Freshwater Haplosporidian Diversity Associated With Invasive Non-Native Bivalves in the River Thames, Including Haplosporidium raabei Infecting Dreissena polymorpha.

Haplosporidians are protistan parasites of aquatic invertebrates. Described here is the first report of the zebra mussel (Dreissena polymorpha) parasite Haplosporidium raabei in the River Thames, London, UK. The prevalence of this protist in the upper tidal Thames, Richmond, was higher than previously reported in other studies from the UK and mainland Europe of infected D. polymorpha populations, with molecular detection of H. raabei DNA in all D. polymorpha specimens collected. Also reported is the molecular detection (DNA and RNA) of H. raabei in other invasive Thames bivalves: the Asian clam (Corbicula fluminea) and quagga mussel (Dreissena bugensis). The molecular sequence of a closely related, but genetically distinct H. raabei-like sequence associated with D. polymorpha is also reported. Two new PCR assays are presented for the specific and sensitive detection of H. raabei and H. raabei-like sequences, enabling independent screening of these two lineages and also confirming the molecular presence of H. raabei in native unionid mussel species. A diversity of novel haplosporidian lineages associated with other invasive bivalves C. fluminea and D. bugensis and associated eDNA from the River Thames is also reported, demonstrating haplosporidian diversity associated with freshwater bivalves that is yet to be explored.