Plerocercoid Rockacestus cf. piriei from the White Sea stickleback and cod: Genetic analysis with description of ultrastructure of the tegument and excretory system.

Plerocercoids of the order Phyllobothriidea were discovered in the gastrointestinal tract of the three-spined stickleback, Gasterosteus aculeatus, and White Sea cod, Gadus morhua marisalbi, in the White Sea. Based on the 28S rDNA sequence, plerocercoids from Gasterosteus aculeatus belong to Rockacestus cf. piriei. A phylogenetic tree was reconstructed to infer their evolutionary relationships. Several aspects of larval biology were described: the prevalence and intensity of infection were quantified in host fishes; movement patterns of the scolex and body were documented in vitro. The maximum infection rate of Rockacestus cf. piriei plerocercoids in Gasterosteus aculeatus was observed in June 2021. Some seasonality of fish infection by these parasites can be linked with three-spined stickleback migration. The detailed morphology of the scolex and microtriches, as well as the ultrastructure of the tegument and excretory system, have been studied using scanning and transmission electron microscopy. Unlike other cestodes, the tegument lacks rod-shaped bodies in its distal cytoplasm and exhibits uniform microtriches, specifically capilliform filitriches. Subtegumental musculature is well developed. The excretory system comprises cyrtocytes and syncytial epithelium forming protonephridial funnels, thin tubules and canals. The cytoplasm of the canal wall contains numerous electron-dense rod-shaped bodies oriented parallel to the surface. Several specific features in the ultrastructure of the tegument and excretory epithelium of the R. cf. piriei plerocercoid have been identified, contributing to the taxonomic and functional understanding of marine cestodes.