Hyperaulax Pilsbry, 1897

Salvador, Rodrigo B. & Cavallari, Daniel C., 2019, Taxonomic revision of the genus Hyperaulax Pilsbry, 1897 (Gastropoda, Stylommatophora, Odontostomidae), Zoosystematics and Evolution 95 (2), pp. 453-463 : 455-456

Genus Hyperaulax Pilsbry, 1897

Bulimulus (Hyperaulax) Pilsbry 1897a: 10; Pilsbry 1897b: 82.

Bonnanius Jousseaume 1900: 39; Schileyko 1999: 337.

Hyperaulax : Pilsbry 1901: 102; Wenz 1923: 729; Thiele 1931: 660; Henderson 1935: 145; Morretes 1949: 153; Oliveira et al. 1981: 350; Parkinson et al. 1987: 29; Schileyko 1999: 320; Salgado and Coelho 2003: 165; Simone 2006: 178; Salvador 2019: 87.

Hyperaulax (Bonnanius) : Pilsbry 1901: 103; Thiele 1931: 661; Morretes 1949: 153; Zilch 1960: 505; Breure 1974: 52; Oliveira et al. 1981: 350.

Hyperaulax (Hyperaulax) : Thiele 1931: 661; Morretes 1949: 153; Zilch 1960: 505; Breure 1974: 109.

Tomigerus (Bonnanius) : Parodiz 1962: 453.

Bonnarius [sic]: Simone 2006: 178.

Type species.

Bulimus (Bulimulus) ridleyi Smith, 1890, by original designation.

Included species.

Hyperaulax ridleyi (Smith, 1890) and H. ramagei (Smith, 1890).

Diagnosis.

Shell bulimoid. Protoconch sculptured by sinuous axial riblets, which can anastomose and fade on abapical region. Umbilicus surrounded by a periumbilical spiral angulation.

Description.

Shell small to medium-sized, bulimoid, with ca 4-5 convex whorls; ground color cream, ochre or brown, with 1-4 lighter-colored spiral bands on lateral portion of whorls; periumbilical region completely or marginally discolored, whitish; apex (especially protoconch) usually of lighter color. Suture well-marked. Protoconch sculptured by numerous fine sinuous axial riblets, transition unclear. Teleoconch overall smooth except for axial growth lines. Aperture ovoid; peristome white, reflected, thickened, and continuous, with 0-4 apertural teeth. Umbilicus perforate, well marked.

Remarks.

After Auffenberg et al. (2015) removed all fossil taxa from Hyperaulax , the genus was left only with two living species, Hyperaulax ridleyi ( Fig. 2 ) and H. ramagei ( Figs 3 , 4 ), with the latter classified in the subgenus or full genus Bonnanius . Both species are known only from Fernando de Noronha Archipelago off north-eastern Brazil.

The genus Bonnanius is considered here synonymous with Hyperaulax as there are no diagnostic characters allowing its clear separation other than its larger shell size. The presence of teeth in the aperture of H. ramagei (previously classified in Bonnanius ) could be used as a diagnostic genus-level character; however, it is well known that odontostomid genera show great inter- and intraspecific variation in the presence and strength of apertural teeth. Moreover, some specimens of H. ridleyi do show a palatal tooth ( Fig. 2A ) similar in position and length to that of H. ramagei . Furthermore, for a genus with only two species, keeping them separated into two distinct subgenera is excessively zealous taxonomy. As for the other conchological characters, Bonnanius share all of them with Hyperaulax , as discussed below.

The protoconch sculpture has always been deemed a good character to define genera in Orthalicoidea and has more recently received large support from molecular studies ( Breure and Romero 2012). The protoconchs of Hyperaulax and Bonnanius are very similar and indicate a close affinity between the two forms: same number of whorls (ca 1¾); same sculpture pattern (sinuous axial riblets, more clearly separate on adapical area of whorl, but anastomosing on abapical area and sometimes fading into scattered dots). There are also some differences on the protoconch, but nothing that would suggest two distinct genera (many genera of Orthalicoidea bear some minor differences in their protoconchs, which helps with species diagnosis; e.g., Salvador and Cavallari 2013; Salvador and Simone 2016). The main difference is that the protoconch of H. ramagei is more flattened and rounded, which makes the riblets a little more spaced; this can be attributed to its shell being larger and wider overall. Furthermore, the protoconch of H. ridleyi has a raised ridge on its middle region. Finally, the protoconch sculpture of museum specimens of H. ramagei is often faded or eroded, which has led to claims of a smooth protoconch in the literature (e.g., Parodiz 1962; Abbott 1989).

