Host–parasite interactions between plerocercoids of Pyramicocephalus phocarum (Fabricius) Monticelli (Cestoda; Diphyllobothriidea) and White Sea cod (Gadus morhua marisalbi Derjugin) were studied, focusing on the tegument and secretory mechanisms and host capsule morphology.Histological and ultrastructural analyses revealed the host’s immune response to infection in the form of a granuloma. The granuloma was found to be composed of fibroblasts, mast cells, neutrophils, macrophages, and epithelioid cells. The parasite produced a protective layer, the cyst, as a structural response to the host immune reaction. Structures involved in the protection against the host included microtriches, extracellular vesicles, tegument organelles, and secretory products of neurons (neuro-exocrine secretion) and specialised eccrine glands. A unique structural adaptation of the P. phocarum plerocercoid was identified, namely the secretory poral cavities exhibiting both apocrine and neuro-exocrine secretion. These findings highlighted specialised adaptations of P. phocarum plerocercoids that may balance isolation from host immunity with access to nutrients, providing new insights into cestode–teleost relationships and the functional diversity of tegumental and neural secretions.
