Carinastele achrosta, 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 : 73-74

Carinastele achrosta sp. nov.

urn:lsid:zoobank.org:act:80AE5B57-5CCA-4EDF-9BAC-47766274BE71

Fig. 29D–F, H

Diagnosis

Shell elevated-turbiniform, height up to 8.7 mm; height> diameter; base narrowly umbilicate; protoconch hexagonally reticulate; teleoconch sculpture of strong spiral cords, nodular on spire whorls; P3 forms carina-like shoulder cord on apical whorls, but subsequently migrates downward and becomes peripheral cord, a strong S3 not present below P3; additional secondary and tertiary cords develop on later whorls, only 2–3 uppermost ones nodular; crests of lower cords with shallow median furrow; base with 10 rounded spiral cords; shell uniformly whitish.

Etymology

From the Greek “ achrostos ” (αχρόστος) – “uncoloured”; in reference to the unpigmented shell.

Material examined

Holotype

WALTERS SHOAL • empty shell; slopes, stn DW4899; 33°09′ S, 44°02′ E; depth 707–720 m; 6 May 2017; MNHN-IM-2000-35705 .

Description

SHELL. Relatively small for the genus (height 8.7 mm), elevated-turbiniform (H/D=1.18); spire prominent (1.35 ×height of aperture), with indented suture; whorls evenly rounded, sculptured by strong spiral cords, the adapical ones nodular; umbilicus narrow; teleoconch of nearly 5.0 whorls. First teleoconch whorl with an axial growth flaw shortly after terminal protoconch varix (at 0.1–0.2 whorl), soon developing a prominent spiral cord midway between sutures, creating a distinct shoulder; two finer spiral lirae between this and adapical suture; shoulder cord becomes carina-like during second whorl (= P3 of Marshall 1988), and lirae above it strengthen to become cords (P1 and P2) ( Fig. 29H ); all spiral cords with low, undulant coronations, those above shoulder linked by broad, low, axial riblets; interval between shoulder carina and abapical suture relatively wide and deeply concave; an additional spiral cord (S1) intercalates between subsutural cords (P1 and P2) during third whorl and P3 becomes less carina-like, migrating down whorl to become peripheral cord; start of fourth whorl with four distinct spiral cords (P1, S1, P2, P3), increasing slightly in size from subsutural one to peripheral one and also becoming less obviously nodular; abapical suture level with and concealing a fifth cord (P4); P4 emerges above suture during fourth whorl, revealing a sixth cord (P5) level with abapical suture; two additional intermediary spiral lirae (T1 and T2) arise during fourth whorl, T1 between P1 and S1 and T2 between S1 and P2; P5 emerges above suture just prior to start of last whorl; at this point adapical part of shell has four strong spiral cords above and including periphery (P1, S1, P2, P3) and two below periphery (P4 and P5), with a further two weaker cords (T1 and T2) in intervals 2 and 3; uppermost cord on last whorl (P1) retaining nodules, but nodules weaker and more uneven on cords below this, those near periphery smooth; profile of cords more rounded on fourth and fifth whorls, their crests with traces of shallow median furrow; cord intervals roundly concave, slightly wider than cords themselves; axial sculpture beyond third whorl comprising only close-set, microscopic, collabral growth-lines, most obvious in cord intervals. Base with 10 evenly-spaced, spiral cords, eight of which originate below insertion of outer lip, the last narrower and spiralling steeply into umbilicus; umbilicus very narrow and deep, partly obscured by reflected upper part of columella lip. Aperture subcircular, roundly quadrate in parieto-columellar region; peristome incomplete; columella somewhat flaring at junction with basal lip; outer lip thin, partly damaged but evidently crenulated by spiral cords; interior somewhat calcined, but retaining traces of nacre.

PROTOCONCH. Surface evidently sculptured with hexagonal network of interconnected ridges, but not examined under SEM as the holotype is the only specimen available.

COLOUR. Uniformly milky-white, cord intervals retaining faint iridescence.

DIMENSIONS. Holotype, height 8.7 mm, diameter 7.4 mm.

OPERCULUM, RADULA AND EXTERNAL ANATOMY. Unknown.

Distribution

Known only from the slopes of Walters Shoal, at a depth of 707–720 m; dredged on coarse sand with solitary corals; living specimens unknown.

Remarks

The single available specimen of Carinastele achrosta sp. nov. is considerably larger and proportionately taller than any of the more than 175 specimens of Bruceina areneformis sp. nov. (height 8.7 mm vs max. 5.4 mm) and it was obtained at a somewhat greater depth. It differs from B. areneformis not only in size and relative height, but also in having more numerous spiral cords that are more rounded in profile and possess a faint median furrow, and it has a narrower umbilicus. In size and shape C. achrosta is closer to the South African Bruceina eos and might be considered an example of the occasional uniformly white specimens of that species (normally bright pink). However, C. achrosta differs fundamentally from species of Bruceina in the ontogeny of the spiral cords (compare Figs 29G and 29H ). In species of Bruceina the shoulder cord of the early whorls (P3) remains more or less at the shoulder and an additional cord (S3) arises beneath this during the second whorl. Such a cord is absent in C. achrosta and with growth P3 changes from being the shoulder cord to become the peripheral cord. Marshall (1988) considered the lack of cord S3 to be an important character separating his genera Carinastele and Bruceina (as Herbertina ) and for this reason I refer the present material to Carinastele . In its overall shape C. achrosta resembles the type species, C. kristelleae , from New Zealand, but in that species the base is flatter and all the supra-peripheral spiral cords are strongly and sharply nodular. Carinastele niceterium (Hedley & May, 1908) and C. jugosa Marshall, 1988 , from Tasmania and south of New Zealand respectively, both have a more conical spire profile and fewer, stronger spiral cords above the periphery that lack nodules. Carinastele jugosa shares with C. achrosta the presence of a narrow median furrow on the crest of the spiral cords.

Figures

Fig. 29. A–C. Bruceina areneformis sp. nov., holotype, height 4.3 mm (MNHN-IM-2000-35699). D–F. Carinastele achrosta sp. nov., holotype, height 8.7 mm (MNHN-IM-2000-35705). G–H. Portions of the above holotypes to illustrate numbering of spiral cords, following Marshall (1988). G. Bruceina areneformis sp. nov.H. Carinastele achrosta sp. nov. The equivalent of P2 is not present inB. areneformis sp. nov.