Integrative Taxonomy of Freshwater Mussels

Abstract

Freshwater mussels (Order Unionida) constitute the most imperilled animal group globally, with approximately 45% of North American species and 56% of European species assessed as threatened or extinct -- yet their taxonomy remains in a state of chronic instability driven by the extreme phenotypic plasticity of shell morphology that has led to widespread over-splitting in the 19th-century literature and wholesale under-splitting where cryptic lineages are masked by convergent shell form. This study applies a fully integrative taxonomic framework to the European freshwater mussel fauna (Families Unionidae and Margaritiferidae), combining mitochondrial COI + 16S barcoding (2,847 individuals from 184 populations), nuclear RAG1 and ITS1 sequencing, shell geometric morphometrics (38 landmarks; 847 shells), and reproductive host fish specificity data for 84 species-level hypotheses across 18 countries. The analysis reduces the nominal European freshwater mussel fauna from 84 traditionally recognised species to 47 validated species after synonymising 34 shell-based taxa with insufficient molecular or biological support, while simultaneously describing 8 new species from cryptic lineages identified by molecular barcoding. The resulting framework -- the first fully integrative taxonomic revision of European freshwater mussels -- provides the stable nomenclatural foundation required for accurate IUCN Red List assessment, national red listing, and conservation prioritisation of the continent's most threatened freshwater invertebrate group.