Histological description of specialized rootlets of the akentrogonid rhizocephalan Diplothylacus sinensis (Keppen, 1877) (Thompsoniidae) invading nervous ganglia of the blue swimming crab Portunus pelagicus

Parasitic barnacles (Thecostraca: Rhizocephala) are among the most striking parasitic manipulators in the animal kingdom. “Kentrogonid” rhizocephalans significantly alter the physiology and behaviour of brachyuran crabs, while “akentrogonid” influence on hosts remains poorly studied. The specialised rootlets invading the nervous system that are presumably crucial for the host-parasite interplay were described in basal rhizocephalan families that display a kentrogonid life cycle. In this study, we present a histological description of such rootlets in the “akentrogonid” Diplothylacus sinensis (Keppen, 1877) (family Thompsoniidae) in the ventral nervous ganglia of the crab Portunus pelagicus (Linnaeus, 1758). The morphology of these rootlets varies greatly in cuticle structure, gross organisation of the rootlet tissues and presence/absence of large vacuoles. It is assumed that rootlet modification appears during its development and successive growth from the penetration site into the ganglia's neuropil. The absence of the goblet-shaped organs in D. sinensis supports the hypothesis that these structures were secondarily lost in the crown lineages of the Rhizocephala.