Epitonium evanidstriatum, 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 : 51-54

Epitonium evanidstriatum new species

( Figures 6A–L , 7 )

Type locality. 40°55’17.4’’S 65°08’07.6’’W, Golfo San Matías , Río Negro Province, Argentina, 7–8 m.

Type material. Holotype (MACN-In 40380) and 3 paratypes (MACN-In 40381) from the type locality; 2 paratypes from 40°55’29.5’’S 65°08’32.7’’W, 3–4 m (MACN-In 40382).

Additional material examined. Argentina: Río Negro Province: 40°48’46.80’’S 65°05’45.18’’W, Las Grutas (MACN-In 40383: 1 sh.) ; 40°50’12.8’’S 65°04’42.2’’W, 10 m (MACN-In 40384: 2 sh.); 40°54’08.3’’S 65°06’28.0’’W, 9 m (MACN-In 40385: 6 sh.); 40°54’21.7’’S 65°06’39.8’’W, 6 m (MACN-In 40386: 1 sh.); 40°56’27.2’S 65°07’58.2’’W, 10–11 m (MACN-In 40387: 11 sh.); 41°38’06.6’’S 65°00’55.9’’W, Playas Doradas, 7 m (MACN-In 40388: 22 sh.); 41°39’03.1’’S 65°00’38.3’’W, 12–14 m (MACN-In 40389: 2 sh.); 41°39’41.3’’S 65°00’32.9’’W, 16 m (MACN-In 40390: 2 sh.); 41°40’17.8’’S 65°00’27.8’’W, 16 m (MACN-In 40391: 2 sh.); 42°00’54.2’’S 65°03’40.9’’W, Puerto Lobos, 10–12 m (MACN-In 40392: 1 sh.).

Known distribution. Only known from Golfo San Matías (40°48’S to 42°00’S), Río Negro Province, Argentina.

Ethymology. evanid - Latin for disappearing + striatum for striae; in reference to the spiral threads that fade in the last whorls of larger specimens.

Diagnosis. Shell broadly conical to narrowly elongate. Protoconch conical, of 3 to 3¼ whorls. Teleoconch with convex whorls, sculptured with lamellate or recurved axial ribs, which decrease in number with growth, and spiral threads, clearly visible in the first whorls but fading in the last whorls of larger specimens. Jaw with a row of flat, distally-blunt denticles at the anterior margin. Teeth with two secondary denticles in the central field, reduced to knobs or completely absent in the outermost teeth.

Description. Shell medium sized (maximum L observed = 11.5 mm), broadly conical to narrowly elongate, thin, white, shiny ( Fig. 6A, C, E, F ). Protoconch conical, of about 300 µm in length and 310 µm in maximum diameter, composed of 3 to 3¼ whorls; whorls slightly convex in outline; usually lost in larger specimens; surface glossy, sculptured with faint axial threads ( Fig. 6D, G, I ). Limit between protoconch and teleoconch clearly defined by a change in sculpture. Teleoconch with up to 7½ whorls, the first ones slightly convex, then gradually increasing in convexity ( Fig. 6A, C, E, F ); suture moderately deep ( Fig. 6J, K ). Last whorl slightly flattened at base. Aperture subovate, expanded at the base. Outer margin thick. Umbilicus absent. Fasciole narrow ( Fig. 6A, C, E, F ).

Teleoconch sculptured with prosocline axial ribs and delicate spiral threads ( Fig. 6A, C, E, F, J ). Axial ribs evenly convex along the whorl, sometimes forming a weak shoulder adapically, but without angulation at the base of last whorl; formed by fusion of several layers. Ribs on adjoining whorls discontinuous, extending well into the suture ( Fig. 6J, K ). Number of ribs consistently decreasing with shell growth: from 18–27 on the first whorls to 13–19 from the fifth whorl onwards ( Fig. 6B ); ribs in the first whorl are relatively low, but gradually increasing in height in subsequent whorls, where they are either erect (lamellar) or slightly to moderately recurved ( Fig. 6B, J, K ); in the last whorl, ribs extending to the umbilical area ( Fig. 6A, C, E, F ). Interspaces between ribs with widelyseparated spiral threads, regularly spaced along the whorl, although somewhat weaker near the suture; in number of 8–9 in the first teleoconch whorl, increasing up to 15–19, but fading from the fourth to sixth whorl onwards ( Fig. 6J, K ). Spiral sculpture not crossing over axial sculpture.

