Cirsotrema, Zelaya, Diego G. & Güller, Marina, 2017

Zelaya, Diego G. & Güller, Marina, 2017, Undercover speciation of wentletraps (Caenogastropoda: Epitoniidae) in the Southwestern Atlantic, Zootaxa 4286 (1), pp. 41-69 : 62-64

“ Cirsotrema ” strebeli new species

( Figures 13 , 14 )

Scalaria magellanica Strebel, 1905: 656 –658 (in part), pl. 23, fig. 44a–e (not fig. 44f) (non Philippi, 1845) Epitonium magellanicum: Castellanos, 1970: 63 –64 (in part)

Type locality. 54°08’45.6’’S 64° 57’36’’W, off Tierra del Fuego Province, Argentina, 110 m.

Type material. Holotype (MACN-In 40393) and 10 paratypes (MACN-In 40394), all of them from the type locality.

Additional material examined. Argentina: Santa Cruz Province: [49°14’57’’S 67°36’50’’W], Punta Desengaño, San Julián (MACN-In 36924: 4 sh.) . Tierra del Fuego Province: 54°26’30’’S 64°53’ W, 111.6 m (MACN-In 25028-1: 1 sh.) ; 54°45’40.6’’S 63°49’06.8’’W, Isla de los Estados (MACN-In 21969: 1 sh.) ; 54°47’S 63°35’W, 133.7 m (MACN-In 22296: 1 sh.); off Tierra del Fuego (MACN-In 40396: 3 spm.) ; 53°38’S 72°22’W, Canal David, 40 m (MACN-In 40395: 4 sh.); 55°01’S 66°42’W, Punta Moat, 15-20 m (MLP-Ma 14173: 1 spm., 1 sh.); 55°03’S 66°37’W, Cabo San Pío, 30-35 m (MLP-Ma 14174: 1 spm., 1 sh.). Burdwood Bank: 54°30’23.4’’S 59°48’39.24’’W, 105 m (MACN-In 40783: 4 sh.); 54°31’40.78’’S 61°27’58.74’’W, 137 m (MACN-In 40784: 1 spm.).

Known distribution. Santa Cruz (49°14’S) to Tierra del Fuego (55°03’S) Provinces, Argentina. Living specimens: 15 to 137 m.

Etymology. The species is dedicated to Hermann Strebel, in recognition for his contributions to the knowledge of the Magellanic molluscs.

Diagnosis. Shell narrowly elongate. Protoconch angulated. Teleoconch sculptured with numerous markedlyrecurved axial ribs, not forming shoulder or coronations; a strong spiral cord at periphery of base; and weak spiral cords between axial ribs. Operculum ovate, paucispiral. Jaw margin with sharply-pointed denticles, followed by hexagonal to lanceolate plates. Most of the teeth with three denticles at the cusp; the outermost ones, with two denticles.

Description. Shell medium sized (maximum L observed = 13.2 mm), slender, moderately solid, white, dull ( Fig. 13 A, B, G, H). Protoconch glossy, of about 630 µm in length and 730 µm in maximum diameter; composed of 1¼ to 1½ whorls ( Fig. 13 D); apical ¼ whorl smooth; subsequent whorl(s) with a strong spiral keel at the middle of whorl or slightly apically displaced, producing an angulated protoconch outline ( Fig. 13 D). Spiral keel gradually increasing in strength, although not reaching protoconch margin. Above and below spiral keel, one or two irregular spiral cords, with bifurcations that form a branched sculpture pattern ( Fig. 13 D). Limit between protoconch and teleoconch delimited by a low cord. Teleoconch with up to 6¾ whorls, markedly convex in outline ( Fig. 13 A, B, G, H). Last whorl flattened at the base. Aperture subovate; peristome continuous, thick, projected at the base. Inner margin of aperture completely attached to last whorl ( Fig. 13 A, B, G, H). Umbilicus absent.

Teleoconch sculptured with prosocline, closely-arranged axial ribs and spiral elements ( Fig. 13 A, B, F–H). Axial ribs usually number 16 to 18 on the first whorls, and 18 to 22 in the last whorl of larger specimens. Ribs on the first whorl are low and narrow; on subsequent whorls, ribs gradually increase in height and become markedly recurved, thus giving the appearance of low but solid elements formed by fusion of several layers ( Fig. 13 C). Ribs evenly curved along the whorls, not forming shoulders, angulations or coronations, except at periphery of base, where they show an abrupt change and extend almost straight, reaching the umbilical area. Ribs of adjoining whorls obliquely aligned, fused. Interspaces of the axial sculpture showing 5 or 6 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 spiral cord that emerges from the insertion of the outer lip of aperture surrounds the base ( Fig. 13 F). Spiral sculpture not crossing over axial sculpture.

Operculum: Thin, ovate, with eccentric nucleus; first whorls slightly sunken; penultimate whorl with edge slightly upraised from last whorl, forming indentations ( Fig. 13 E). Outside surface with 19 to 22 wide, irregular bars per 0.1 mm, obliquely oriented with respect to growth lines ( Fig. 13 I). Colour: light brown.

Jaw ( Fig. 14 D, E): Anterior margin with a row of strong, sharp, triangular denticles; followed by numerous rows of densely-pitted, hexagonal plates, which gradually become lanceolate.

