The Pericardial Body of Ciona intestinalis Contains Hemocytes and Degenerating Muscle Cells, But No Parasites

PURPOSE: A ventral heart positioned posterior to the branchial basket and equipped with a pericardium is homologous in tunicates and their sister group, the craniates, yet the tunicate model organism Ciona intestinalis features a pericardial body, a structure peculiar to few ascidian species. Here, we set out to distinguish between two competing hypotheses regarding the function of the pericardial body found in the literature: (H(1)) The pericardial body performs a role in the removal of dysfunctional myocardial cells, and (H(2)) it is a specialized niche of the immune system involved in defense against parasites. METHODS: We used histological techniques, transmission electron microscopy, and PCR-based gene sequencing to investigate whether individual ascidians parasitized with apicomplexan protists show signs of infections within the pericardial body. RESULTS: In individuals of C. intestinalis from the German North Sea infested with apicomplexan protists, the pericardial body contains numerous myocardial cells in various stages of degeneration while no remnants of parasitic cells could be identified. CONCLUSION: Thus, we conclude that H(2)—the pericardial body is a specialized niche of the immune system involved in defense against parasites—can be refuted. Rather, our observations support H(1), the hypothesis that the pericardial body performs a role in the removal of dysfunctional myocardial cells. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11686-020-00323-x.