Multiple Introductions of the Pestiferous Land Snail Theba pisana (Müller, 1774) (Gastropoda: Helicidae) in Southern California

SIMPLE SUMMARY: In Southern California, USA, the introduced white Italian land snail, Theba pisana, is prolific and locally pestiferous. To better understand its diversity and infer its parent population(s), we collected it from Los Angeles and San Diego counties and generated and analyzed gene sequence data (CO1, 16S, ITS2) that we compared between localities and to T. pisana CO1 barcodes from around the world. We also compared the morphology of the jaw, radula, and reproductive systems in T. pisana from Los Angeles and San Diego Counties. We found that T. pisana living at several sites in Los Angeles County in 2019–2020 had a single origin and were most similar in CO1 DNA sequence, based on available data, to specimens from Malta. Theba pisana collected from one site in San Diego County differed from Los Angeles T. pisana and were most similar in CO1 barcode sequence to specimens from Morocco. Jaw and mucous gland morphology also differed between Los Angeles and San Diego populations, but it is unclear if these traits are unique to lineages of T. pisana or if they change during a snail’s lifetime. We discuss how Los Angeles and San Diego T. pisana lineages may have arrived in Southern California and anticipate that the genetic data and morphological observations generated by this study will inform future studies of T. pisana where it is native and introduced. ABSTRACT: The terrestrial land snail Theba pisana is circum-Mediterranean in native range and widely introduced and pestiferous in regions around the world. In California, USA, T. pisana has been recorded intermittently since 1914, but its source population(s) are unknown, and no morphological or molecular analyses within or between California populations have been published. Therefore, we compared molecular data (CO1, 16S, ITS2) and internal morphology (jaw, radula, reproductive system) in T. pisana collected from Los Angeles and San Diego counties in 2019–2020. DNA barcode (CO1 mtDNA) analysis revealed that T. pisana from Los Angeles County was most similar to T. pisana from the Mediterranean island of Malta, and northern San Diego County-collected specimens were most similar to T. pisana from Morocco. Morphology of the jaw and mucous glands also differed between Los Angeles and San Diego populations, but it is unclear if traits are lineage-specific or artifacts of ontogeny. Several pathways of introduction into Southern California are possible for this species, but evidence for intentional vs. accidental introduction of present populations is lacking. Subsequent investigation(s) could use the data generated herein to assess the provenance of T. pisana elsewhere in California and/or worldwide and inform analyses of reproductive biology and systematics in this widespread species.