Spiralix (Burgosia) aenigmatica, Álvarez & Quiñonero-Salgado & López-Soriano & Glöer, 2024

Spiralix (Burgosia) aenigmatica View in CoL sp. nov.

( Figs. 1-2 View FIGURE 1 View FIGURE2 )

Type material. Holotype RMNH.MOL.350846 (Naturalis) . Paratypes: 4 s. in RMNH.MOL.350847 (Naturalis). 5 s. in SQS coll. 12 s. in JFMA coll .

Type locality. Fuente de Sevilla , Zufre (Huelva); 29S X734085 Y4190034. 05⁄10⁄2022; 16⁄12⁄2022; 09⁄01⁄2023; 14⁄05⁄2023 , JFMA leg. 375m a.s.l. (Fig. 10A). The spring of the type locality flows out of limes- tone, and is collected in an artificial vaulted basin, which eventually feeds an irrigation pool “ alberca ” used for water supply to a neighbouring orchard .

Etymology. The specific name refers to the enigma represented by the large gap in the distribution range of the subgenus, until now only known from the Cantabrian Mountains and surrounding areas, in the north of Spain, very far from the locality where the new species was found.

Description. Shell small, translucent, fragile, subcylindrical and turriculated, with deep sutures and 3–3.5 convex whorls with orthocline growth lines. Size of about 1.0 mm high and 0.5 mm wide. Teleoconch microsculpture formed by irregularly arranged cuneiform marks. Protoconch microsculpture formed by angular depressions, irregularly arranged. Umbilicus deep. Aperture oval, aligned with the vertical axis, 0.4 mm high and 0.3 mm wide.

Dimensions. Holotype 0.90 mm high and 0.52 mm wide. For complete size ranges see Table 1 and Fig. 3. This is the smallest species in this genus.

Habitat. Stygobitic.

Differentiating characters. Spiralix (Burgosia) burgensis Boeters, 2003 has a similar shape, but has larger whorls, making it higher ( 1.24–1.44 mm compared to 0.86–1.01 mm) and wider (apex 0.73–0.85 mm compared to 0.47–0.64 mm; last whorl 0.88–0.96 mm vs. 0.58–0.71 mm). The ovoid aperture is slanted instead of vertical.

Spiralix (Burgosia) affinitatis Boeters, 2003 View in CoL is quite different, having much more inflated whorls, resulting in an overall ovoid outline. Its length ( 1.07–1.24 mm) and width (apex 0.66–0.76; last whorl 0.78–0.93mm) fit in between Spiralix aenigmatica View in CoL sp. nov. and S. burgensis View in CoL . This species also has a slanted aperture.

Boeters (2003) did not elaborate on the microsculpture of the species he described, but Quiñonero-Salgado et al. (2017) presented SEM images of the six thenknown species, including the two species mentioned above. From this, it is evident that the microsculpture of the shell of S. aenigmatica View in CoL sp. nov. is characteristic for the subgenus Burgosia : irregular cuneiform marks on the teleoconch, best seen on the first teleoconch whorls, and the protoconch with a pitted sculpture of angular depressions.

Some other species in the nominal genus Spiralix from the eastern Iberian Peninsula have to some degree similar marks at the initial whorl of the teleoconch (see Corbella et al., 2014), but they are much less apparent and developed than in Burgosia (detailed conchological redescription of this genus by Quiñonero-Salgado et al., 2017). So, the subgenus classification of the new species should be considered as tentative, until new evidence (anatomical or molecular) allows for more reliable criteria.

Remarks. The family Moitessieriidae includes only strict stygobiont species. Within this family, in Spain the genus Spiralix Boeters, 1972 is represented by nine species or subspecies in the Valencian Community, all included in the nominate subgenus Spiralix , and another seven species of the subgenus Burgosia Boeters, 2003 restricted to the north of the Iberian Peninsula. Of these seven species in the second subgenus, two are endemic to Asturias ( Quiñonero-Salgado et al., 2018a), two are from Cantabria, another two are exclusively found in the province of Burgos (northern part of the Castilla y León Autonomous Community), and one more is found both in Burgos and the Basque Country. While many other Moitessieriidae species are known in Catalonia, Aragón and the Valencian Community, little is known about this family in the rest of Spain, to date with no known representatives in the whole centre and southern part.

The finding of a species of the subgenus Burgosia in Andalusia enlarges its distribution range about 600 km to the south from all previously known species. Such a big gap is quite surprising for a subgenus that presumably has very limited dispersion capability. Further exploratory research is needed to determine whether there is a biogeographic explanation for this distribution, or if it is caused by a lack of knowledge of the stygobiont malacofauna in a large part of the Iberian Peninsula. The huge increase in our knowledge of stygobiont molluscs in Iberia over the last two decades makes the second explanation more likely.