Ultrastructure of the renopericardial complex in Hypselodoris tricolor (Gastropoda, Nudibranchia)

The histology and ultrastructure of the renopericardial complex of Hypselodoris tricolor (Gastropoda, Nudibranchia, Doridoidea) have been investigated by means of semithin serial sections and transmission electron microscopy (TEM). The examinations revealed a functional metanephridial system comprising a monotocardian heart with ventricle and auricle in a spacious pericardium that is linked with the single, large kidney by a renopericardial duct with prominent ciliation towards its opening. Podocytes as the site of ultrafiltration were not only detected in the auricular epicardium, but also line the entire outer pericardial epithelium. The cuboidal, highly vacuolated excretory cells of the kidney epithelium with extensive basal infoldings and an apical microvillous border indicate secretory and reabsorptive activity. Solitary rhogocytes (pore cells) of the connective tissue and haemocoel represent additional loci of ultrafiltration with a fine structure identical to that of the podocytes (slits between cytoplasmic processes, bridged by fine diaphragms and covered by extracellular matrix). The presence of podocytes situated in the epicardial wall of the auricle is regarded as plesiomorphic for the Mollusca and is confirmed for the Nudibranchia. An additional, extensive and separate ultrafiltration site in the outer pericardial wall is not known from any other taxon of the Mollusca and strongly suggests a significantly increased ultrafiltration activity in H. tricolor.