DNA and Dissection Untangle a 200-Year-Old Mystery of Mediterranean Snails

On the Greek island of Corfu, two species of small land snails have been hiding in plain sight for nearly two centuries. Now, an international team of malacologists has finally put their identities on firm ground, redescribing Monacha claustralis and Monacha parumcincta with the aid of modern genetics, painstaking dissection, and a deep dive into museum archives dating back to the 1830s. The study, published in the journal ZooKeys, also sorts out a tangle of allied species from southern Italy and Sicily, resolving nomenclatural puzzles that have confused naturalists since the early days of European conchology.

The genus Monacha belongs to the hygromiid snails, a group of air-breathing pulmonate gastropods whose shells are notoriously conservative in shape. Species that look almost identical on the outside can differ dramatically in the architecture of their reproductive organs, and the two Corfu species are a textbook case. Shells of Monacha claustralis and its look-alike Monacha cartusiana do not differ in any reliable way, yet their distal genitalia and mitochondrial DNA tell entirely different stories. The same holds for Corfiot Monacha parumcincta versus the Italian snails long assigned to that name.

To disentangle the mess, the researchers employed an integrative approach, combining morphology with molecular data. Specimens were collected from topotypical localities, meaning the very places where the species were first described. Shells were measured for height, diameter, aperture proportions and whorl number, while bodies were dissected under light microscopes to map structures such as the vagina, vaginal appendix, digitiform glands, bursa copulatrix, flagellum, epiphallus and penis. Nucleotide sequences of mitochondrial genes COI and 16S rDNA, along with nuclear ITS2 flanked by fragments of the 5.8S and 28S rDNA genes, were compared using Maximum Likelihood and Bayesian phylogenetic methods, with the widespread species Trochulus hispidus serving as an outgroup to root the trees.

The results were unambiguous. Monacha claustralis, distinguished from Monacha cartusiana by a longer distal vagina, the absence of a lateral vaginal sac, a vaginal appendix inserted near the distal end of the vagina and a longer flagellum, forms its own clade in mitochondrial analyses. Monacha parumcincta, meanwhile, turned out to be genuinely Corfiot: its Italian relatives are a separate species, now properly called Monacha manfredonica, a name originally proposed by Kobelt in 1891 for snails from the Gargano peninsula in Apulia. The Italian species lacks the refringent ring found in the proximal vagina of the Corfiot form and has a C-shaped cross-section of the central duct of the penial papilla, among other anatomical differences.

Both Corfu species faced a serious nomenclatural obstacle: the original type specimens, deposited by the nineteenth-century German malacologist Rossmässler, could not be found in his surviving collection at the Senckenberg Forschungsinstitut in Frankfurt. Curators confirmed in 2024 that no specimens labelled Helix claustralis or Helix parumcincta from Corfu or Dalmatia remain. Under the International Code of Zoological Nomenclature, the team therefore designated neotypes, newly chosen reference specimens that anchor each name to a precisely defined species. The neotype of Monacha claustralis, catalogued as SMF 383165, was collected near Molos cemetery on Corfu in October 2022; that of Monacha parumcinctus, SMF 383166, comes from Paleokastritsa on the same island. Crucially, both neotypes are characterised not only by shell and anatomy but also by DNA sequences deposited in GenBank, a practice that should prevent future confusion.

The historical detective work behind these designations was itself considerable. The name Helix claustralis first appeared in 1828 as a manuscript label by the shell dealer Franz A. Ziegler, mentioned only in synonymy by Menke, and was first validly established by Rossmässler in 1834 for a snail from Corfu. Because Menke merely listed the name as a synonym of a variant without using it as valid, authorship rightly belongs to Rossmässler, correcting an earlier interpretation by Hausdorf. A similarly convoluted history surrounds Helix parumcincta, whose identity was shaped by debates over Férussac’s poorly described Helix olivieri of 1821, a name that may only be fully resolved when a forthcoming edition of the Code addresses names proposed with descriptions for variants but not for the main form.

The revision did not stop at the two Corfu species. The team also redescribed Monacha cartusiana, the oldest known species of the genus, first named by Müller in 1774, whose syntypes survive in the Natural History Museum of Denmark. They examined Monacha gregaria from western Sicily and the Egadi islands, an easily recognised species lacking both digitiform glands and the vaginal appendix, and Monacha rizzae from Syracuse in Sicily, redescribed from topotypical specimens. Two further entities, Monacha cf. consona from the Nebrodi Mountains of north-eastern Sicily and an as-yet-unnamed large species from central and southern Calabria, were documented morphologically and, in part, genetically, with the Calabrian snail appearing as a well-separated lineage in COI phylogenies and probably representing another species in need of formal description.

The findings have practical implications beyond taxonomy. Monacha cartusiana has been spreading eastward across Europe in recent decades, with confirmed anatomical and molecular records from Germany, Poland, Slovakia, Hungary, Austria and the Balkans, and it has even been reported as a significant agricultural pest in Egypt and Saudi Arabia, though the species involved in those reports remain uncertain. The problem is compounded by evidence of hybridisation: in central and eastern Europe, researchers have found specimens whose reproductive anatomy matches one species while their mitochondrial DNA matches the other, suggesting interbreeding between Monacha claustralis and Monacha cartusiana in areas where one or both have been introduced. One Corfu specimen even carried an ITS2 sequence identical to that of Monacha cartusiana. Accurate identification, the authors stress, therefore requires both anatomical examination and molecular data, since neither alone is foolproof in regions of overlap.

The study is a reminder of how much biodiversity remains cryptic within familiar-looking animals, and of how classical museum collections continue to underpin modern science. By uniting Rossmässler’s fragile old shells, dissection trays, GenBank accessions and the fine print of the nomenclatural Code, the researchers have turned a two-hundred-year-old naming headache into a stable framework for studying the evolution, biogeography and invasive spread of Mediterranean land snails. Much work remains, particularly in Calabria and Sicily where several lineages still await formal names, but the foundation is now firmly in place.

Subject of Research: Integrative taxonomy and redescription of hygromiid land snail species of the genus Monacha from Corfu, southern Italy and Sicily

Article Title: Exploring Monacha species from the island of Corfu (NW Greece) continues: redescription of M. claustralis , M. parumcincta , and some allied Italian species ( Gastropoda , Eupulmonata , Hygromiidae )

Article References: Manganelli, G., Pieńkowska, J. R., Barbato, D., Benocci, A., Sosnowska-Kałasz, K., Lesicki, A., & Giusti, F. (2026). Exploring Monacha species from the island of Corfu (NW Greece) continues: redescription of M. claustralis, M. parumcincta, and some allied Italian species (Gastropoda, Eupulmonata, Hygromiidae). ZooKeys, 1290, 85-145. https://doi.org/10.3897/zookeys.1290.183303

Image Credits: AI Generated

DOI: 10.3897/zookeys.1290.183303

Keywords: Monacha, land snails, Gastropoda, Hygromiidae, Corfu, taxonomy, neotype, integrative taxonomy, DNA barcoding, Sicily, southern Italy, ZooKeys