Cirsotrema

Zelaya, Diego G. & Güller, Marina, 2017, Undercover speciation of wentletraps (Caenogastropoda: Epitoniidae) in the Southwestern Atlantic, Zootaxa 4286 (1), pp. 41-69 : 59-61

“ Cirsotrema ” ctenodentatum new species

( Figures 11 , 12 )

Epitonium magellanicum latecostatum: Castellanos, 1970: 64 –65 (in part), pl. 4, fig. 13 (non Strebel, 1908)

Type locality. 54°47’54’’S 65°14’42’’W, Bahía Buen Suceso , Tierra del Fuego Province, Argentina, 18 m .

Type material. Holotype (MACN-In 40401) and one paratype (MACN-In 40402) from the type locality.

Additional material examined. Argentina: Malvinas / Falkland Islands: Lively Island (MACN-In 10138: 1 sh.; MM EE 7942.2: 11 sh.). Tierra del Fuego Province: 52°55’S 75°00’W, 92-101 m ( USNM 870443 : 1 sh.) ; 54°50’S 64°01’W, south of Isla de los Estados , 153.6 m (MACN-In 22733: 1 spm.) ; 54°41’35’’S 64°01’37’’W, 50.2 m (MACN-In 22614: 1 sh.); 55°41’S 66°34’W, 25 m (MACN-In 24975: 6 sh.); off Tierra del Fuego (MACN-In 40403: 3 spm.). Burdwood Bank : 54°30’23.4’’S 59°48’39.24’’W, 105 m (MACN-In 40782: 1 spm.).

Known distribution. Tierra del Fuego Province, Argentina; living specimens: 18 to 153.6 m.

Etymology. The name of the species refers to the comb-like morphology of the radular teeth, whose cusps are composed of several denticles.

Diagnosis. Shell elongated to broadly conical; protoconch of 2 rounded whorls; teleoconch sculptured with numerous, reflexed ribs, forming shoulders; interspaces between ribs with spiral cords; a stronger spiral cord delimiting the base. Operculum ovate, paucispiral. Jaw with several rows of polygonal plates at the anterior part. All radular teeth with three or four denticles at the cusp.

Description. Shell large (maximum L observed = 25.0 mm, apex missing), narrowly elongated to broadly conical, with gradated whorls; thick, white, chalky ( Fig. 11 A, B, E, F, H). Protoconch of about 1,000 µm in length and 700 µm in maximum diameter, composed of 2 evenly-rounded whorls ( Fig. 11 D), usually lost in larger specimens. Protoconch sculpture unknown, eroded in the available specimens. Limit between protoconch and teleoconch readily discernible, marked by a simple growth scar. Teleoconch with up to 7 whorls; whorls convex in outline ( Fig. 11 A, B, E, F, H); adjacent whorls attached; suture deep, crossed by axial sculpture. Last whorl flattened at the base. Aperture ovate; peristome continuous, thick, slightly auriculate at the base ( Fig. 11 A, B, E, F, H). Inner margin of aperture either completely attached to last whorl or partially detached, in the last case originating a narrow umbilicus. Fasciole usually well-developed.

Teleoconch sculptured with numerous, slightly- to markedly-prosocline axial ribs and spiral elements ( Fig. 11 A, B, E, F–H). Axial ribs usually 16 to 19 per whorl, but up to 25; either constant in number throughout whorls or with a greater number of ribs in the second and third teleoconch whorls. Ribs on the first whorl are low, narrow and evenly arcuate; in subsequent whorls, ribs become stronger, gradually increasing in height and slightly reflexed, formed by fusion of several layers ( Fig. 11 A–C, E–H). Apically, ribs projecting perpendicular to columellar axis, forming a straight shoulder that gradates the whorls ( Fig. 11 B, E, H), becoming convex anteriorly. Ribs of adjoining whorls obliquely aligned and fused, extending to umbilical area in last whorl, markedly changing their curvature at periphery of base ( Fig. 11 A, B, E, F, H). Interspaces of the axial sculpture showing 6–8 lower but wide spiral cords, separated by extremely narrow interspaces, where secondary spiral cords and small, spirally aligned punctae are present. Spiral sculpture clearly visible in all whorls, but missing at the base. A strong basal spiral cord emerging from the insertion of the outer lip of aperture, delimits the base ( Fig. 11 G). Spiral sculpture not crossing over axial sculpture.

