has an unusual pitted (shagreened) micro-sculpture on the shoulder and in the umbilicus. There is also some resemblance to an unidentified species of Moelleriopsis Bush, 1897 from Réunion figured by Warén (1992: fig. 31a), but that has a less exsert protoconch with distinct spiral sculpture and a strongly channelled suture ( Warén 1992: fig. 26c).

The 15 shells of Mikro crassus sp. nov. available are all damaged to some extent and most are juvenile. They all came from a single station and many evince the remains of a dark brown periostracum-like superficial deposit ( Fig. 33D–E ), possibly of a ferruginous nature, suggesting that the species may live in an unusual and highly specialised microhabitat associated with anoxic conditions ( Warén & Bouchet 1989; Warén 1991). A similar superficial layer is evident in the Antarctic Lacuna abyssicola Melvill & Standen, 1912 , a species which Engl (2012) referred to Moelleriopsis on account of “the shape and sculpture of the shell”, at the same time renaming it Moelleriopsis poppei Engl, 2012 on account of secondary homonymy with the type species of the genus. However, details of protoconch morphology were not provided and so a comparison with the characteristic protoconch of Moelleriopsis detailed by Warén (1992) is not possible. Further confirmation of this generic transfer is needed. The protoconch of Mikro crassus sp. nov. does not exhibit the spiral ribs and channelled suture distinctive of that of Moelleriopsis .

Figures

Fig. 33. Mikro crassussp. nov. A–B. Holotype, apertural and oblique basal views (apex missing), diameter 2.9 mm (MNHN-IM-2000-38066). C. Apical view of juvenile specimen with intact apex, diameter 1.5 mm (paratype, MNHN-IM-2000-38067). D–E. Specimen with brown superficial deposit, diameter 1.7 mm (paratype, MNHN-IM-2000-38067). F. Protoconch of C, scale bar =100 μm. G. Composite image of holotype superimposed on apex of C, diameter 2.9 mm.