Suppl material to CTOZ - Unveiling Icelandic land snail fauna: new discoveriesand biogeographical insights

Iceland is the westernmost stronghold of European flora and fauna in the North Atlantic and

provides a model system for studying post-glacial colonisation. We conducted quantitative sampling

of terrestrial gastropods at 97 natural sites in Iceland between 2016 and 2024, documenting

29 species. Local diversity ranged from zero to 14 species (10 excluding slugs), with an average of

five species (four excluding slugs). Of the species recorded, Carychium tridentatum (Risso, 1826),

Euconulus alderi (Gray, 1840), Vertigo lilljeborgi (Westerlund, 1871) and V. substriata (Jeffreys, 1833)

are new to Iceland and all occur in minerotrophic fens along the south coast. Their distribution is

highly localised, with C. tridentatum and V. lilljeborgi occurring at only one and two sites respectively.

Their absence in similar habitats in the north, except for a single record of V. lilljeborgi,

suggests a climate-driven limitation. Phylogenetic reconstruction and haplotype networks based

on mitochondrial DNA confirm their European origin and are consistent with previously studied

species. Genetic similarities between Icelandic populations and those from the British Isles

and mainland Europe support the hypothesis of natural dispersal via migratory birds. Our results

emphasise the role of climate and habitat heterogeneity in shaping the Icelandic gastropod fauna,

while suggesting that most species arrived postglacially through passive dispersal mechanisms.

This study expands the knowledge of Icelandic biodiversity and underscores the importance of

wetlands as refugia for rare species in this extreme environment.