Marteilia cocosarum in the Wash Estuary, UK: links to mass mortalities of cockles Cerastoderma edule and relationship to closely related species.

Globally, Marteilia spp. parasites have been associated with significant mass mortality events in populations of commercially important bivalve molluscs, frequently resulting in large-scale fishery collapses and substantial socio-economic impacts. The Wash Estuary, UK, supports several bivalve fisheries, and among these, common cockles Cerastoderma edule have suffered unusually high mortalities since 2008. We investigate potential causes of these mortalities, and confirm infection with M. cocosarum, strongly associated with cockle moribundity, also confirming its presence in archived samples collected in 2009. Molecular and light microscopy screening of samples collected during mortality events in 2021, including healthy (buried) and moribund (weak, unable to bury) cockles, indicated high prevalence of M. cocosarum in moribund cockles (PCR incidence up to 95%) in contrast to healthy cockles (up to 42%), suggesting an association between cockle moribundity and Marteilia infection. Analysis of the full ribosomal RNA array identified consistently different nucleotides between M. cocosarum infections in the Wash (denoted as genotype WE1) and those in Wales (denoted genotype WA1). A total of 83% of infections in the Wash could be identified as M. cocosarum WE1 and 12% as M. cocosarum WA1, with both genotypes recovered from 5% of infected animals. Histopathologically, M. cocosarum WE1 infects the gill, mantle and connective tissues, identical to observations of M. cocosarum infecting Welsh cockles. Ongoing cockle mortalities in the Wash raise concerns regarding the sustainability of this resource ecologically and economically. Additional measures may be required to reduce the spread of this pathogen, noting that its distribution beyond the Wash and Wales is currently unknown.