Ultrastructure of an opecoelid daughter sporocyst, Podocotyle sp. (Digenea: Opecoelidae): comparative analysis of the somatic tissues and new insights into the organization of the nervous system

Digenea is a group of widespread parasitic flatworms with a complex life cycle including a successive change of parthenogenetic and hermaphroditic generations. Daughter sporocysts are among the least studied parthenitae in terms of the ultrastructure of their body wall and nervous system. Here we present an ultrastructural and immunocytochemical study of Podocotyle sp. daughter sporocyst (Opecoelidae), parasitizing in Littorina obtusata Linnaeus, 1758 from the White Sea. Our results focus on the structure of the body wall, birth pore, excretory and nervous systems, and include new data on the taxonomic affiliation based on the partial cox1 mtDNA sequence. The daughter sporocyst of Podocotyle sp. has ultrastructural similarities with different plagiorchiid and diplostomid digeneans and also possesses some specific features. In the studied species, the birth canal is epithelialized; the body wall is composed of nine types of somatic cells and includes a well-developed central nervous system. Neurons of the latter form specific hemidesmosome-like attachment sites, which we found in digeneans for the first time. The obtained results are necessary to complement the comparative morphological analysis of daughter parthenogenetic generations in different digenean lineages.