Radula: With numerous teeth per row, each having a well-developed basal denticle ( Fig. 14 A); innermost teeth and outermost tooth shorter than the others. Teeth of the central field usually with three (an apical and two secondary) denticles, similar in length and solidness ( Fig. 14 B); an additional smaller, knob-like, proximal secondary denticle, sometimes present. Secondary denticles usually reducing in size and number (up to one) in outer teeth ( Fig. 14 C).

Remarks. “ Cirsotrema ” strebeli n. sp. resembles “ Cirsotrema ” magellanicum and “ Cirsotrema ” ctenodentatum n. sp. by having a solid shell, sculptured with prominent axial ribs, low but solid spiral elements in the interspaces, and a strong spiral cord surrounding the base. Despite these overall similarities, “ Cirsotrema ” strebeli n. sp. consistently has more markedly recurved axial ribs, which consequently give the appearance of lower and wider elements. In addition, the ribs in “ Cirsotrema ” strebeli n. sp. are uniformly curved below the suture, lacking either the flat slope or the straight shoulder, as in the cases of “ Cirsotrema ” magellanicum and “ Cirsotrema ”ctenodentatum n. sp., respectively. Another very distinctive character is the protoconch, which is angulated in “ Cirsotrema ” strebeli n. sp., and evenly conic in “ Cirsotrema ” magellanicum and “ Cirsotrema ” ctenodentatum n. sp. Both “ Cirsotrema ” magellanicum and “ Cirsotrema ” ctenodentatum n. sp. reach larger sizes than “ Cirsotrema ” strebeli n. sp. The anterior part of the jaw in “ Cirsotrema ” strebeli n. sp. also differs from that of “ Cirsotrema ” magellanicum and “ Cirsotrema ” ctenodentatum n. sp.; in the former, there is a clear difference between the marginal (denticles) and the subsequent (plates) elements, whereas in the latter two species, the margin and the subsequent rows are composed of similar polygonal plates. The operculum of “ Cirsotrema ” strebeli n. sp., like that of “ Cirsotrema ” ctenodentatum n. sp. is ovate and paucispiral, while “ Cirsotrema ” magellanicum has a circular and multispiral operculum. Also similar to “ Cirsotrema ” ctenodentatum n. sp. is the morphology of the radula, with most of the teeth having three denticles at the cusp, although in the case of “ Cirsotrema ” strebeli n. sp., the outermost teeth show two denticles. On the contrary, in “ Cirsotrema ” magellanicum the number of denticles is reduced outwards, showing only one in the outermost teeth (the apical denticle).

Judging from the excellent figures provided by Strebel (1905: fig. 44a–d) there is no doubt that the author had “ Cirsotrema ” strebeli n. sp. in hand, although he misidentified it as the nominotypical Scalaria magellanica . Strebel (1905) figured specimens of “ Cirsotrema ” strebeli n. sp. from Punta Arenas [53°10’S 70°56’W], Puerto Harris [53°50’S 70°27’W] and Strait le Maire [54°50’S 64°55’W], although none of these lots is currently preserved (B. Hausdorf, pers. com. 9/2014; A. Warén, pers. com. 7/2014). All of these figured specimens are small (less than 12.3 mm L) and show numerous and low axial ribs, which project roundly below the suture. Curiously, Strebel (1905) also included under the studied material of Scalaria magellanica two larger specimens (22.2 mm L) from Lively Island, Malvinas / Falkland, for which he did not mention illustrations but also provided a detail of axial ribs ( Strebel 1905: fig. 44e), which was not referred to when mentioning the previous lots. This detail of the ribs, strikingly differs from the ribs of “ Cirsotrema ” strebeli n. sp., in being considerably wider. Unfortunately, the above-mentioned material from Lively Island could not be traced at the Manchester Museum to confirm its identity (H. McGhie, pers. com. 8/2014). If the details of these ribs actually correspond to the specimens from Lively Island, then this lot could correspond to “ Cirsotrema ” ctenodentatum n. sp. The large size of these specimens, and the finding herein of additional material of this species at the same place, could support this hypothesis.

One specimen of “ Cirsotrema ” strebeli n. sp. (currently MACN-In 25028-1) was found in a mixed lot, among specimens previously identified by Castellanos (1970) as Epitonium magellanicum (MACN-In 25028).

Figures

FIGURE 13. “ Cirsotrema ” strebeli new species, shell and operculum. A, F. Holotype (MACN-In 40393). F. Detail of teleoconch sculpture at last whorl. B – E, H, I. Specimens from off Tierra del Fuego (MACN-In 40396). I. Detail of operculum sculpture. G. Specimen from Cabo San Pío (MLP-Ma 14174). Scale bars: A, B = 1 mm; C, F = 500 µm; D, E = 200 µm; G = 2 mm; H = 5 mm; I = 20 µm.

FIGURE 14. “ Cirsotrema ” strebeli, radula and jaw. Specimen from off Tierra del Fuego (same specimen as in Fig. 12 E, I: MACN-In 40396). A – C. Radula. A. Half radular field. B. Inner teeth. C. Cusps of intermediate teeth. D, E. Jaw. D. Anterior inner surface. E. Detail of plates. Scale bars: A = 50 µm, B, C = 10 µm, D = 20 µm, E = 5 µm.