Quo vadis, clausiliid taxonomy?

Hartmut Nordsieck (XI.2020)

In 2020 two papers, which will influence the taxonomy of Clausiliidae, have been published:
Koch et al. (2020) and
De Mattia et al. (2020).
In the first paper the Alopia species from Bucegi Mountains, Romania, are lumped to a (mega)species A. livida, because the taxa concerned hybridize to a large extent.
This will influence the species taxonomy of the genus Alopia as a whole and other genera mainly of Alopiinae with frequent interspecies hybridization.
In the second paper the genus Montenegrina was split into numerous species, because the taxa concerned differ in mtDNA sequences and inner genital characters.
This will influence the species taxonomy of other genera of Alopiinae, in which the inner genital characters vary to a comparable degree.

The papers are reported as follows:

Koch et al. (2020):
The paper deals with the relationship of the Alopia taxa from Bucegi Mountains (A. straminicollis, A. livida, A. nixa, A. fussi, A. pomatias). In the COI tree (fig. 1) three clades appear, A. straminicollis, A. livida + A. nixa + A. fussi, and A. pomatias. Within the livida clade only A. nixa comes out as monophyletic subclade, A. livida and A. fussi are intermingled. Within the straminicollis clade s. straminicollis and s. monacha are sister groups. In the AFLP network (fig. 2) only A. pomatias is clearly separated. The remaining taxa form neighbouring clusters ordered according to distribution: from S to N s. monacha, two livida clusters (l. nubila, l.kimakowiczi), several fussi clusters, among them the compact nixa cluster, two livida clusters (l. bipalatalis, l. hypula), s. straminicollis. The straminicollis and the neighbouring livida cluster each are connected by intermediary specimens.
Thus, gene flow between all taxa of thelivida group is shown. A. straminicollis and A. livida hybridize where they meet, therefore they are ranked as subspecies. Within A. livida l. nubila and l. hypula are connected by l. kimakowiczi and l. bipalatalis, which share genes with A. nixa and A. fussi, therefore only two livida subspecies are accepted. A. nixa and A. fussi, in spite of their considerable genital differences to the other taxa, are because of gene flow also classified as subspecies. The Alopia taxa from Bucegi Mountains except of A. pomatias are regarded as one polytypic species A. livida. Absence of sufficient barriers against gene flow is judged as more important than morphological differences.
The examination is restricted to the Bucegi Mountains; other Alopia species with similar relationships are not considered. This concerns especially the relationship of A. livida and A. glorifica from the neighbouring Piatra Craiului Mountains. The sampling is incomplete, because the examined material comes only from a part of the mountains. In shell and genital morphology the authors rely on former works (e. g., Grossu 1981); the determination of the specimens for DNA study was not controlled by genital examinations.

De Mattia et al. (2020):
The authors carried out an integrative study (of mtDNA and genital characters) of the Montenegrina material, on which the work of Fehér & Szekeres (2016) and Mason et al. (2020) was based. The examination of the genital organs of more than 80% of the taxa was not restricted to outer genital characters as usual, but also inner characters were considered, such as inner sculpturing of male and female end ducts and shape and dimension of penial papilla.
Based on this examination an expanded genital diagnosis of the genus was presented, but without considering the frequency of character states. The genital characters of the species taxa were described and figured in detail. The differences in these characters within the species or species groups (clades) were discussed, but not those to other taxa of the genus. Thus no attempt was made to clear up the relationships between the species or species groups within the genus with the help of genital characters. For the taxonomic rank the position in the DNA tree, though only based on mtDNA markers, was in general given more weight than the genital characters. The latter were only used to split up the species or species groups defined by the DNA analysis. If there are genital similarities contrary to the position in the tree, they were regarded as homoplasies or plesiomorphies.
As result of the study the system of Fehér & Szekeres (2016) was considerably changed. Of 106 species taxa 37 subspecies became species, because of their position in the DNA tree and (or) their genital characters. So, the species number increased from 29 to 66. On the other hand, several taxa much differing in shell or genital characters were left together within a species. The extreme splitting within the new system (species number more than doubled) is problematic for two reasons:
Taxa ranks should be equivalent in all groups of European Clausiliidae.
The taxonomic importance of the used inner genital characters for species definition must be controlled by comparison with other groups of Alopiinae and Clausiliidae on the whole, before they can be used for revisions.

Both working groups have their own methods – lumping or splitting of taxa.
Lumping of taxa in genera, in which hybridization is frequent, because species meet.
Splitting of taxa in genera, in which hybridization is rare, because species do not meet.
In different groups hybridization can be frequent, rare or absent (see website article on hybridization). The right methods must be practicable for all clausiliid groups.

References

Fehér, Z. & Szekeres, M. (2016): Taxonomic revision of the rock-dwelling door-snail genus Montenegrina Boettger, 1877 (Mollusca, Gastropoda, Clausiliidae). – ZooKeys, 599: 1-137.

De Mattia, W., Fehér, Z., Mason, K., & Haring, E. (2020): An integrative approach to the taxonomy and systematics within the genus Montenegrina Boettger, 1877 (Mollusca, Gastropoda, Clausiliidae). – Journal of Zoological Systematics and Evolutionary Research, 58 (3): 691-808.

Grossu, A. V. (1981): Gastropoda Romaniae. Ordo Stylommatophora, 3. Suprafamiliile Clausiliacea şi Achatinacea. – 269 pp. Bucureşti (Universitatea din Bucureşti, Facultatea de Biologie).

Koch, E. L., Neiber, M. T., Walther, F. & Hausdorf, B. (2020): Patterns and processes in a non-adaptive radiation: Alopia (Gastropoda, Clausiliidae) in the Bucegi Mountains. – Zoologica Scripta, 49 (3): 280-294.

Mason, K., Fehér, Z., Bamberger, S., Reier, S., Szekeres, M., Sattmann, H., Kruckenhauser, L., De Mattia, W. & Haring, E. (2020): New insights into and limitations of the molecular phylogeny in the taxon-rich land snail genus Montenegrina (Mollusca: Gastropoda: Clausiliidae). – Journal of Zoological Systematics and Evolutionary Research, 58 (3): 662-690.