A Novel Paramyxean Parasite Marteilia Myarenae Sp. Nov. Infecting Softshell Clams Mya Arenaria in European Waters

Marteilia spp. parasites affect some marine bivalve species around the world, causing important economic losses and severe ecological effects. The present study reports, for the first time, the infection of the clam Mya arenaria by a protozoan parasite of the genus Marteilia, proposing a novel species in the genus: Marteilia myarenae sp. nov. Early infections were characterized by the occurrence of pre-sporulated stages of the parasite in the epithelium of the stomach; as the infection progressed, parasite sporulated stages were also observed in the epithelia of the digestive gland tubules. This species is characterized by primary cells containing at least 6 sporangia; each sporangium containing at least 7 spores. Although the number of spores per sporangia was compatible with M. octospora, both species have ultrastructural morphological differences in size and abundance of the inclusion bodies present in the sporangia. Moreover, phylogenetic analysis of the rDNA ITS1 and IGS regions indicates that this parasite is genetically distinct from other Marteilia spp. Susceptibility to infection is influenced by clam age/size, with juvenile clams showing higher prevalence than adult clams; but not by sex, with males and females being equally affected. The temporal dynamics of the disease presented a consistent annual pattern, characterized by annual minima of prevalence in winter, whereas peak values were detected in late spring-summer and autumn months. This information deeps into the phylogeny, ecology and epidemiology of Marteilia spp., contributing to better understand the impact and associated risks related with these parasites.