Take the fish taxi! Phylogeny and palaeo-biogeography of the Margaritiferidae

Today, eleven species of Margaritiferidae occur scattered over Eurasia, North America and northern Africa. Margaritiferidae likely originate from the extinct Silesunionidae. Although identification of fossil shells is problematic, numerous Mesozoic and Cenozoic fossils are quite confidently assigned to the Margaritiferidae, the earliest ones from the Late Triassic of China.

Results of molecular phylogenetic analysis of all extant species, based on two mitochondrial and three nuclear markers, were processed with BEAST to infer divergence times. Assuming mean substitution rates, a Miocene origin for the Margaritiferidae was calculated. Using fossils for calibration, the results indicate substitution rates ten times lower than usual and intercontinental relationships at four different levels.

How come? Margaritiferidae occur exclusively in freshwater. Their larvae undergo a parasitic phase, attaching to the gills of a host fish, which acts as a vector for distribution. Most host fish of margaritiferids are amphidromous. It is thus unlikely that land bridges promote propagation. It is further unlikely that mussel larvae survive travel in saline waters. We propose that Margaritiferidae spread during phases of high freshwater runoff or seasonal freshwater layering. This model has general implications for the phylogeography of the Unionida, which all depend on the ‘fish taxi’ for propagation.