An alien intermediate host snail, Radix rubiginosa (Gastropoda: Lymnaeidae), on Unguja Island, Zanzibar.

In June 2025, two unusual populations of lymnaeid snails were encountered within peri-urban freshwater habitats of Unguja Island, Zanzibar (United Republic of Tanzania). These were noted within a wider malacological survey for freshwater intermediate host snails across twenty-one locations. Using an integrative taxonomic approach combining conchology and anatomy (n = 21) with mitochondrial 16S rDNA (n = 8) and mitochondrial cox1 (n = 3) sequence information, these lymnaeids were identified as Radix rubiginosa (Michelin, 1831). This is a first observation of this Southeast Asian alien species in East Africa, adding to its prior reports in South Africa. While no intrapopulation DNA sequence diversity was detected, very high DNA sequence similarities [16S rDNA(>99.4%) and cox1 (>99.6%] were noted, foremost with a Southeast Asian aquarium population. Upon field collection and then later by laboratory inspections (n=150), no trematode larvae were observed either upon cercarial shedding or by dissections. Furthermore, ad hoc parasitological screening of faeces from local cattle (n = 10) yielded no trematode eggs. Nonetheless, we raise a new regional surveillance flag for snail-borne diseases potentially transmissible by this alien intermediate host in Zanzibar.