Operculum: Thin, subovate, paucispiral, with eccentric nucleus ( Fig. 6H ). Adjacent whorls without raised edges. Outside surface sculptured with 34 to 36 irregular threads per 0.1 mm, obliquely oriented with respect to growth lines ( Fig. 6L ). Colour: light brown, translucent.

Jaw ( Fig. 7 D, E): Anterior margin with a row of flat, distally-blunt denticles, followed by a series of irregular, pitted plates.

Radula: With numerous teeth per row, all of them showing an obscure basal denticle. Teeth of the central field short and stout, with an acute apical denticle and one or two, variably-developed secondary denticles ( Fig. 7 A). Subsequent teeth with a markedly-elongate, narrow blade, and an upturned, acute apical denticle ( Fig. 7 B, C). A minute, knob-like secondary denticle sometimes present. Outermost teeth slightly smaller ( Fig. 7 B).

Remarks. The variability in shell outline described above for E. evanidstriatum n. sp. is size-related: smaller specimens are broadly conical, and gradually become narrowly elongate with growth. This variation is associated with the change in the incremental rate of whorls, which in the first teleoconch whorl is high (with an increase in width of 50–70% from the last protoconch whorl to the first teleoconch whorl), but in subsequent whorls gradually decreases (to about 20% from the sixth to seventh whorls), showing a particularly marked decrease from the first to the second teleoconch whorl. This change in the incremental rate of whorls gives origin to a narrowly elongated aspect of the shell outline of larger specimens.

In shell outline, Epitonium evanidstriatum n. sp. closely resembles that referred to as Epitonium candeanum (d’Orbigny, 1842) by different authors (e.g., Clench & Turner 1951; Rios 1994; Redfern 2000). Although the concept of E. candeanum is variable among different authors (e.g., d’Orbigny 1842; Clench & Turner 1951; Diaz Merlano & Puyana Hegedus 1994; Redfern 2000), there is a general agreement on the presence of fine, secondary axial sculpture running across the spiral threads, between the strong, primary axial ribs. This condition clearly differs from that present in E. evanidstriatum n. sp., where this secondary sculpture is completely absent. Furthermore, E. evanidstriatum n. sp. is narrower and with less inflated whorls than E. candeanum .

In E. evanidstriatum n. sp. the spiral sculpture fades towards the last teleoconch whorls. The vanishing condition of spirals has been previously described for a few species: Surrepifungium costulatum ( Kiener, 1838) , S. oliverioi (Bonfitto & Sabelli, 2000) , S. patamakanthini Gittenberger & Gittenberger, 2005 , all from the Indo-West Pacific, and Epitonium apiculatum (Dall, 1889) , described from the East coast of the United States ( Fig. 6 M). However, in these species the spiral sculpture disappears completely at some point, whereas in E. evanidstriatum n. sp. the spirals fade in the last whorls of larger specimens but are still visible under high magnification (SEM). In addition, Epitonium evanidstriatum n. sp. reaches larger sizes than E. apiculatum , but smaller than Surrepifungium costulatum , S. oliverioi and S. patamakanthini (11.5 mm vs. 4.5 mm vs. 44.0 mm, 20.0 mm and 22.8 mm, respectively); but even when comparing similarly-sized specimens, E. evanidstriatum n. sp. shows a consistently more narrowly-elongated outline than the other four species, which are widely conical all along their ontogeny. In addition, Epitonium apiculatum differs from E. evanidstriatum n. sp. by having a greater number of whorls at the same size (9 fide Clench & Turner 1952 in 4.5 mm long specimens vs. 7 whorls in E. evanidstriatum ); fewer axial ribs on the fourth and fifth teleoconch whorl (10–11 vs. 14–20 in E. evanidstriatum ) and the whorls only attached by axial ribs ( Clench & Turner 1952). Additional characters for distinguishing Epitonium evanidstriatum n. sp. from Surrepifungium costulatum , S. oliverioi and S. patamakanthini come from the radula and jaw: the latter three species have all radular teeth with a single denticle at the cusp (the apical denticle), and the anterior margin of the jaw with several rows of acute, slender denticles. In fact, these two characters were used by Gittenberger & Gittenberger (2005) to reunite these species under the genus Surrepifungium . On the contrary, E. evanidstriatum n. sp. has the anterior margin of the jaw with a single row of blunt denticles, and the teeth of the central field of the radula with three denticles. The radula and jaw of Epitonium apiculatum remain unknown.

