| Summary: | Apicomplexans comprise a group of unicellular, often highly pathogenic, obligate parasites infecting both vertebrates and invertebrates, exploiting either one (monoxenous) or two hosts (heteroxenous) to complete a full reproductive life cycle. Their pathogenicity varies considerably between species and/or their hosts but as most species are obligate, intracellular parasites they cause some level of pathology in all cases. In relation to abnormal mortality events experienced in scallop populations in the North Atlantic, four different species of scallops, i.e. Iceland scallop, queen scallop, king scallop and sea scallop, were examined for infectious agents, with the aim of shedding light on these events. Most emphasis was made on the Iceland scallop stock in Icelandic waters that unexpectedly collapsed during the 2000s. Its health status was annually monitored for 14 years. In addition to the scallops, whelks and other mollusc species were examined at later stages in the study, in context with presumable life cycle of a pathogen observed. Two different apicomplexan species, both of which appeared to be previously unknown, were identified: 1) Margolisiella islandica, a novel species infecting the heart auricles and highly prevalent in Iceland scallop, without causing any significant pathology, but absent in the three other scallop species. It is a monoxenous species, with all life stages present in a single host, i.e. the Iceland scallop. 2) Initially anonymous species, infecting muscular- and connective tissues, was found in all four scallop species examined. Except for scallops from the UK, infections were generally heavy, causing severe histopathological changes leading to significant reduction in the general condition of the scallops, especially adductor muscles, which become abnormally reduced and discoloured. Furthermore, it hampers normal gonad development, at least in the Iceland scallop. Molecular studies and in situ hybridization, revealed that it is conspecific with Merocystis kathae, an apicomplexan ...
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