Operculum: Thin, ovate, with eccentric nucleus; paucispiral, with edge of adjacent coils with raised edges ( Fig. 11 I). Outside surface sculptured with about 23 regular bars per 0.1 mm, obliquely oriented with respect to growth lines ( Fig. 11 J). Colour: light brown.

Jaw ( Fig. 12 D, E): Anterior part uniformly paved with several rows of superimposed polygonal, bluntlypointed, pitted plates that continue to the margin.

Radula: With numerous teeth per row, each with a well-developed basal denticle ( Fig. 12 A). Teeth of the central field with stout blades; subsequent teeth with elongate, strong blades, reducing in size near the outer margin ( Fig. 12 A, B). Cusps similar along all radular field, comprising a short, acute apical denticle and usually two longer, blunt secondary denticles ( Fig. 12 C). An additional, smaller proximal denticle, sometimes present, varying from a knob to a well-developed structure, always smaller than the other denticles ( Fig. 12 C).

Remarks. In shell morphology, “ Cirsotrema ” ctenodentatum n. sp. closely resembles “ Cirsotrema ” magellanicum , from which it differs by having consistently more, lower and more markedly recurved axial ribs, which result in narrower interspaces. Another difference between these species arises in the straight shoulder of the axial ribs of “ Cirsotrema ” ctenodentatum n. sp., which results in a markedly-gradated outline of the whorls. In “ Cirsotrema ” magellanicum , instead, the axial ribs slope apically. The distinction between “ Cirsotrema ” ctenodentatum n. sp. and “ Cirsotrema ” magellanicum becomes even more evident when comparing the operculum and radula: “ Cirsotrema ” ctenodentatum has a thin, ovate, paucispiral operculum, whereas that of “ Cirsotrema ” magellanicum is thick, subcircular and multispiral. With regard to the radula, “ Cirsotrema ” ctenodentatum n. sp. has three / four denticles in the cusp of outer teeth, while in “ Cirsotrema ” magellanicum these teeth invariably show a single denticle (the apical denticle). Another similar species to “ Cirsotrema ” ctenodentatum n. sp. is “ Cirsotrema ” strebeli n. sp. A comparison between these taxa is provided in the Remarks section of the following species.

Castellanos (1970) included under the name Epitonium magellanicum latecostatum , one lot belonging to this species (MACN-In 22733).

Figures

FIGURE 11. “ Cirsotrema ” ctenodentatum new species, shell and operculum. A, G, I. Holotype (MACN-In 40401). G. Detail of teleoconch sculpture at last whorl. B, H. Specimens from off Tierra del Fuego (MACN-In 24975). C, E. Specimen from off Tierra del Fuego (MACN-In 22614). D, F. Paratype (MACN-In 40402). J. Detail of operculum sculpture, specimen from off Tierra del Fuego (MACN-In 40403). Scale bars: A = 2 mm; B, C, E, H = 5 mm; D = 200 µm; F, G, I = 1 mm; J = 50 µm.

FIGURE 12. “ Cirsotrema ” ctenodentatum, radula and jaw. A – D. Holotype (same specimen as in Fig. 10 A: MACN-In 40401). E. Specimen from off Tierra del Fuego (MACN-In 40403). A – C. Radula. A. Half radular field. B. Outer teeth. C. Cusps of intermediate teeth. D, E. Jaw. D. Anterior inner surface. E. Detail of plates. Scale bars: A = 50 µm; B = 20 µm; C, D = 10 µm; E = 5 µm.