The interna of the rhizocephalan Peltogaster reticulata: Comparative morphology and ultrastructure.

Specialized morphology of diverse parasitic crustaceans reflects their adaptations to an endoparasitic lifestyle. Rhizocephalan barnacles are one of the most highly modified obligatory parasites of other crustaceans. Comprehension of the functional morphology of rhizocephalans could elucidate the main evolutionary trends not only inside parasitic barnacles, but in parasitism as a whole. Despite that, the available morphological information on the rhizocephalans is very fragmented. In this study, we examined the organization and ultrastructural features in different parts of the interna of Peltogaster reticulata (fam. Peltogastridae). The main trunk cuticle is much thicker than that of the side branches due to the different functions of these body parts. The central lumen in the main trunk is lined by an extracellular matrix, while the side branches are not. Muscular fibers are only present in the body wall of the main trunk, where they are organized as a "wicker basket". Furthermore, functional differentiation can be found at the ultrastructural level in the cells of the rootlets: there are distinct cell types both in hypodermal and axial cell layers. The rootlets of P. reticulata are covered by a network of the host's neurons and capillaries.

Specialised rootlets of Sacculina pilosella (Rhizocephala: Sacculinidae) used for interactions with its host's nervous system.

Parasitic rhizocephalan barnacles induce morphological, physiological, and behavioural changes in their hosts. The mechanisms of these intimate host-parasite interactions remain unknown. We have shown previously that rootlets of the internae of Peltogasterella gracilis and Peltogaster paguri penetrate the ganglion's envelope of their hermit crab hosts and form specialised structures in the ganglion periphery, the so-called goblet-shaped organs. Here, we examine the gross morphology and ultrastructure of these goblet-shaped organs in the interna of Sacculina pilosella. They consist of three layers of cells; in the intermediate layer of the organs, unusual lamellar bodies and muscle cells were found. Extensive degeneration of the host nervous tissue was observed in the funnel of the goblet-shaped organs. We conclude that the ability to penetrate into the host's nervous tissue could be a common trait in rhizocephalans. The goblet-shaped organs may play a key role in the host-parasite relationships by enabling the parasite to influence the host via hormones and neurotransmitters.