Rhizocephalan barnacles are a unique group of endoparasitic crustaceans. In their extreme adaptation to endoparasitism, rhizocephalan adults have lost almost all features of their free-living relatives but acquired an outstanding degree of control over the body of their hosts (mostly decapods). The subtle influence exercised by rhizocephalans on the physiology, morphology and behaviour of their hosts is a vivid example of the most intimate host-parasite interactions but their mechanisms are very poorly known. In this study we examined the morphology and the adaptive ultrastructure of the organs invading the nervous system of the host in two rhizocephalan species from the families Peltogastridae, (Peltogaster paguri) and Peltogasterellidae (Peltogasterella gracilis). We found two essentially different types of structures involved in interactions of these two rhizocephalans with the nervous system of their hosts: modified rhizocephalan rootlets lying inside the ganglia and the neural fibres of the host enlacing the trophic rootlets of the parasites. We suggest that both these structures may be highly specialized tools allowing the parasite to interact with the host on the humoral level via neuromediators, hormones, attractants and trophic factors.

Rhizocephala is a group of unique and highly specialized parasitic crustaceans. Due to specialization in endoparasitism adult rhizocephalans have lost almost all the traits of free-living crustaceans. Despite large amount of data on different aspects of their biology there is still insufficient knowledge of their interna morphology. The muscular system in interna has been found and described only for Peltogaster paguri from the family Peltogastridae. The aim of our study is to visualize and describe muscular system in the interna of Polyascus polygenea and Sacculina pilosella (family Sacculinidae). Representatives of both species have similar type of muscular system, which is significantly different from that in P. paguri. The muscular system of the sacculinids consists of muscular rosettes located within the body wall of each rootlet. Muscle fibers in these rosettes are oriented in different directions. Tiny muscular processes connect the rosettes to each other. These contractile elements form a joint net-like muscular system. We suggest that this muscular system provides peristaltic movement of the roots and transports nutrients within the interna.

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