Specimens of Epitonium evanidstriatum n. sp. could be confused with the sympatric E. georgettinum , particularly if seen under low magnification. However, the former clearly differs by having spiral sculpture, and the adjacent whorls in tight contact instead appearing as “detached”. In addition, the protoconch of Epitonium evanidstriatum n. sp. is smaller than that of E. georgettinum (300 vs. 500 µm long), even when having a greater number of whorls (3–3¼ vs. 2½); the number of axial ribs in the teleoconch of E. evanidstriatum n. sp. decreases with growth, instead of being constant, as it is the case of E. georgettinum ; and the first teleoconch whorl shows almost twice the number of ribs than those present in E. georgettinum (18–27 vs. 12–15). Another difference arises in the morphology of the operculum, which is paucispiral in Epitonium evanidstriatum n. sp. and multispiral in E. georgettinum . Concerning the radula, Epitonium evanidstriatum n. sp. shows the outermost teeth with minute or absent secondary denticles, whereas in E. georgettinum the outer teeth have two well-developed secondary denticles.

The only other species sympatric with E. evanidstriatum n. sp. having both axial and spiral sculpture is E. striatellum . Epitonium evanidstriatum n. sp. differs from this species by having a deeper suture, and the axial sculpture comprises fewer, more widely separated, higher and stronger ribs. Epitonium evanidstriatum n. sp. also has weaker and fewer spiral threads, which fade in the last whorls, whereas in E. striatellum the spiral sculpture is regularly present along all the shell whorls. The aperture in E. striatellum is larger and more markedly ovate, and this species reaches larger sizes than E. evanidstriatum n. sp. (28.5 vs. 11.5 mm long, respectively).

Figures

FIGURE 6. A – L. Epitonium evanidstriatum new species, shell and operculum. A, B. Holotype (MACN-In 40380). C, D, F, G, J. Specimens from 40 ° 56 ' 27.2 ' S 65 ° 07 ' 58.2 '' W (MACN-In 40387). E, K. Paratype (MACN-In 40381). H, I, L. Paratype (MACN-In 40382). I. Detail of protoconch sculpture. J, K. Detail of teleoconch sculpture between second and third whorls (J) and last two whorls (K); L. Detail of operculum sculpture. M. Lectotype of Scala apiculata (USNM 94890). Scale bars: A – C, E, M = 2 mm; D, G, J = 200 µm; F, H, K = 1 mm; I = 5 µm; L = 20 µm.

FIGURE 7. Epitonium evanidstriatum, radula and jaw. Paratype (same specimen as Fig. 6 H, I, L: MACN-In 40382). A – C. Radula: A. Central field. B. Outer teeth. C. Detail of cusps of intermediate teeth. D, E. Jaw. D. Anterior inner surface; E. Detail of marginal denticles. Scale bars: A = 10 µm; B = 20 µm; C, D = 5 µm; E = 1 µm.