Further observations on <i>Renicola parvicaudatus</i> (Digenea, Renicolidae): structure of the ‘paletot’ and body wall composition of the daughter sporocyst

Abstract Adult flukes of the family Renicolidae parasitize the kidneys and ureters of aquatic birds. Daughter parthenitae of renicolids are sac-like sporocysts covered by a layer of host tissue, often described as a paletot. Using the example of Renicola parvicaudatus, the intramolluscan stages of which parasitize the common periwinkle, we present the first detailed description of the microanatomy of daughter sporocysts in the Renicolidae. The combination of electron and confocal microscopy allowed us to obtain data on the structure of the ‘paletot’, tegument, musculature, brood lining, parenchymal, and nerve cells. Ultrastructural observation revealed eight types of somatic cells that exhibit different specializations in the body wall. Although the studied sporocysts share many structural similarities with parthenitae of various digeneans, they also possess distinct features. These include the presence of an epithelial lining of the birth canal and a significant reduction of the nervous system. Our results also demonstrated that the encapsulation response prevails in the host–parasite interface of the Littorina littorea – R. parvicaudatus system. Modified host hyalinocytes aggregate into a paletot-like layer only in the anterior region, while degrading hyalinocytes, myofibroblast-like cells, and extracellular filaments surround the rest of the sporocyst surface.