Other conchological characters are very similar in both H. ridleyi and H. ramagei : the roughly pentagonal shape of the aperture and its positioning in relation to the body whorl, the long palatal tooth (absent in most H. ridleyi specimens), the shape of the umbilicus and the periumbilical spiral angulation, the unsculptured teleoconch (except for growth striations), and the periostracum color (brown with at least one white spiral band).

The classification of Hyperaulax in Odontostomidae has been well supported in the literature, with just a few different classification schemes. For instance, Schileyko (1999) argued in favor of Bulimulidae because H. ridleyi has no teeth; however, not all odontostomids actually have teeth and some specimens of H. ridleyi do show a faint palatal tooth, as discussed above and already remarked by Pilsbry (1901). Moreover, H. ridleyi has the typical channel-like structure on the junction of the parietal and palatal regions of the peristome. Schileyko (1999), however, maintained Bonnanius (and hence H. ramagei ) in Odontostomidae .

In any event, there are other conchological characters favoring an allocation within Odontostomidae , such as the elevated embryonic whorls ( Auffenberg et al. 2015) and the protoconch sculpture (wavy riblets, similar to Plagiodontes Doering, 1876; Pizá and Cazzaniga 2016). The overall shell shape of Hyperaulax is very similar to Tomigerus Spix, 1827 and Biotocus Salgado & Leme, 1990, but with a different position of the aperture in relation to the body whorl and a different structure of the umbilical region; also, the protoconch of Tomigerus is smooth. The periostracum color is also similar to what is seen in Tomigerus (e.g., T. clausus Spix, 1827, T. matthewsi Salgado & Leme, 1991, and T. rochai Ihering, 1905), but a striped pattern can also be found in species of Moricandia Pilsbry & Vanatta, 1898 and even Anostoma Waldheim, 1807. The dentition of Hyperaulax (mainly of H. ramagei ) is similar to that of Burringtonia Parodiz, 1944 and also Anctus angiostomus (Wagner, 1827). Finally, the channel-like structure on the junction of the parietal and palatal regions of the peristome is virtually identical to what is observed in some species of Cyclodontina Beck, 1837, Spixia Pilsbry & Vanata, 1898, and Anostoma Waldheim, 1807.

Our analysis of interspecific affinities using the barcoding region of the COI marker has grouped Hyperaulax ( H. ridleyi only) with Tomigerus , as expected by our morphological analysis, with a posterior probability of 0.997. However, these two species were grouped with Simpulopsis Beck, 1837 (family Simpulopsidae ) in our Bayesian tree, instead of being grouped with other odontostomids. This is likely due to the fact that COI alone is not sufficient to solve family-level relationships among stylommatophoran snails ( Breure and Romero 2012), despite being sufficient to capture the relationship of close species-level taxa. Based on morphological data, we retain Hyperaulax (and Tomigerus ) in the family Odontostomidae .

It is curious that another orthalicoid lineage, from the other side of South America, evolved an uncannily similar shell shape to Hyperaulax : Naesiotus wolfi (Reibisch, 1892), from the Galapagos (lectotype ZSM 47.950, paralectotype NHMUK 1894.6.8.7). Furthermore, N. wolfi is within the size range of Hyperaulax , and has a similar color pattern to H. ridleyi , including the median white spiral band. The protoconch, naturally, is different ( Naesiotus Albers, 1850 has very fine and well-defined axial striae), alongside other more general shell features: higher spire, different proportion of body whorl to spire, and a larger number of whorls (ca 6½). In any event, this is a remarkable case of convergent evolution on islands and deserves further investigation.

Figures

Figure 2. Hyperaulax ridleyi. A-E. Lectotype, NHMUK 1888.6.27.106. F-I. Paralectotypes 1 to 4 (in order), NHMUK 1888.6.27.107 - 110. J. Extreme form, large and with thick callus, NHMUK 1888.6.27.88 - 94. K. Weathered specimen, ZMS unnumbered (ex Heimburg colln.). L. Protoconch detail (same specimen from K).

Figure 3. Hyperaulax ramagei. A-E. Lectotype, NHMUK 1988.6.24.163. F-J. Paralectotypes 1 to 5 (in order), including large forms in apparent sub-fossil state, NHMUK 1988.6.24.164 - 170. K. Specimen without the white spiral bands, NHMUK 1902.10.16.4.

Figure 4. Hyperaulax ramagei. A-C. Syntype of Bonnanius bouvieri, MNHN-IM- 2000 - 28020 (MNHN). D. Reproduction of " Turbine 44 " of Buonanni (1861), the holotype of Bonnanius bonnanius. E. Protoconch detail, NHMUK 1902.10.16.4.